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Operating Manual
CCD Camera Models
ST-7E, ST-8E and ST-9E
Santa Barbara Instrument Group
1482 East Valley Road • Suite 33
PO Box 50437
Santa Barbara, CA 93150
Phone (805) 969-1851 · Fax (805) 969-4069
Web:<www.sbig.com> · Email:<sbig@sbig.com>
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Table of Contents
1.
1.1.
1.2.
Introduction .................................................................................................................1
Road Map of the Documentation ...............................................................................1
Quick Tour....................................................................................................................1
1.2.1. CCDOPS Software ....................................................................................2
1.2.2. CCD Camera .............................................................................................3
2.
2.1.
2.2.
Introduction to CCD Cameras...................................................................................5
Cameras in General .....................................................................................................5
How CCD Detectors Work..........................................................................................5
2.2.1. Full Frame and Frame Transfer CCDs ....................................................6
Camera Hardware Architecture .................................................................................6
CCD Special Requirements.........................................................................................8
2.4.1. Cooling.......................................................................................................8
2.4.2. Double Correlated Sampling Readout ....................................................9
2.4.3. Dark Frames ..............................................................................................9
2.4.4. Flat Field Images .....................................................................................10
2.4.5. Pixels vs. Film Grains..............................................................................10
2.4.6. Guiding ....................................................................................................11
Electronic Imaging .....................................................................................................12
Black and White vs. Color.........................................................................................13
2.3.
2.4.
2.5.
2.6.
3.
At the Telescope with a CCD Camera....................................................................15
Step by Step with a CCD Camera ............................................................................15
Attaching the Camera to the Telescope ...................................................................15
Establishing a Communications Link ......................................................................16
Focusing the CCD Camera........................................................................................16
Finding and Centering the Object ............................................................................18
Taking an Image ........................................................................................................18
Displaying the Image ................................................................................................18
Processing the Image .................................................................................................18
Advanced Capabilities...............................................................................................19
3.9.1. Crosshairs Mode (Photometry and Astrometry)..................................19
3.9.2. Sub-Frame Readout in Focus .................................................................19
3.9.3. Track and Accumulate............................................................................20
3.9.4. Autoguiding and Self Guiding ..............................................................20
3.9.5. Auto Grab ................................................................................................21
3.9.6. Color Imaging..........................................................................................21
3.1.
3.2.
3.3.
3.4.
3.5.
3.6.
3.7.
3.8.
3.9.
4.
Camera Hardware .....................................................................................................23
System Components..................................................................................................23
Connecting the Power ...............................................................................................23
Connecting to the Computer ....................................................................................23
Connecting the Relay Port to the Telescope ............................................................23
4.4.1 Using Mechanical Relays ...........................................................................24
4.1.
4.2.
4.3.
4.4.
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4.5.
Modular Family of CCD Cameras............................................................................26
4.6 Connecting the older model CFW-6 filter wheel to the Camera ..................................30
4.7 Battery Operation .............................................................................................................31
5.
Advanced Imaging Techniques ..............................................................................33
Lunar and Planetary Imaging...................................................................................33
Deep Sky Imaging .....................................................................................................33
Terrestrial Imaging ....................................................................................................33
Taking a Good Flat Field ...........................................................................................34
Building a Library of Dark Frames...........................................................................34
Changing the Camera Resolution ............................................................................34
Flat Fielding Track and Accumulate Images...........................................................35
Tracking Functions ....................................................................................................36
5.1.
5.2.
5.3.
5.4.
5.5.
5.6.
5.7.
5.8.
6.
Accessories for your CCD Camera..........................................................................39
Cooling Booster..........................................................................................................39
Tri-color Imaging .......................................................................................................40
Camera Lens Adapters and Eyepiece Projection ....................................................40
Focal Reducers ...........................................................................................................40
AO-7 and Lucy-Richardson Software ......................................................................40
SGS - Self-Guided Spectrograph...............................................................................41
Third Party Products and Services ...........................................................................41
6.7.1. Windows Software..................................................................................41
6.7.2. Image Processing Software.....................................................................41
6.7.3. Getting Hardcopy ...................................................................................41
SBIG Technical Support.............................................................................................42
6.1.
6.2.
6.3.
6.4.
6.5.
6.6.
6.7.
6.8.
7.
Common Problems ...................................................................................................43
Glossary......................................................................................................................47
Appendix A - Connector ad Cables ........................................................................51
8.
A.
A.1. Appendix A - Connector Pinouts .............................................................................51
A.2. SBIG Tracking Interface Cable (TIC-78) ...................................................................51
B.
B.1.
B.2.
Appendix C - Maintenance ......................................................................................53
Cleaning the CCD and the Window ........................................................................53
Regenerating the Desiccant.......................................................................................53
C.
Appendix C - Capturing a Good Flat Field............................................................55
C.1.
Technique 55
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Section 1 - Introduction
1.
Introduction
Congratulations and thank you for buying one of Santa Barbara Instrument Group's CCD
cameras. The model ST-7E, ST-8E and ST-9E are SBIG's fourth generation CCD cameras and
represent the state of the art in CCD camera systems with their low noise and advanced
capabilities, including Kodak's new Blue Enhanced E series of CCDs. We feel that these
cameras will expand your astronomy experience by being able to easily take images like the
ones you've seen in books and magazines, of structure never seen through the eyepiece. SBIG
CCD cameras offer convenience, high sensitivity, and advanced image processing techniques
that film just can't match. While CCDs will probably never replace film in its large format,
CCDs allow a wide range of scientific measurements and have established a whole new field of
amateur astronomy that is growing by leaps and bounds.
The ST-7E, ST-8E and ST-9E cameras include an exciting new feature: self-
guiding (US Patent 5,525,793). These cameras have two CCDs inside; one for guiding and a
large one for imaging. The low noise of the read out electronics virtually guarantees that a
usable guide star will be within the field of the guiding CCD for telescopes with F/ numbers
F/ 6.3 or faster. The relay output plugs directly into most recent commercial telescope drives.
As a result, you can take hour long guided exposures with ease, with no differential deflection
of guide scope relative to main telescope, and no radial guider setup hassles, all from the
computer keyboard. This capability, coupled with the phenomenal sensitivity of the CCD, will
allow the user to acquire observatory class images of deep sky images with modest apertures!
The technology also makes image stabilization possible through our AO-7, or self-guided
spectroscopy with our SGS.
1.1. Road Map of the Documentation
This manual describes the ST-7E, ST-8E and ST-9E CCD Camera Systems from Santa Barbara
Instrument Group. For users new to the field of CCD Astronomy, Sections 2, 3 and 4 offer
introductory material about CCD Cameras and their applications in Astronomy. Users who are
familiar with CCD cameras may wish to skip section 2 and browse through sections 3 and 4,
reading any new material.
Thoroughly experienced SBIG customers may wish to jump right to the separate
Software Manual, which gives detailed and specific information about the SBIG software.
Sections 5 and 6 offer hints and information about advanced imaging techniques and
accessories for CCD imaging that you may wish to read after your initial telescope use of the
CCD camera. Finally, section 7 may be helpful if you experience problems with your camera,
and the Appendices provide a wealth of technical information about these systems.
1.2. Quick Tour
This section is a quick guided tour of the CCD Camera System you have just purchased. If
you're like most people you want to get started right away and dispense with the manual. Use
this section as a guide for learning about your new system.
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Section 1 - Introduction
1.2.1. CCDOPS Software
Follow the instructions below to run the CCDOPS software and display and process sample
images included on the distribution diskette.
•
Install the software onto your hard disk. For Windows this involves running
the Setup.exe file on the first diskette. For Macintosh or DOS this involves
copying the contents of the floppy disk to a folder or directory on your hard
disk.
CCDOPS for Windows or Macintosh
•
•
Double-click on the CCDOPS icon to launch the program.
Use the Open command in the File menu to load one of the sample images.
A window showing the exposure time, etc. will appear. Click in it to make it
disappear. The image will show up in its own window.
•
Try using the crosshairs. Use the Crosshairs command in the Display menu.
Use the mouse to move the crosshair around in the image and see the pixel
values.
•
•
•
•
Close the crosshairs and try inverting the image. Click the Invert item in the
Contrast window.
Try the photo display mode. Use the Photo Mode command in the Display
menu. Click the mouse to return to the menus.
Load up the other sample images and display them using the photo display
mode. You have to close any existing image first.
If you find that the display is too dark or bright, try setting Auto Contrast in
the Contrast window or adjust the background and range parameters to
achieve the best display. You may have to hit the Apply button in the
Contrast window to see changes in the Background and Range
CCDOPS for DOS
•
You should make the CCDOPS floppy or directory the active directory, then
give DOS the CCDOPS command.
•
•
•
Use the Open command in the File menu to load one of the sample images.
Use the Image command in the Display menu to display the image.
Try using the crosshairs. Hit the 'X' key. Use the mouse or arrow keys to
move the crosshair around in the image and see the pixel values.
•
•
Quit the crosshairs and try inverting the image. Hit the Esc key to quit the
Crosshairs mode, then hit the 'N' key.
Try the photo display mode. Exit the display mode by hitting the Esc key
and then use the Display Image command again, this time selecting the
Photo Display mode instead of the Analysis mode.
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Section 1 - Introduction
•
•
Load up the other sample images and display them using the photo display
mode.
If you find that the display is too dark or bright, try setting Auto Contrast in
the display menu or adjust the background and range parameters to achieve
the best display. Usually your monitor brightness and contrast want to be
set fairly high.
Note: Full daylight at F/22 will saturate these cameras with the shortest exposure.
With a camera lens start out in dim room light. For full sunlight you will need
a neutral density filter.
1.2.2. CCD Camera
Unfortunately there really aren't many shortcuts you can take when using the CCD camera to
capture images. The instructions below refer you to various sections of the manual.
•
•
•
Insert the CCD Camera into the telescope and focus on a star (refer to
Sections 3.2 and 3.3).
Find some relatively bright object like M51, the Ring Nebula (M57) or the
Dumbbell Nebula (M27) (refer to section 3.5).
Take a 1 minute exposure using the Grab command with the Dark frame
option set to Also (refer to Section 3.6).
•
•
Display the image (refer to Section 3.7).
Process the image (refer to Section 3.8).
If you happen to have purchased a camera lens adapter for your CCD Camera you can use that
to take images in the daytime. Additionally you could make a small pinhole aperture out of a
piece of aluminum foil after wrapping it around the camera's nosepiece.
•
•
•
•
•
Shut down the F stop all the way to F/ 16 or F/ 22.
Set the focus based upon the object and the markings on the lens.
Take a 1 second exposure with the Grab command.
Display the image (refer to Section 3.7).
Process the image (refer to Section 3.8).
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Section 2 - Introduction to CCD Cameras
2.
Introduction to CCD Cameras
This section introduces new users to CCD (Charge Coupled Device) cameras and their
capabilities and to the field of CCD Astronomy and Electronic Imaging.
2.1. Cameras in General
The CCD is very good at the most difficult astronomical imaging problem: imaging small, faint
objects. For such scenes long film exposures are typically required. The CCD based system has
several advantages over film: greater speed, quantitative accuracy, ability to increase contrast
and subtract sky background with a few keystrokes, the ability to co-add multiple images
without tedious dark room operations, wider spectral range, and instant examination of the
images at the telescope for quality. Film has the advantages of a much larger format, color, and
independence of the wall plug (the SBIG family of cameras can be battery operated in
conjunction with a laptop computer, though, using a power inverter). After some use you will
find that film is best for producing sensational large area color pictures, and the CCD is best for
planets, small faint objects, and general scientific work such as variable star monitoring and
position determination.
2.2. How CCD Detectors Work
The basic function of the CCD detector is to convert an incoming photon of light to an electron
which is stored in the detector until it is read out, thus producing data which your computer
can display as an image. It doesn't have to be displayed as an image. It could just as well be
displayed as a spreadsheet with groups of numbers in each cell representing the number of
electrons produced at each pixel. These numbers are displayed by your computer as shades of
gray for each pixel site on your screen thus producing the image you see. How this is
accomplished is eloquently described in a paper by James Janesick and Tom Elliott of the Jet
Propulsion Laboratory:
"Imagine an array of buckets covering a field. After a rainstorm, the buckets are
sent by conveyor belts to a metering station where the amount of water in each
bucket is measured. Then a computer would take these data and display a
picture of how much rain fell on each part of the field. In a CCD the "raindrops"
are photons, the "buckets" the pixels, the "conveyor belts" the CCD shift registers
and the "metering system" an on-chip amplifier.
Technically speaking the CCD must perform four tasks in generating an image.
These functions are 1) charge generation, 2) charge collection, 3) charge transfer,
and 4) charge detection. The first operation relies on a physical process known
as the photoelectric effect - when photons or particles strikes certain materials
free electrons are liberated...In the second step the photoelectrons are collected in
the nearest discrete collecting sites or pixels. The collection sites are defined by
an array of electrodes, called gates, formed on the CCD. The third operation,
charge transfer, is accomplished by manipulating the voltage on the gates in a
systematic way so the signal electrons move down the vertical registers from one
pixel to the next in a conveyor-belt like fashion. At the end of each column is a
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Section 2 - Introduction to CCD Cameras
horizontal register of pixels. This register collects a line at a time and then
transports the charge packets in a serial manner to an on-chip amplifier. The
final operating step, charge detection, is when individual charge packets are
converted to an output voltage. The voltage for each pixel can be amplified off-
chip and digitally encoded and stored in a computer to be reconstructed and
displayed on a television monitor."1
Readout Register
Output
Y=1
Amplifier
Y=N
X=1
X=M
Figure 2.1 - CCD Structure
2.2.1. Full Frame and Frame Transfer CCDs
In the ST-7E, ST-8E and ST-9E, the CCD is read out electronically by shifting each row of pixels
into a readout register at the Y=0 position of the CCD (shown in Figure 2.1), and then shifting
the row out through an amplifier at the X=0 position. The entire array shifts up one row when
a row is shifted into the readout register, and a blank row is inserted at the bottom. The
electromechanical shutter built into the camera covers the CCD during the readout to prevent
streaking of the image. Without a shutter the image would be streaked due to the fact that the
pixels continue to collect light as they are being shifted out towards the readout register. CCDs
with a single active area are called Full Frame CCDs.
For reference, the ST-5C, ST-237, STV and ST-6 CCD cameras use a different type of
CCD which is known as a Frame Transfer CCD. In these devices all active pixels are shifted
very quickly into a pixel array screened from the light by a metal layer, and then read out. This
makes it possible to take virtually streak-free images without a shutter. This feature is typically
called an electronic shutter.
2.3. Camera Hardware Architecture
This section describes the ST-7E, ST-8E and ST-9E CCD cameras from a systems standpoint. It
describes the elements that comprise a CCD camera and the functions they provide. Please
refer to Figure 2.2 below as you read through this section.
1
"History and Advancements of Large Area Array Scientific CCD Imagers", James Janesick, Tom
Elliott. Jet Propulsion Laboratory, California Institute of Technology, CCD Advanced Development
Group.
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Section 2 - Introduction to CCD Cameras
TE Cooler
Clock
Trackin g CCD
Micro-
con troller
Drivers
PC
In terface
16 Bit
A/ D
Pream p
Telescope
In terface
Sh u tter
Im agin g CCD
Desktop
Power
Su pply
Host Com pu t er
Parallel In terface
Figure 2.2 - CCD System Block Diagram
As you can see from Figure 2.2, the ST-7E, ST-8E and ST-9E are completely self contained.
Unlike our previous products, the ST-7E, ST-8E and ST-9E contain all the electronics in the
optical head. There is no external CPU like the ST-5C, ST-237, ST-6 and STV.
At the "front end" of any CCD camera is the CCD sensor itself. As we have already
learned, CCDs are a solid state image sensor organized in a rectangular array of regularly
spaced rows and columns. The ST-7E, ST-8E and ST-9E use two CCDs, one for imaging (Kodak
KAF series) and one for tracking (TI TC211, like the ST-4/ 4X). Table 2.1 below lists some
interesting aspects of the CCDs used in the various SBIG cameras.
Array
Number of
Pixels
Camera
Tracking CCD
ST-5C
ST-237
STV
ST-6
ST-7E
ST-8E
ST-9E
CCD
Dimensions
2.6 x 2.6 mm
3.2 x 2.4 mm
4.7 x 3.6 mm
4.7 x 3.0 mm
8.6 x 6.5 mm
Pixel Sizes
13.75 x 16 µ
10 x 10 µ
7.4 x 7.4 µ
14.8 x 14.8 µ
23 x 27 µ
TC211
TC255
TC237
TC237
TC241
192 x 164
320 x 240
640 x 480
320 x 200
375 x 242
765 x 510
KAF0401E 6.9 x 4.6 mm
KAF1602E 13.8 x 9.2 mm
KAF0261E 10.2 x 10.2 mm
9 x 9 µ
1530 x 1020 9 x 9 µ
512 x 512 20 x 20 µ
Table 2.1 - Camera CCD Configurations
The CCD is cooled with a solid-state a thermoelectric (TE) cooler. The TE cooler pumps heat
out of the CCD and dissipates it into a heat sink which forms part of the optical head's
mechanical housing. In the ST-7E and ST-8E cameras this waste heat is dumped into the air
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Section 2 - Introduction to CCD Cameras
using passive radiators and a small fan, making the design and operation of the heads simple
and not inconvenienced by requirements for liquid recirculation cooling. The ST-9E includes
SBIG's secondary TE/ Liquid cooling booster.
Since the CCD is cooled below 0°C, some provision must be made to prevent frost from
forming on the CCD. The ST-7E, ST-8E and ST-9E cameras have the CCD/ TE Cooler mounted
in a windowed hermetic chamber sealed with an O-Ring. The hermetic chamber does not need
to be evacuated, another "ease of use" feature we employ in the design of our cameras. Using a
rechargeable desiccant in the chamber keeps the humidity low, forcing the dew point below the
cold stage temperature.
Other elements in the self contained ST-7E, ST-8E and ST-9E include the preamplifier
and an electromechanical shutter. The shutter makes taking dark frames a simple matter of
pushing a button on the computer and provides streak-free readout. Timing of exposures in
ST-7E, ST-8E and ST-9E cameras is controlled by this shutter.
The Clock Drivers and Analog to Digital Converter interface to the CCD. The Clock
Drivers convert the logic-level signals from the microcontroller to the voltage levels and
sequences required by the CCD. Clocking the CCD transfers charge in the array and is used to
clear the array or read it out. The Analog to Digital Converter (A/ D) digitizes the data in the
CCD for storage in the Host Computer.
The microcontroller is used to regulate the CCD's temperature by varying the drive to
the TE cooler. The external Power Supply provides +5V and ±12V to the cameras. Finally, the
cameras contain a TTL level telescope interface port to control the telescope and the optional
CFW-6A motorized color filter wheel.
Although not part of the CCD Camera itself, the Host Computer and Software are an
integral part of the system. SBIG provides software for the ST-7E, ST-8E and ST-9E cameras for
the IBM PC and Compatible computers (all cameras, DOS and Windows) and the ST-7E/ 8E are
also supported by the Macintosh. The software allows image acquisition, image processing,
and auto guiding with ease of use and professional quality. Many man-years and much
customer feedback have gone into the SBIG software and it is unmatched in its capabilities.
2.4. CCD Special Requirements
This section describes the unique features of CCD cameras and the special requirements that
CCD systems impose.
2.4.1. Cooling
Random readout noise and noise due to dark current combine to place a lower limit on
the ability of the CCD to detect faint light sources. SBIG has optimized the ST-7E, ST-8E and
ST-9E to achieve readout noises below 20 electrons rms for two reads (light - dark). This will
not limit most users. The noise due to the dark current is equal to the square root of the
number of electrons accumulated during the integration time. For these cameras, the dark
current is not significant until it accumulates to more than 280 electrons. Dark current is
thermally generated in the device itself, and can be reduced by cooling. All CCDs have dark
current, which can cause each pixel to fill with electrons in only a few seconds at room
temperature even in the absence of light. By cooling the CCD, the dark current and
corresponding noise is reduced, and longer exposures are possible. In fact, for roughly every
5°C of additional cooling, the dark current in the CCD is reduced to half. The ST-7E and ST-8E
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Section 2 - Introduction to CCD Cameras
have a single stage TE cooler and a temperature sensing thermistor on the CCD mount to
monitor the temperature. The ST-9E has a supplemental second stage cooling booster with
water cooling as an option (described in section 6.1). The microcontroller controls the
temperature at a user-determined value for long periods. As a result, exposures hours long are
possible, and saturation of the CCD by the sky background typically limits the exposure time.
The temperature regulation feature of the ST-7E, ST-8E and ST-9E also means that one dark
frame can be used for similar exposures on several nights. At 0 °C the dark current in the ST-
7E and ST-8E, high-resolution mode, is only 36 electrons per minute! The ST-9E, with it's
bigger pixels, has roughly 15 times this amount of dark current, 5 times due to the larger pixel
area and 3 times due to the inherent higher bulk dark current in the device. That's why we
include the cooling booster with the ST-9E.
The sky background conditions also increase the noise in images, and in fact, as far as
the CCD is concerned, there is no difference between the noise caused by dark current and that
from sky background. If your sky conditions are causing photoelectrons to be generated at the
-
rate of 100 e / pixel/ sec, for example, increasing the cooling beyond the point where the dark
current is roughly half that amount will not improve the quality of the image. This very reason
is why deep sky filters are so popular with astrophotography. They reduce the sky
background level, increasing the contrast of dim objects. They will improve CCD images from
very light polluted sights.
2.4.2. Double Correlated Sampling Readout
During readout, the charge stored in a pixel is stored temporarily on a capacitor. This capacitor
converts the optically generated charge to a voltage level for the output amplifier to sense.
When the readout process for the previous pixel is completed, the capacitor is drained and the
next charge shifted, read, and so on. However, each time the capacitor is drained, some
residual charge remains.
This residual charge is actually the dominant noise source in CCD readout electronics.
This residual charge may be measured before the next charge is shifted in, and the actual
difference calculated. This is called double correlated sampling. It produces more accurate
data at the expense of slightly longer read out times (two measurements are made instead of
one). The ST-7E, ST-8E and ST-9E utilize double correlated sampling to produce the lowest
-
-
possible readout noise. At 13e to 15e rms per read these cameras are unsurpassed in
performance.
2.4.3. Dark Frames
No matter how much care is taken to reduce all sources of unwanted noise, some will remain.
Fortunately, however, due to the nature of electronic imaging and the use of computers for
storing and manipulating data, this remaining noise can be drastically reduced by the
subtraction of a dark frame from the raw light image. A dark frame is simply an image taken
at the same temperature and for the same duration as the light frame with the source of light to
the CCD blocked so that you get a "picture" of the dark. This dark frame will contain an image
of the noise caused by dark current (thermal noise) and other fixed pattern noise such as read
out noise. When the dark frame is subtracted from the light frame, this pattern noise is
removed from the resulting image. The improvement is dramatic for exposures of more than a
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Section 2 - Introduction to CCD Cameras
minute, eliminating the many "hot" pixels one often sees across the image, which are simply
pixels with higher dark current than average.
2.4.4. Flat Field Images
Another way to compensate for certain unwanted optical effects is to take a "flat field image"
and use it to correct for variations in pixel response uniformity across the area of your dark-
subtracted image. You take a flat field image of a spatially uniform source and use the
measured variations in the flat field image to correct for the same unwanted variations in your
images. The Flat Field command allows you to correct for the effects of vignetting and
nonuniform pixel responsivity across the CCD array.
The Flat Field command is very useful for removing the effects of vignetting that may
occur when using a field compression lens and the fixed pattern responsivity variations present
in all CCDs. It is often difficult to visually tell the difference between a corrected and
uncorrected image if there is little vignetting, so you must decide whether to take the time to
correct any or all of your dark-subtracted images. It is always recommended for images that
are intended for accurate photometric measurements.
Appendix D describes how to take a good flat field. It's not that easy, but we have
found a technique that works well for us.
2.4.5. Pixels vs. Film Grains
Resolution of detail is determined, to a certain degree, by the size of the pixel in the detector
used to gather the image, much like the grain size in film. The pixel size of the detector in the
ST-7E and ST-8E is 9 x 9 microns (1 micron = 0.001mm, 0.04 thousandths of an inch) and in the
ST-9E it's 20 x 20 microns. However, the effects of seeing are usually the limiting factor in any
good photograph or electronic image. On a perfect night with excellent optics an observer
might hope to achieve sub-arcsecond seeing in short exposures, where wind vibration and
tracking error are minimal. With the average night sky and good optics, you will be doing well
to achieve stellar images in a long exposure of 3 to 6 arcseconds halfwidth. This will still result
in an attractive image, though.
Using an ST-7E or ST-8E camera with their 9 micron pixels, an 8" f/ 10 telescope will
produce a single pixel angular subtense of 0.9 arcsecond. A 8" f/ 4 telescope will produce
images of 2.5 arcseconds per pixel. If seeing affects the image by limiting resolution to 6
arcseconds, you would be hard pressed to see any resolution difference between the two focal
lengths as you are mostly limited by the sky conditions. However, the f/ 4 image would have a
larger field of view and more faint detail due to the faster optic. The ST-9, with its 20 micron
pixels would have the same relationship at roughly twice the focal length or a 16 inch f/ 10
telescope. See table 4.4 for further information.
A related effect is that, at the same focal length, larger pixels collect more light from
nebular regions than small ones, reducing the noise at the expense of resolution. While many
people think that smaller pixels are a plus, you pay the price in sensitivity due to the fact that
smaller pixels capture less light. The ST-9E with its large 20 x 20 micron pixels captures five
times as much light as the ST-7E and ST-8E's 9 micron square pixels. For this reason we
provide binning on the ST-7E and ST-8E that configure the camera for 18 or 27-micron square
pixels. Binning is selected using the Camera Setup Command. It is referred to as resolution
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Section 2 - Introduction to CCD Cameras
2
2
2
(High = 9µ pixels, Medium = 18µ pixels, Low = 27µ pixels). When binning is selected the
electronic charge from groups of 2 by 2 or 3x3 pixels is electronically summed in the CCD
before readout.. This process adds no noise. Binning should be used if you find that your
stellar images have a halfwidth of more than 3 pixels. If you do not bin, you are wasting
sensitivity without benefit.
The halfwidth of a stellar image can be determined using the crosshairs mode. Find the
peak value of a relatively bright star image and then find the pixels on either side of the peak
where the value drops to 50% of the peak value (taking the background into account, if the star
is not too bright). The difference between these pixel values gives the stellar halfwidth.
Sometimes you need to interpolate if the halfwidth is not a discrete number of pixels.
Another important consideration is the field of view of the camera. For comparison, the
diagonal measurement of a frame of 35mm film is approximately 43mm, whereas the diagonal
dimension of the ST-7E chip is approximately 8 mm. The relative CCD sizes for all of the SBIG
cameras and their corresponding field of view in an 8" f/ 10 telescope are given below:
Camera
Tracking CCD
ST-5C
ST-237
STV
ST-6
ST-7E
ST-8E
ST-9E
Array Dimensions
2.64 x 2.64 mm
3.20 x 2.40 mm
4.74 x 3.55 mm
4.74 x 2.96 mm
8.63 x 6.53 mm
6.89 x 4.59 mm
13.8 x 9.18 mm
10.2 x 10.2 mm
36 x 24 mm
Diagonal
3.73 mm
4.00 mm
5.92 mm
5.58 mm
10.8 mm
8.28 mm
16.6 mm
14.4 mm
43 mm
Field of View at 8" f/ 10
4.5 x 4.5 arcminutes
5.6 x 4.2 arcminutes
8.2 x 6.1 arcminutes
8.2 x 5.1 arcminutes
14.6 x 11 arcminutes
11.9 x 7.9 arcminutes
23.8 x 15.8 arcminutes
17.6 x 17.6 arcminutes
62 x 42 arcminutes
35mm
Table 2.2 - CCD Array Dimensions
2.4.6. Guiding
Any time you are taking exposures longer than several seconds, whether you are using a film
camera or a CCD camera, the telescope needs to be guided to prevent streaking. While modern
telescope drives are excellent with PEC or PPEC, they will not produce streak-free images
without adjustment every 30 to 60 seconds. The ST-7E, ST-8E and ST-9E allow simultaneous
guiding and imaging, called self-guiding (US Patent 5,525,793). This is possible because of the
unique design employing 2 CCDs. One CCD guides the telescope while the other takes the
image. This resolves the conflicting requirements of short exposures for guiding accuracy and
long exposures for dim objects to be met, something that is impossible with single CCD
cameras. Up to now the user either had to set up a separate guider or use Track and
Accumulate to co-add several shorter images. The dual CCD design allows the guiding CCD
access to the large aperture of the main telescope without the inconvenience of off-axis radial
guiders. Not only are guide stars easily found, but the problems of differential deflection
between guide scope and main scope eliminated.
Track and Accumulate is another SBIG patented process (US #5,365,269) whereby short
exposures are taken and added together with appropriate image shifts to align the images. It is
supported by the ST-7E, ST-8E and ST-9E camera software, but will generally not produce as
good as results as self guiding, where the corrections are more frequent and the accumulated
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Section 2 - Introduction to CCD Cameras
readout noise less. It is handy when no connection to the telescope drive is possible. SBIG is
proud to make self-guiding available to the amateur, making those long exposures required by
the small pixel geometry of the ST-7 and ST-8 easy to achieve!
2.5. Electronic Imaging
Electronic images resemble photographic images in many ways. Photographic images are
made up of many small particles or grains of photo sensitive compounds which change color or
become a darker shade of gray when exposed to light. Electronic images are made up of many
small pixels which are displayed on your computer screen to form an image. Each pixel is
displayed as a shade of gray, or in some cases a color, corresponding to a number which is
produced by the electronics and photo sensitive nature of the CCD camera. However,
electronic images differ from photographic images in several important aspects. In their most
basic form, electronic images are simply groups of numbers arranged in a computer file in a
particular format. This makes electronic images particularly well suited for handling and
manipulation in the same fashion as any other computer file.
An important aspect of electronic imaging is that the results are available immediately.
Once the data from the camera is received by the computer, the resulting image may be
displayed on the screen at once. While Polaroid cameras also produce immediate results,
serious astrophotography ordinarily requires hypersensitized or cooled film, a good quality
camera, and good darkroom work to produce satisfying results. The time lag between
exposure of the film and production of the print is usually measured in days. With electronic
imaging, the time between exposure of the chip and production of the image is usually
measured in seconds.
Another very important aspect of electronic imaging is that the resulting data are
uniquely suited to manipulation by a computer to bring out specific details of interest to the
observer. In addition to the software provided with the camera, there are a number of
commercial programs available which will process and enhance electronic images. Images may
be made to look sharper, smoother, darker, lighter, etc. Brightness, contrast, size, and many
other aspects of the image may be adjusted in real time while viewing the results on the
computer screen. Two images may be inverted and electronically "blinked" to compare for
differences, such as a new supernova, or a collection of images can be made into a large mosaic.
Advanced techniques such as maximum entropy processing will bring out otherwise hidden
detail.
Of course, once the image is stored on a computer disk, it may be transferred to another
computer just like any other data file. You can copy it or send it via modem to a friend, upload
it to your favorite bulletin board or online service, or store it away for processing and analysis
at some later date.
We have found that an way to obtain a hard copy of your electronic image is to
photograph it directly from the computer screen. You may also send your image on a floppy
disk to a photo lab which has digital photo processing equipment for a professional print of
your file. Make sure the lab can handle the file format you will send them. Printing the image
on a printer connected to your computer is also possible depending on your software/ printer
configuration. There are a number of software programs available, which will print from your
screen. However, we have found that without specialized and expensive equipment, printing
images on a dot matrix or laser printer yields less than satisfactory detail. However, if the
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Section 2 - Introduction to CCD Cameras
purpose is simply to make a record or catalog the image file for easy identification, a dot matrix
or laser printer should be fine. Inkjet printers are getting very good, though.
2.6. Black and White vs. Color
The first and most obvious appearance of a CCD image is that it is produced in shades of gray,
rather than color. The CCD chip used in SBIG cameras itself does not discriminate color and
the pixel values that the electronics read out to a digital file are only numbers proportional to
the number of electrons produced when photons of any wavelength happen to strike its
sensitive layers.
Of course, there are color video cameras, and a number of novel techniques have been
developed to make the CCD chip "see" color. The most common way implemented on
commercial cameras is to partition the pixels into groups of three, one pixel in each triplet
"seeing" only red, green or blue light. The results can be displayed in color. The overall image
will suffer a reduction in resolution on account of the process. A newer and more complicated
approach in video cameras has been to place three CCD chips in the camera and split the
incoming light into three beams. The images from each of the three chips, in red, green and
blue light is combined to form a color image. Resolution is maintained. For normal video
modes, where there is usually plenty of light and individual exposures are measured in small
fractions of a second, these techniques work quite well. However, for astronomical work,
exposures are usually measured in seconds or minutes. Light is usually scarce. Sensitivity and
resolution are at a premium. The most efficient way of imaging under these conditions is to
utilize all of the pixels, collecting as many photons of any wavelength, as much of the time as
possible.
In order to produce color images in astronomy, the most common technique is to take
three images of the same object using a special set of filters and then recombine the images
electronically to produce a color composite or RGB color image. SBIG offers as an option a
motorized color filter wheel. The accessory is inserted between the telescope and the CCD
head. An object is then exposed using a red filter. The wheel is commanded to insert the green
filter in place, and another image taken. Finally a blue image is taken. When all three images
have been saved, they may be merged into a single color image using SBIG or third party color
software.
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Section 3 - At the Telescope with a CCD Camera
3.
At the Telescope with a CCD Camera
This section describes what goes on the first time you take your CCD camera out to the
telescope. You should read this section throughout before working at the telescope. It will
help familiarize you with the overall procedure that is followed without drowning you in the
details. It is recommended you first try operating the camera in comfortable, well lit
surroundings to learn its operation.
3.1. Step by Step with a CCD Camera
In the following sections we will go through the steps of setting up and using your CCD
camera. The first step is attaching the camera to the telescope. The next step is powering up
the camera and establishing a communication link to your computer. Then you will want to
focus the system, find an object and take an image. Once you have your light image with a
dark frame subtracted, you can display the image and process the results to your liking. Each
of these steps is discussed in more detail below.
3.2. Attaching the Camera to the Telescope
ST-7E, ST-8E and ST-9E cameras are similar in configuration. The CCD head attaches to the
telescope by slipping it into the eyepiece holder. A fifteen foot cable runs from the head to the
host computer's parallel port. The camera is powered by a desktop power supply. Operation
from a car battery is possible using the optional 12V power supply or with a 12V to 110V power
inverter.
Connect the CCD head to the parallel port of your computer using the supplied cable
and insert the CCD Camera's nosepiece into your telescope's eyepiece holder. Fully seat the
camera against the end of the draw tube so that once focus has been achieved you can swap out
and replace the camera without having to refocus. Orient the camera so that the CCD's axes
are aligned in Right Ascension and Declination. Use Figure 3.1 below showing the back of the
optical head as a guide for the preferred orientation. Any orientation will work, but it is
aggravating trying to center objects when the telescope axes don't line up fairly well with the
CCD axes.
Next, connect the power cable and plug in the desktop power supply. The red LED on
the rear of the camera should glow and the fan should spin indicating power has been applied
to the unit. We recommend draping the cables over the finderscope, saddle or mount to
minimize cable perturbations of the telescope, and guard against the camera falling out of the
drawtube to the floor. We also recommend using the T-Ring attachments for connecting the
camera to the telescope, as the cameras are heavy.
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Section 3 - At the Telescope with a CCD Camera
RA
DEC
*
Figure 3.1 Orientation of the Optical Head Viewed from Back.
(Pixel 1,1 is at the upper left in this view)
3.3. Establishing a Communications Link
When the CCDOPS program is initiated it will automatically attempt to establish a link to the
camera. This involves identifying the type of CCD head. If the software is successful the
"Link" field in the Status Window is updated to show the type of camera found. If the camera
is not connected, powered up, or the LPT port setting has not yet been properly set, a message
will be displayed indicating that the software failed to establish a link to the camera. If this
happens, use the Communications Setup command in the Misc menu to configure the
CCDOPS software for the LPT port you are using. Then use the Establish COM Link command
in the Camera Menu to establish communications with the camera.
Note: It is not necessary to have a camera connected to your computer to run the software and
display images already saved onto disk. It is only necessary to have a camera connected
when you take new images.
Once the COM link has been established you may need to set the camera's setpoint
temperature in the Camera Setup command. The ST-7E, ST-8E and ST-9E power up regulating
to whatever temperature the CCD is at, which in this case will be the ambient temperature.
Use the Camera Setup command and choose a setpoint temperature approximately 30°C below
the ambient temperature. Type in the setpoint, set the temperature control to active, and hit
ENTER.
3.4. Focusing the CCD Camera
Focusing a CCD camera can be a tedious operation, so a few hints should be followed. Before
using the software to focus the camera the first time you should place a diffuser (such as scotch
tape or ground glass) at the approximate location of the CCD's sensitive surface behind the
eyepiece tube and focus the telescope on the moon, a bright planet or a distant street lamp.
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Section 3 - At the Telescope with a CCD Camera
This preliminary step will save you much time in initially finding focus. The approximate
distance behind the eyepiece tube for each of our CCD cameras is listed in Table 3.1 below:
Camera
ST-5C
ST-237/ STV
ST-6
ST-7E/ 8E/ 9E
Distance
Diffuser
0.660 inch
0.680 inch
0.560 inch
0.920 inch
Table 3.1 - Camera Back Focus
Back Focus Distance
from Table 3.1
To achieve fine focus, insert the CCD head into the eyepiece tube, taking care to seat it,
and then enter the CCDOPS FOCUS mode. The Focus command automatically displays
successive images on the screen as well as the peak brightness value of the brightest object in
the field of view. Point the telescope at a bright star. Center the star image in the CCD, and
adjust the focus until the star image is a small as can be discerned. Next, move the telescope to
a field of fainter stars that are dimmer so the CCD is not saturated. Further adjust the focus to
maximize the displayed star brightness in counts and minimize the star diameter. This can be
tedious. It helps considerably if a pointer or marker is affixed to the focus knob so you can
rapidly return to the best focus once you've gone through it.
An exposure of 1 to 3 seconds is recommended to smooth out some of the atmospheric
effects. While you can use the Full frame mode to focus, the frame rate or screen update rate
can be increased significantly by using Planet mode. In Planet mode the Focus command takes
a full image and then lets you position a variable sized rectangle around the star. On
subsequent images the Planet mode only digitizes, downloads, and displays the small area you
selected. The increase in frame rate is roughly proportional to the decrease in frame size,
assuming you are using a short exposure.
The telescope focus is best achieved by maximizing the peak value of the star image.
You should be careful to move to a dimmer star if the peak brightness causes saturation. The
saturation levels of the various resolution modes are shown in Table 3.2 below. Another point
you should also be aware of is that as you approach a good focus, the peak reading can vary by
30% or so. This is due to the fact that as the star image gets small, where an appreciable
percentage of the light is confined to a single pixel, shifting the image a half a pixel reduces the
peak brightness as the star's image is split between the two pixels. The Kodak CCD pixels are
so small that this is not likely to be a problem.
Resolution
High Res
Saturation Counts
~20,000 for ST-7E/ 8E ABG Cameras,
~40,000 for ST-7E/ 8E Non ABG cameras,
~65,000 for ST-9E
Med/ Low Res 65,000
Table 3.2 - Saturation Values
Once the best focus is found, the focusing operation can be greatly shortened the second time
by removing the CCD head, being careful not to touch the focus knob. Insert a high power
eyepiece and slide it back and forth to find the best visual focus, and then scribe the outside of
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Section 3 - At the Telescope with a CCD Camera
the eyepiece barrel. The next time the CCD is used the eyepiece should be first inserted into
the tube to the scribe mark, and the telescope visually focused and centered on the object. At
f/ 6 the depth of focus is only 0.005 inch, so focus is critical. An adapter may be necessary to
allow the eyepiece to be held at the proper focus position. SBIG sells extenders for this
purpose.
3.5. Finding and Centering the Object
Once best focus is achieved, we suggest using "Dim" mode to help center objects. This mode
gives a full field of view, but reduces resolution in order to increase the sensitivity, and
digitization and download rate. If you have difficulty finding an object after obtaining good
focus, check to be sure that the head is seated at best focus, then remove the head and insert a
medium or low power eyepiece. Being careful not to adjust the focus knob on the telescope,
slide the eyepiece in or out until the image appears in good focus. Then visually find and center
the object, if it is visible to the eye. If not, use your setting circles carefully. Then, re-insert the
CCD head and use FOCUS mode with an exposure time of about ten seconds, if it is dim.
Center the object using the telescope hand controls.
Note: With a 10 second exposure, objects like M51 or the ring nebula are easily detected with
modest amateur telescopes. The cores of most galactic NGC objects can also be seen.
3.6. Taking an Image
Take a CCD image of the object by selecting the Grab command and setting the exposure time.
Start out with the Image size set to full and Auto Display and Auto contrast enabled. The
camera will expose the CCD for the correct time, and digitize and download the image. One
can also take a dark frame immediately before the light image using the Grab command.
Because the ST-7E/ ST-8E/ ST-9E have regulated temperature control, you may prefer to
take and save separate dark images, building up a library at different temperatures and
exposure times, and reusing them on successive nights. At the start it's probably easiest to just
take the dark frames when you are taking the image. Later, as you get a feel for the types of
exposures and setpoint temperatures you use, you may wish to build this library of dark
frames.
3.7. Displaying the Image
The image can be displayed on the computer screen using the graphics capability of your PC.
Auto contrast can be selected and the software will pick background and range values which
are usually good for a broad range of images or the background and range values can be
optimized manually to bring out the features of interest.
The image can also be displayed as a negative image, or can be displayed with
smoothing to reduce the graininess. Once displayed, the image can be analyzed using
crosshairs, or can be cropped or zoomed to suit your tastes.
3.8. Processing the Image
If not done already, images can be improved by subtracting off a dark frame of equal exposure.
You will typically do this as part of the Grab command although it can also be done manually
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Section 3 - At the Telescope with a CCD Camera
using the Dark Subtract command. By subtracting the dark frame, pixels which have higher
dark current than the average, i.e., "hot" pixels, are greatly suppressed and the displayed image
appears much smoother. Visibility of faint detail is greatly improved.
The CCDOPS program also supports the use of flat field frames to correct for vignetting
and pixel to pixel variations, as well as a host of other image processing commands in the
Utility menu. You can smooth or sharpen the image, flip it to match the orientation of
published images for comparison, or remove hot or cold pixels.
3.9. Advanced Capabilities
The following sections describe some of the advanced features of SBIG cameras. While you
may not use these features the first night, they are available and a brief description of them is in
order for your future reference.
3.9.1. Crosshairs Mode (Photometry and Astrometry)
Using the crosshair mode2 enables examination of images on a pixel by pixel basis for such
measurements as Stellar and Diffuse Magnitude, and measurement of stellar positions. The 16
bit accuracy of SBIG systems produces beautiful low-noise images and allows very accurate
brightness measurements to be made. With appropriate filters stellar temperature can be
measured.
In the crosshair mode, you move a small cross shaped crosshair around in the image
using the keyboard or the mouse. As you position the crosshair, the software displays the pixel
value beneath the crosshair and the X and Y coordinates of the crosshair. Also shown is the
average pixel value for a box of pixels centered on the crosshair. You can change the size of the
averaging box from 3x3 to 31x31 pixels to collect all the energy from a star.
3.9.2. Sub-Frame Readout in Focus
The Focus command offers several frame modes for flexibility and increased frame throughput.
As previously discussed, the Full frame mode shows the entire field of view of the CCD with
the highest resolution, digitizing and displaying all pixels.
The "Dim" mode offers the same field of view but offers higher frame rates by reducing
the image's resolution prior to downloading. The resolution is reduced by combining a
neighboring block of pixels into a "super pixel". This reduces the download and display times
proportionately, as well as improving sensitivity. It is great for finding and centering objects.
The Planet mode is suggested if high spatial resolution is desired for small objects like
planets. The Planet mode allows you to select a small sub-area of the entire CCD for image
acquisition. The highest resolution is maintained but you don't have to waste time digitizing
and processing pixels that you don't need. Again, the image throughput increase is
proportional to the reduction in frame size. It can be entered from Auto mode.
2
Under DOS the Crosshairs mode is accessed through the Display Image command in the Analysis
mode.
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Section 3 - At the Telescope with a CCD Camera
Another aspect of the Focus command and its various modes is the Camera Resolution3
setting in the Camera Setup command. Briefly, the Resolution setting allows trading off image
resolution (pixel size) and image capture time while field of view is preserved. High resolution
with smaller pixels takes longer to digitize and download than Low resolution with larger
pixels. The cameras support High, Medium, Low and Auto resolution modes. The Auto mode
is optimized for the Focus command. It automatically switches between Low resolution for
Full frame mode to provide fast image acquisition, and High resolution for Planet mode to
achieve critical focus. While Auto resolution is selected all images acquired using the GRAB
command will be high resolution.
3.9.3. Track and Accumulate
An automatic Track and Accumulate mode (SBIG patented) is available in CCDOPS which
simplifies image acquisition for the typical amateur with an accurate modern drive. These
drives, employing PEC or PPEC technology and accurate gears, only need adjustment every 30
to 120 seconds. With Track and Accumulate the software takes multiple exposures and
automatically co-registers and co-adds them. The individual exposures are short enough such
that drive errors are not objectionable and the accumulated image has enough integrated
exposure to yield a good signal to noise ratio.
Operationally the camera will take an exposure, determine the position of a preselected
star, co-register and co-add the image to the previous image, and then start the cycle over
again. Up to 64 images can be co-added, and the software even allows making telescope
corrections between images to keep the object positioned in the field of view. The resulting
exposure is almost as good as a single long exposure, depending on the exposure used and sky
conditions. The great sensitivity of the CCD virtually guarantees that there will be a usable
guide star within the imaging CCD's field of view. This feature provides dramatic performance
for the amateur, enabling long exposures with minimal setup!
3.9.4. Autoguiding and Self Guiding
The CCDOPS software allows the ST-7E, ST-8E and ST-9E cameras to be used as autoguiders
and self-guiders through the commands in the Track menu. While these systems are not stand-
alone like the ST-4, but require a host computer, they can accurately guide long duration
astrophotographs and CCD images with equal accuracy. Their sensitivity is much greater than
an ST-4, and the video display easier to use.
When functioning as an autoguider, the CCD camera repeatedly takes images of a
guide star, measures the star's position to a fraction of a pixel accuracy, and corrects the
telescope's position through the hand controller. While autoguiding alleviates the user of the
tedious task of staring through an eyepiece for hours at a time, it is by no means a cure to
telescope drive performance. All the things that were important for good manually guided
exposures still exist, including a good polar alignment, rigid tubes that are free of flexure and a
fairly good stable mount and drive corrector. Remember that the function of an auto guider is
to correct for the small drive errors and long term drift, not to slew the telescope.
3
The Resolution setting in the Camera Setup command combines pixels before they are digitized.
This is referred to as on-chip binning and offers increases in frame digitization rates.
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Section 3 - At the Telescope with a CCD Camera
One of the reasons that SBIG autoguiders are often better than human guiders is that,
rather than just stabbing the hand controller to bump the guide star back to the reticule, it gives
a precise correction that is the duration necessary to move the guide star right back to its
intended position. It knows how much correction is necessary for a given guiding error
through the Calibrate Track command. The Calibrate Track command, which is used prior to
autoguiding, exercises the telescope's drive corrector in each of the four directions, measuring
the displacement of a calibration star after each move. Knowing the displacement and the
duration of each calibration move calibrates the drive's correction speed. Once that is known,
the CCD tracker gives the drive corrector precise inputs to correct for any guiding error.
When self guiding is selected by invoking the Self Guiding command under the Track
Menu, the computer prompts the user for the exposure time for the tracking and imaging
CCDs. Once these are entered, the computer takes and displays an image with the tracking
CCD, and the user selects a guide star using the mouse. Guide stars that are bright, but not
saturating, and isolated from other stars are preferred. Once the star is selected, the computer
starts guiding the telescope. When the telescope corrections settle down (usually once the
backlash is all taken up in the declination drive) the user starts the exposure by striking the
space bar. The computer then integrates for the prescribed time while guiding the telescope,
and downloads the image for display.
A calibration star should be chosen that is relatively bright and isolated. The calibration
software can get confused if another star of comparable brightness moves onto the tracking
CCD during a move. The unit will self guide on much fainter stars. Tests at SBIG indicate that
the probability of finding a usable guide star on the tracking CCD is about 95% at F/ 6.3, in
regions of the sky away from the Milky Way. If a guide star is not found the telescope position
should be adjusted, or the camera head rotated by a multiple of 90 degrees to find a guide star.
We recommend that the user first try rotating the camera 180 degrees. Rotating the camera will
require recalibration of the tracking function.
3.9.5. Auto Grab
The Auto Grab command allows you to take a series of images at a periodic interval and log the
images to disk. This can be invaluable for monitoring purposes such as asteroid searches or
stellar magnitude measurements. You can even take sub-frame images to save disk space if
you don't need the full field of view.
3.9.6. Color Imaging
The field of CCD color imaging is relatively new but expanding rapidly. Since all SBIG
cameras are equipped with monochromatic CCDs, discriminating only light intensity, not
color, some provision must be made in order to acquire color images. SBIG offers a color filter
wheel, the CFW-8, which provides this capability.
The color filter wheel allows remotely placing interference filters in front of the CCD in
order to take multiple images in different color bands. These narrow band images are then
combined to form a color image. With the SBIG system, a Red, Green and Blue filter are used
to acquire three images of the object. The resulting images are combined to form a tri-color
image using the CCDCOLOR software.
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Section 3 - At the Telescope with a CCD Camera
Color imaging places some interesting requirements on the user that bear mentioning.
First, many color filters have strong leaks in the infrared (IR) region of the spectrum, a region
where CCDs have relatively good response. If the IR light is not filtered out then combining
the three images into a color image can give erroneous results. If your Blue filter has a strong
IR leak (quite common) then your color images will look Blue. For this reason, SBIG
incorporates an IR blocking filter stack with the three color band filters.
Second, since you have narrowed the CCD's wavelength response with the interference
filters, longer exposures are required to achieve a similar signal to noise compared to what one
would get in a monochrome image with wide spectral response. This is added to the fact that
tri-color images require a higher signal to noise overall to produce pleasing images. With black
and white images your eye is capable of pulling large area detail out of random noise quite
well, whereas with color images your eye seems to get distracted by the color variations in the
noisy areas of the image. The moral of the story is that while you can achieve stunning results
with CCD color images, it is quite a bit more work.
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Section 4 - Camera Hardware
4.
Camera Hardware
This section describes the modular components that make up the CCD Camera System and
how they fit into the observatory, with all their connections to power and other equipment.
4.1. System Components
The ST-7E, ST-8E and ST-9E CCD cameras consist of four major components: the CCD Sensors
and Preamplifier, the Readout/ Clocking Electronics, the Microcontroller, and the power
supply. All the electronics are packaged in the optical head in these cameras with an external
desktop power supply.
The CCDs, Preamplifier, and Readout Electronics are mounted in the front of the optical
head. The optical head interfaces to the telescope through a 1.25 inch draw tube, sliding into
the telescope's focus mechanism. The placement of the preamplifier and readout electronics
close to the CCD is necessary to achieve good noise performance. The Microcontroller is
housed in the rear of the Optical Head along with the interface logic to the PC and Telescope.
4.2. Connecting the Power
The desktop power supply is designed to run off wall voltage, and both domestic (110 VAC)
and European (220 VAC) versions are available. In the field however, battery operation may be
the most logical choice. In that case you need to use the optional 12V power supply or a
12VDC to 110 VAC power inverter.
4.3. Connecting to the Computer
The ST-7E, ST-8E and ST-9E CCD Cameras are supplied with a 15 foot cable to connect the
system to the host computer. The connection is between the camera and the Host Computer's
parallel LPT port. The camera will operate properly with up to 50 feet of cable to the computer.
The computer's parallel port is easily identified since it is a 25 pin Female D connector.
4.4. Connecting the Relay Port to the Telescope
The ST-7E, ST-8E, and ST-9E camera systems can be used as autoguiders where the telescope's
position is periodically corrected for minor variations in the RA and DEC drives. The host
software functions as an autoguider in three modes: the Track mode, the Track and
Accumulate mode (SBIG patented), and Self Guided mode (SBIG patented).
In the Track mode and Self Guided mode the host software corrects the telescope as
often as once every 2 seconds to compensate for drift in the mount and drive system. The host
software and the CCD camera operate in tandem to repeatedly take exposures of the
designated guide star, calculate its position to a tenth of a pixel accuracy, and then
automatically activate the telescope's controller to move the star right back to its intended
position. It does this tirelessly to guide long duration astrophotographs.
In the Track and Accumulate mode the software takes a series of images and
automatically co-registers and co-adds the images to remove the effects of telescope drift.
Typically you would take ten 1 minute "snapshots" to produce an image that is comparable to a
single 10 minute exposure except that no guiding is required. The reason no guiding is
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Section 4 - Camera Hardware
required is that with most modern telescope mounts the drift over the relatively short 1 minute
interval is small enough to preserve round star images, a feat that even the best telescope
mounts will not maintain over the longer ten minute interval. The Track and Accumulate
software does allow correction of the telescope position in the interval between snapshots to
keep the guide star grossly positioned within the field of view, but it is the precise co-
registration of images that accounts for the streakless images.
The host software and the CCD camera control the telescope through the 9-pin
Telescope port on the camera. This port provides active low open collector signals to the
outside world. By interfacing the camera to the telescope's controller the CPU is able to move
the telescope as you would: by effectively closing one of the four switches that slews the
telescope.
Note: You only need to interface the camera' s Telescope port to your telescope if you are
planning on using the camera system as an autoguider or selfguider, or feel you need to
have the Track and Accumulate command make telescope corrections between images
because your drive has a large amount of long term drift.
Some recent model telescopes (like the Celestron Ultima and the Meade LX200) have
connectors on the drive controller that interface directly to the camera's TTL level Telescope
port. All that's required is a simple cable to attach the 9 pin Telescope port to the telescope's
telephone jack type CCD connector. SBIG includes its TIC-78 (Tracking Interface Cable
Adapter) for this express purpose although it is easy to modify a standard 6-pin telephone
cable for interface to the Telescope port (see Appendix A for specific pin outs, etc.). The TIC-78
plugs into the 9-pin port on the ST-7E/ 8E/ 9E, and a standard phone cable, which we supply,
connects the adapter to the telescope drive. Note: phone cables come in a few variations. We
use the six-pin cable, and the pin order is reversed left to right relative to the connector from
one end to the other. This is identical to what is typically sold at Radio Shack stores as an
extension cable.
4.4.1 Using Mechanical Relays
Older telescopes generally require modifying the hand controller to accept input from the
camera's Telescope port. The difficulty of this task varies with the drive corrector model and
may require adding external relays if your drive corrector will not accept TTL level signals. We
maintain a database of instructions for the more popular telescopes that we will gladly share
with you. For a minimal charge will also modify your hand controllers if you feel you do not
have the skills necessary to accomplish such a task. We sell a mechanical relay box that
interfaces to the ST-7E/ 8E/ 9E, and will interface to the older drives. Contact SBIG for more
information.
In general, the camera has four signals that are used in tracking applications. There is
one output line for each of the four correction directions on the hand controller (North, South,
East and West). Our previous cameras had internal relays for the telescope interface, but with
the proliferation of TTL input telescopes the relays were removed in the ST-7 and ST-8 (we do
offer an external relay adapter accessory). The following paragraphs describe the general-
purpose interface to the telescope which involves using external relays.
In our older camera models and in the optional relay adapter accessory, each of the
relays has a Common, a Normally Open, and a Normally Closed contact. For example, when
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Section 4 - Camera Hardware
the relay is inactivated there is a connection between the Common and the Normally Closed
contact. When the relay is activated (trying to correct the telescope) the contact is between the
Common and the Normally Open contacts.
If your hand controller is from a relatively recent model telescope it probably has four
buttons that have a "push to make" configuration. By "push to make" we mean that the
switches have two contacts that are shorted together when the button is pressed. If that's the
case then it is a simple matter of soldering the Common and Normally Open leads of the
appropriate relay to the corresponding switch, without having to cut any traces, as shown in
Figure 4.1 below.
B: Modified Push to Make Switch
A: Unmodified Push to Make Switch
common
relay
c
switch
nc
no
switch
normally open
Figure 4.1 - Push to Make Switch Modification
Another less common type of switch configuration (although it seems to have been used more
often in older hand controllers) involve hand controller buttons that use both a push to make
contact in conjunction with a push to break contact. The modification required for these
switches involves cutting traces or wires in the hand controller. Essentially the relay's
Normally Open is wired in parallel with the switch (activating the relay or pushing the hand
controller button closes the Normally Open or Push to Make contact) while at the same time the
Normally Closed contact is wired in series with the switch (activating the relay or pushing the
hand controller button opens the Normally Closed or the Push to Break contact). This type of
switch modification is shown in Figure 4.2 below.
A: Unmodified Push to Make/ Break Switch
B: Modified Push to Make/ Break Switch
common
c
common
c
c
relay
switch
nc
no
nc
no
nc
no
normally open
normally closed
normally open
normally closed
Figure 4.2- Push to Make/Break Modification
The last type of hand controller that is moderately common is the resistor joystick. In this
joystick each axis of the joystick is connected to a potentiometer or variable resistor. Moving
the joystick handle left or right rotates a potentiometer, varying the resistance between a central
"wiper" contact and the two ends of a fixed resistor. The relays can be interfaced to the joystick
as shown in Figure 4.3 below. Essentially the relays are used to connect the wire that used to
attach to the wiper to either end of the potentiometer when the opposing relays are activated.
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Section 4 - Camera Hardware
A
+ relay
c
- relay
c
A
wiper
B
nc
no
nc
no
C
B
C
potentiometer
B: Modified Joystick
Figure 4.3 - Joystick Modification
A: Unmodified Joystick
A slight variation on the joystick modification is to build a complete joystick eliminator as
shown in Figure 4.4 below. The only difference between this and the previous modification is
that two fixed resistors per axis are used to simulate the potentiometer at its mid position. You
do not need to make modifications to the joystick; you essentially build an unadjustable
version. This may be easier than modifying your hand controller if you can trace out the
wiring of your joystick to its connector.
A
- relay
+ relay
c
c
A
nc
no
wiper
B
no
nc
C
B
C
R
potentiometer
R/ 2
R/ 2
A: Unmodified Joystick
B: Joystick Eliminator
Figure 4.4- Joystick Eliminator
4.5. Modular Family of CCD Cameras
With the introduction of the ST-6 CCD Camera in 1992 SBIG started a line of high quality, low
noise, modular CCD cameras. The ST-7E, ST-8E and ST-9E are a second family of modular
CCD cameras.
The benefits of a modular line of CCD Cameras are manyfold. Users can buy as much
CCD Camera as they need or can afford, with the assurance that they can upgrade to higher
performance systems in the future. With a modular approach, camera control software like
CCDOPS can easily support all models. This last point assures a wide variety of third party
software. Software developers can produce one package for the many users across the model
line instead of different packages for each of the cameras.
While the SBIG cameras have many similarities, there are also important differences
between the products. Table 4.2 below highlights the differences from a system's standpoint:
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Section 4 - Camera Hardware
A/ D
Temperature
Electromechanical
Electronic
Shutter
Camera
Resolution Regulation
Shutter/ Shutter
Wheel/ Vane
Shutter Wheel
Shutter Wheel
Shutter Wheel
Vane
ST-5C
ST-237
STV
ST-6
ST-7E/ 8E/ 9E
16 bits
12 bits
10+2 bits
16 bits
16 bits
Closed Loop
Closed Loop
Closed Loop
Closed Loop
Closed Loop
0.01 second
0.01 second
0.001 second
0.01 second
None
Shutter
Table 4.2 - System Features
How these features affect the average user are discussed in the paragraphs below:
A/ D Resolution - This is a rough indication of the camera's dynamic range. Higher precision
A/ D Converters are able to more finely resolve differences in light levels, or for
larger CCDs with greater full well capacities, they are able to handle larger total
charges with the same resolution.
Temperature Regulation - In an open loop system like the original ST-4 the CCD cooling is
either turned on or turned off. While this provides for adequate cooling of the
CCD, the CCD's temperature is not regulated which makes it important to take
dark frames in close proximity to the associated light frame. Closed loop
systems regulate the CCD's temperature to an accuracy of ±0.1° C making dark
frames useful over longer periods.
Electromechanical Vane - Having the vane in the ST-6 means the host software can effectively
"cover the telescope" and take dark frames remotely, without the user having to
get up and physically cover the telescope.
Electromechanical Shutter - Having the shutter in the ST-7E/ 8E/ 9E gives streak-free readout
and allows taking dark frames without having to cover the telescope. While the
minimum exposure is 0.11 seconds, repeatability and area uniformity are
excellent with SBIG's unique unidirectional shutter.
Shutter Wheel - The Shutter Wheel, used in conjunction with the camera's Electronic shutter,
allows you to cover the CCD for taking dark frames and in the case of the
ST-5C/ 237 allows replacement with a mini internal color filter wheel.
Electronic Shutter - Having an electronic shutter involves having a CCD with a frame transfer
region. These CCDs actually have an array that has twice the number of rows
advertised, where the bottom half is open to the light (referred to as the Image
Area), and the top half is covered with a metalization layer (referred to as the
Storage Area). In frame transfer CCDs at the end of the exposure, the pixel data
from the Image Area is transferred into the Storage Area very rapidly where it
can be read out with a minimum of streaking.
In addition to the system level differences between the various cameras, Table 4.3 below
quantifies the differences between different CCDs used in the cameras:
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Section 4 - Camera Hardware
CCD Used Number of Pixel
Array
Dimension
Read
Noise
Full Well
Capacity
Camera
Pixels
Dims.
-
-
Tracking TC-211
CCD
192 x 164
13.75 x 16µ 2.6 x 2.6mm
12e rms
150Ke
-
-
ST-5C
ST-237
STV
TC-255
TC-237
TC-237
TC-241
320 x 240
640 x 480
320 x 200
375 x 242
10 x 10µ
7.4 x 7.4µ
14.8 x 14.8
23 x 27µ
9 x 9µ
3.2 x 2.4mm
4.7 x 3.6mm
4.7 x 3.0mm
8.8 x 6.6mm
6.9 x 4.6mm
20e rms
50Ke
-
-
15e rms
20Ke
-
20Ke-
17e rms
-
-
ST-6
30e rms
400Ke
-
-4
-4
ST-7E
ST-8E
ST-9E
KAF0401E 765 x 510
15e- rms
50Ke / 100Ke
-
KAF1602E 1530 x 1020 9 x 9µ
13.8 x 9.2mm 15e- rms
50Ke / 100Ke
KAF0261E 512 x 512 20 x 20µ
10.2 x 10.2mm 13e- rms
200Ke-
Table 4.3- CCD Differences
How these various specifications affect the average user is described in the following
paragraphs:
Number of Pixels - The number of pixels in the CCD affects the resolution of the final images.
The highest resolution device is best but it does not come without cost. Larger
CCDs cost more money and drive the system costs up. They are harder to cool,
require more memory to store images, take longer to readout, etc. With PCs and
Macintosh computers offering graphics resolutions of 320 x 200 to 640 x 480 with
good grey scale, the CCDs used in the SBIG cameras offer a good trade off
between cost and resolution, matching the computer's capabilities well.
Pixel Dimensions - The size of the individual pixels themselves really plays into the user's
selection of the system focal length. Smaller pixels and smaller CCDs require
shorter focal length telescopes to give the same field of view that larger CCDs
have with longer focal length telescopes. Smaller pixels can give images with
higher spatial resolution up to a point. When the pixel dimensions (in
arcseconds of field of view) get smaller than roughly half the seeing, decreasing
the pixel size is essentially throwing away resolution. Another aspect of small
pixels is that they have smaller full well capacities.
For your reference, if you want to determine the field of view for a pixel
or entire CCD sensor you can use the following formula:
8.12x size (µm)
Field of view (arcseconds) =
focal length (inches)
20.6x size(um)
Field of view (arcseconds) =
focal length(cm)
where size is the pixel dimension or CCD dimension in millimeters and the focal
4
The Kodak CCDs (KAF0400 and KAF1600) are available with or without Antiblooming Protection.
Units with the Antiblooming Protection have one-half the full well capacity of the units without it.
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Section 4 - Camera Hardware
length is the focal length of the telescope or lens. Also remember that 1° = 3600
arcseconds.
Read Noise - The readout noise of a CCD camera affects the graininess of short exposure
images. For example, a CCD camera with a readout noise of 30 electrons will
give images of objects producing 100 photoelectrons (very dim!) with a Signal to
Noise (S/ N) of approximately 3 whereas a perfect camera with no readout noise
would give a Signal to Noise of 10. Again, this is only important for short
exposures or extremely dim objects. As the exposure is increased you rapidly
get into a region where the signal to noise of the final image is due solely to the
exposure interval. In the previous example increasing the exposure to 1000
photoelectrons results in a S/ N of roughly 20 on the camera with 30 electrons
readout noise and a S/ N of 30 on the noiseless camera. It is also important to
note that with the SBIG CCD cameras the noise due to the sky background will
exceed the readout noise in 15 to 60 seconds on the typical amateur telescopes.
Even the $30,000 priced CCD cameras with 10 electrons of readout noise will not
produce a better image after a minute of exposure!
Full Well Capacity - The full well capacity of the CCD is the number of electrons each pixel can
hold before it starts to loose charge or bleed into adjacent pixels. Larger pixels
hold more electrons. This gives an indication of the dynamic range the camera is
capable of when compared to the readout noise, but for most astronomers this
figure of merit is not all that important. You will rarely takes images that fill the
pixels to the maximum level except for stars in the field of view. Low level
nebulosity will almost always be well below saturation. While integrating
longer would cause more build up of charge, the signal to noise of images like
these is proportional to the square-root of the total number of electrons. To get
twice the signal to noise you would have to increase the exposure 4 times. An
-
ST-5C with its relatively low full well capacity of 50,000e could produce an
image with a S/ N in excess of 200!
Antiblooming - All the SBIG CCD cameras have antiblooming protection. The TI CCDs used in
the ST-5C, ST-237 and ST-6 have it built into the CCDs and the Kodak CCDs
used in the ST-7E and ST-8E have Antiblooming versions of the CCDs available.
Blooming is a phenomenon that occurs when pixels fill up. As charge continues
to be generated in a full pixel, it has to go somewhere. In CCDs without
antiblooming protection the charge spills into neighboring pixels, causing bright
streaks in the image. With the CCDs used in the SBIG cameras the excess charge
can be drained off saturated pixels by applying clocking to the CCD during
integration. This protection allows overexposures of 100-fold without blooming.
The ST-9E does not have antiblooming protection but the large pixels can
hold a great deal of charge. It's not a limiting problem.
From the telescope's point of view, the different models offer differing fields of view for a given
focal length, or turned around, to achieve the same field of view the different models require
differing focal lengths. Tables 4.4 and 4.5 below compare the fields of view for the cameras at
several focal lengths, and vice versa.
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Section 4 - Camera Hardware
C8, 8" f/ 10
LX200, 10" f/ 35
14" f/ 11
Pixel
Field of
View
Pixel
Size
Field of
View
Pixel
Size
Field of
View
Size
Camera (arcmins)
Tracking 4.2x4.2
CCD
(arcsecs) (arcmins)
(arcsecs) (arcmins)
(arcsecs)
0.7x0.8
1.3x1.5
11.7x11.7
3.7x4.3
2.3x2.3
ST-5C
ST-237
STV
5.4x4.1
8.0x6.0
8.0x5.0
14.6x11.1
11.9x7.9
23.8x15.8
17.6x17.6
1.0x1.0
0.75x0.75 21.6x16.2
14.4x10.8
2.7x2.7
2.0x2.0
4.0x4.0
6.2x7.3
2.4x2.4
2.4x2.4
5.3x5.3
2.8x2.1
4.0x3.0
4.0x2.5
7.6x5.7
6.1x4.1
12.2x8.2
9.1x9.1
0.5x0.5
0.38x0.38
0.75x0.75
1.2x1.4
0.5x0.5
0.5x0.5
1.1x1.1
1.5x1.5
2.3x2.7
0.9x0.9
0.9x0.9
2.0x2.0
21.6x13.5
38.9x29.5
31.2x20.8
62.4x41.6
46.2x46.2
ST-6
ST-7E
ST-8E
ST-9E
Table 4.4 - Field of View
Focal Length to fill Focal Length to fill
Object
Moon
Size
0.5°
Tracking CCD
ST-7E
275mm =
11 inches
760mm =
30 inches
Jupiter
40 arcseconds
13000mm =
510 inches
34000mm =
1350 inches
M51-Whirlpool
Galaxy
8 x 5
arcminutes
1040mm =
41 inches
3700mm =
145 inches
M27-Dumbell
Nebula
8.5 x 5.5
arcminutes
"
"
M57-Ring Nebula
1.3 x 1
arcminutes
6400mm =
250 inches
23000mm =
900 inches
Table 4.5 - Focal Length Required
From these numbers you can deduce that the popular C8, an 8" f/ 10 telescope will nicely frame
many popular objects with the ST-7E whereas a much shorter system (f/ 3, perhaps achieved
with a focal reducer) will frame the same objects for the tracking CCD. Another point to bear
in mind is that, except for planetary images, you'll rarely take images where the pixel size in
seconds of arc is down near the seeing limit. Most objects are relatively large, where the field of
view is more important than whether the individual pixels are less than half the seeing.
4.6 Connecting the older model CFW-6 filter wheel to the Camera
The SBIG CFW-6 filter wheel can be connected to the camera using the relay port
adapter provided with the camera. The adapter plugs into the 9 Pin connector on the
ST-7E/ 8E/ 9E head. The phone cable connection connects to the telescope drive to adjust the
telescope position when collecting images using Self-Guiding, and the short cable with three
5
f/ 6.3 with the Meade star digonal focal reducer.
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Section 4 - Camera Hardware
wires connects to the CFW-6. The black wire of the three-wire group mates to the brown or
black wire of the CFW-6.
4.7 Battery Operation
The ST-7E/ 8E/ 9E can be operated off of a 12 volt car or marine battery using a the
optional 12V power supply or using a power inverter. We have used the Radio Shack model
22-132A, 12 volt DC to 115 volt AC Portable Power Inverter (140 watt) with good success. The
camera draws 2..2 amps from the battery with this inverter, which should enable an evening's
operation from a single battery. We recommend a separate battery for the camera; using your
vehicle's battery with the engine running may add undesirable readout noise, and using your
vehicle's battery without the engine running may result in a long walk home in the dark!
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Section 5 - Advanced Imaging Techniques
5.
Advanced Imaging Techniques
With practice, you will certainly develop methods of your own to get the most from your CCD
camera. In this section we offer some suggestions to save you time getting started in each of
the different areas outlined below, but these suggestions are by no means exhaustive.
5.1. Lunar and Planetary Imaging
When imaging the moon using most focal lengths available in astronomical telescopes, you will
note that the moon's image typically fills the CCD. The image is also very bright. The best way
to image the moon is to use neutral density filters to attenuate the light.
You may also try an aperture mask to reduce the incoming light. If you feel that an
aperture mask reduces resolution to an unacceptable degree, consider using a full aperture
solar filter. This will result in an optimum exposure of only a few seconds. Another way to
reduce the incoming light is to increase the effective focal length of your telescope by using a
barlow lens or eyepiece projection. This is very desirable for planetary imaging since it also
increases the image size.
5.2. Deep Sky Imaging
Ordinarily, with telescopes of 8" aperture or larger, a ten second exposure in focus mode, with
a dark frame subtracted, will show most common deep sky objects except for the very faintest.
This is a good starting point for finding and centering deep sky objects. If you find ten seconds
to be insufficient, a one minute exposure will clearly show nearly any object you wish to image,
particularly if 2x2 or 3x3 binning is selected. Using the Grab command, exposures of five
minutes will generally give you a clear image with good detail. Of course, longer exposures
are possible and desirable, depending on your telescope's tracking ability or your desire to
guide. Once you have determined the longest exposure possible with your particular drive
error, try Track and Accumulate exposures of a duration less than your measurable error. We
have found that exposures of thirty seconds to two minutes are best depending on the focal
length of telescope one is using. With the self guiding feature of the ST-7e/ 8E/ 9E one can take
long integrations while the internal tracking CCD guides the telescope.
We highly recommend that you initially pursue deep sky imaging with a fast telescope,
or focal reducer to produce a final F number of F/ 6.3 or faster at the CCD. The sensitivity
advantage is considerable!
5.3. Terrestrial Imaging
An optional accessory for the SBIG cameras is the camera lens adapter. These accessories are
made to accommodate most popular camera models. You may attach a camera lens in place of
your telescope and use the CCD camera for very wide angle images of the night sky or for
terrestrial views in daylight. Begin with a tenth second exposure at f/ 16 for scenes at normal
room light and adjust as necessary for your conditions.
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Section 5 - Advanced Imaging Techniques
5.4. Taking a Good Flat Field
If you find that flat field corrections are necessary due to vignetting effects, CCD sensitivity
variations, or for more accurate measurements of star magnitudes, try either taking an image of
the twilight sky near the horizon or take an image of a blank wall or neutral grey card. The
Kodak CCDs may have a low contrast grid pattern visible in the sky background. A flat field
will eliminate this.
Finding areas of the sky devoid of stars is very difficult after twilight. Therefore, you
should take flat field images of the night sky after sunset, but long before you can see any stars.
If this is not possible, take an image of a featureless wall or card held in front of the telescope.
However, if using this second method, be sure that the wall or card is evenly illuminated.
Appendix D describes how to do this. You will know if the flat field is good if the sky
background in your images has little variation across the frame after flat fielding, displayed
using high contrast (a range of 256 counts is good for showing this).
If you plan on flat fielding Track and Accumulate images you should also refer to
section 6.8. Since the same flat field is added to itself a number of times, be sure that you do
not saturate the flat field image by starting with pixel values too high. Typically try to keep the
pixel values between 10% to 20% of saturation for this purpose. For single flat field images, try
to keep the values to approximately 50% of saturation.
5.5. Building a Library of Dark Frames
The ST-7e/ 8E/ 9E have regulated temperature control, and therefore it is possible to duplicate
temperature and exposure conditions on successive nights. You can set the camera TE cooler
temperature to a value comfortably within reach on your average night, and then take and save
on disk a library of dark frames for later use. This is a good project for a rainy night. We
recommend you build a file of 5, 10, 20 ,40, and 60 minute dark frames at zero degrees
Centigrade for a start. Otherwise you will find yourself wasting a clear night taking hour long
dark frames!
Note: Dark frames taken the same night always seem to work better. The adaptive dark
subtract will help if the ambient temperature changes slightly.
5.6. Changing the Camera Resolution
The Camera Setup command allows you to select the resolution mode you wish to use for
taking and displaying images. The ST-7E/ 8E/ 9E cameras have High, Medium, Low and Auto
modes. The High Resolution mode is the best for displaying the greatest detail since it utilizes
the maximum number of pixels for your particular camera. The Medium Resolution Mode
operates by combining 2x2 pixels giving the same field of view as High Resolution Mode, but
with 1/ 4 the resolution. This results in significantly faster digitization and download times.
Also, in Medium Resolution Mode, with larger pixels and comparable readout noise there is a
better signal to noise ratio for very dim diffuse objects. This improved signal to noise ratio
combined with faster digitization and download times makes Medium Resolution Mode ideal
for finding and centering dim objects, and for imaging most objects. Additionally, a Low
resolution mode is provided which bins the CCD 3x3 before readout. Low resolution mode is
sensational for displaying faint nebulosity with short exposure times. In Auto Resolution
Mode, the camera and software will always use High Resolution for all imaging and display
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Section 5 - Advanced Imaging Techniques
functions except when you are in Full Frame Focus Mode. It will then automatically switch to
Low Resolution Mode. If you further select Planet Mode for focusing, the camera will switch
back to High Resolution on the selected box area. The small pixel size, is best for critical
focusing. Planet mode will result in fast digitization and download times since only a small
portion of the frame is read out.
In general, you should pick a binning mode that yields stars with two to three pixels full
width at half maximum. This is easily measured by using the crosshairs to determine the peak
brightness of a relatively bright star, and determining the number of pixels between the 50%
values on either side of the peak. More than 3 pixels per stellar halfwidth merely wastes
sensitivity without improved resolution.
5.7. Flat Fielding Track and Accumulate Images
This section gives the step by step procedure for flat field correcting images taken using the
Track and Accumulate command.
Flat field correcting images allow the user to remove the effects of CCD response non-
uniformity (typically less than a few percent) and optical vignetting which for some optical
systems can be as much as a 50% effect from center to edge. The CCDOPS software allows flat
field correcting images using the Flat Field command, but some preparation must be made to
use that command with Track and Accumulate images. Essentially you must prepare a special
flat field correction image for Track and Accumulate images. This special preparation is
necessary to have the same set of alignment and co-addition operations apply to the flat field
file that have occurred in acquiring the Track and Accumulate image. In general, the following
procedure should be followed when flat field correction of Track and Accumulate images is
desired:
1. Take a normal flat field image using the Grab command. You can use the dusk
sky or a neutral gray or white card held in front of the telescope. Try to adjust
the illumination and/ or exposure so that the build up of light in the image
yields values that when co-added several times will not overflow 65,000 counts.
The number of times the image will need to be co-added without overflowing is
set by how many snapshots you intend to use in Track and Accumulate. A good
goal is to try and attain a maximum level in the flat field image of 1,000 to 2,000
counts which will allow co-addition 32 times without overflow.
Note: You will have to take a new flat field image anytime you change the optical
configuration of your telescope such as removing and replacing the optical head in the
eyepiece holder.
2. Save the flat field image on your disk using the Save command. In the following
discussions this flat field file will be referred to as FLAT.
3. Take your Track and Accumulate image using the Track and Accumulate
command and save it on the disk using the Save command. In the following
discussions this Track and Accumulate image file will be referred to as IMAGE.
4. Immediately after saving the IMAGE use the Save Track List command on the
PC or activate the Track List window on the Mac and use the Save command to
save the Track and Accumulate track list. The track list is a file that describes
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Section 5 - Advanced Imaging Techniques
what alignment operations were done to the individual components of IMAGE
to achieve the end result. In the following discussions this track list file will be
referred to as TRACK.
5. Repeat steps 3 and 4 as many times as desired for all the objects you wish to
image, each time choosing a set of corresponding new names for the IMAGE and
TRACK files.
6. You will now create a combined flat field image for each Track and Accumulate
image you captured. Invoke the Add by Track List command. The software will
bring up a file directory dialog showing all the track list files. Select the TRACK
file corresponding to the image you wish to correct. The software will load the
TRACK file and present you with another file directory dialog showing all the
images. Select the appropriate FLAT image. The software will align and co-add
the FLAT image using the same operations it performed on the Track and
Accumulate image. Finally save the combined flat field image using the Save
command. In the following discussions this combined flat field image will be
referred to as COMBINED-FLAT. Repeat this step for each of the TRACK files
using a corresponding name for the COMBINED-FLAT image.
7. You will now flat field correct the Track and Accumulate image with the
combined flat field image. Use the Open command to load the IMAGE file, then
use the Flat Field command. The software will present you with a file directory
dialog where you should select the corresponding COMBINED-FLAT image.
After the software has finished correcting the image you can view the results
and save the flat field corrected image with the Save command. This image will
be referred to as the CORRECTED-IMAGE file. Repeat this step for each of the
IMAGE files using the corresponding COMBINED-FLAT image.
At SBIG we have adopted the following naming convention for our various image and related
files. If it helps you organize your files please feel free to adopt it or any method you feel helps
sort out the process of naming files:
Image type
Name
Uncorrected image (IMAGE)
Flat field file (FLAT)
Track list file (TRACK)
Combined flat field (COMBINED-FLAT)
Flat field corrected image (CORRECTED-IMAGE)
XXXXXXXX.
FLATXXXX.
XXXXXXXX.TRK
FLATXXXX.C
XXXXXXXX.F
(blank extension)
(blank extension)
5.8. Tracking Functions
The CCDOPS software allows your ST-7E/ 8E/ 9E to be used as an autoguider or self guided
imager. It does not function as a stand-alone autoguider like the ST-4, but instead requires
using a PC to perform the function. These cameras have considerably better sensitivity than
the ST-4.
CCD autoguiders alleviate you from having to stare down the eyepiece for hours at a
time while guiding astrophotographs. They are not the end-all, cure-all approach to telescope
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Section 5 - Advanced Imaging Techniques
mechanical problems, though. You still need a good polar alignment and a rigid mount
between the guide scope and the main scope or you need to use an off-axis guider, with all its
inherent difficulties. A good declination drive, free of backlash, is desirable although not
absolutely necessary. Finally, modern drive correctors with periodic error correction (PEC) or
permanent periodic error correction (PPEC) will ease the difficulty of achieving good results.
The moral of the story is don' t count on the CCD autoguider to fix all your problems.
The better the drive, the better results you will obtain.
Using the CCD as an autoguider requires interfacing the CPU's relay port to your hand
controller (as discussed in section 4.4) and then training the CCD system on your telescope.
This is done with the Calibrate Track command. Focus your system and then find and center
on a moderately bright calibration star (1000 to 20,000 peak counts will do) without any nearby
neighboring stars of similar brightness. Then execute Calibration command. It takes a
sequence of five images. In the first image it determines the pixel position of the calibration
star. In the four subsequent images, the software, in sequence, activates each of the telescope's
four correction directions, measuring the displacement of the calibration star. From this
calibration information, the software is able to calculate a precise correction when the guide star
moves away from its intended position.
At the start, you should pick a calibration star with roughly the same declination as the
intended object since the telescope's correction speeds vary with declination. As you get used
to the tracking functions you can calibrate on a star near the celestial equator and have the
software adjust for different declinations for you.
The tracking functions in CCDOPS are accessed through the Track menu. The Calibrate
Track command, as described above, is used to calibrate your telescope's drive corrector. Once
that is done, the Track command, which you would use for autoguiding astrophotographs,
allows you to select a guide star in the field of view, and then repeatedly takes images,
measuring the guide star's position and hence guiding error, and corrects the telescope.
The Track and Accumulate command, discussed in section 3.9.2, also has provisions for
making telescope corrections between images. This is necessary only if your drive has a large
amount of long term drift, which results in Track and Accumulate images that are reduced in
width.
Finally, the Tracking Parameters command allows you to fine tune the CCDOPS
tracking performance. You can deactivate the RA or DEC corrections or even deactivate both, a
feature that can be used to monitor the uncorrected tracking accuracy of your drive. You can
also fine tune the correction speeds if you find the telescope is consistently over or under
correcting.
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Section 6 - Accessories for your CCD Camera
6.
Accessories for your CCD Camera
This section briefly describes the different accessories available for your CCD camera.
6.1. Cooling Booster
The cooling booster, which is included with the ST-9E and is an option for the ST-7E and ST-8E,
is a small module that gets installed inside the back compartment of the camera. This section
tells you how to best use the Cooling Booster.
SBIG understands that water-cooling is a major annoyance. We do not recommend
using water cooling in the field due to the aggravation of dragging around buckets of water
and finding more electrical power. For users that have fixed sites, or small observatories, the
aggravation is more tolerable. The major problem one must deal with is routing the rather
heavy water tubes off the mount to minimize perturbations to the mount during tracking. In
general, try to route tubes (and wires) over the mount, rather than just let the tubes dangle
from the end of a long tube. Water-cooling is probably not necessary for most users when the
air temperature is below 10 degrees C (50 degrees F), since the dark current is fairly low
already. Think of it as a summertime accessory! We do not recommend use of water cooling
below freezing temperatures, where antifreeze must be added to the water. It is simply not
necessary then. There is no problem with using the cooling booster with only air cooling in the
winter, though.
The cooling booster can be used either without the flowing water or with it. Without
flowing water the cooling improvement is about 6 degrees C. With it the cooling improvement
is about 15 degrees C. If you plan to use it without the water then you should disconnect the
hoses from the camera and shake out the water trapped in the heat sink. Disconnecting the
hoses will reduce the potential perturbation to your telescope mount.
To operate the cooling booster without water cooling, mount the camera to the telescope
as before and simply plug the auxiliary 12 volt supply jack into the connection on the camera
back plate. Turn on the TE cooling to 100% by giving it a target temperature of –50 degrees.
After 10 minutes examine the camera temperature, and reset the set point to 3 degrees C above
the current temperature. This 3 degree temperature margin will enable the ST-7E/ 8E/ 9E to
regulate the temperature accurately.
To operate the camera with water cooling, the procedure is the same except that the
water flow must be established before mounting the camera to the telescope, since the water
pumps have limited pressure capability. To do this, put the camera at the same level as the
water reservoir. Connect all the hoses, and make sure the water return goes back into the
reservoir. Push the ¼ inch internal diameter (ID) hoses onto the nipples on the back of the
camera so they seal. Attach one hose to the nipple onto the reducing connector that adapts the
¼ inch ID hose to the ½ inch diameter hose from the pump.
Turn on the pump, and let the flow establish itself through the hoses. Next, mount the
camera to the telescope. If you always keep the return hose outlet near the reservoir level the
pump will have no problem raising the water 2 meters (6 feet) off the floor. The limited
pressure capacity of the pump is only a problem when you let the water fall back into the
reservoir from a significant height above it, such a 0.3 meter (12 inches). Lastly, check for
leaks!
When using water cooling, avoid the temptation to put ice in the water to get the
camera even colder. As the cooling booster is designed, the camera will not be cooled below
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Section 6 - Accessories for your CCD Camera
ambient temperature if ambient temperature water is used. If colder water is used, the head
may fog or frost up, depending on the dew point. . The exposed electronics inside the ST-7/ 8
will get wet, and corrode. The hoses will start dripping condensation, and you will have a
mess. Keep the ice for a cold drink!
At the end of the evening, stop the pump, and raise the outlet hose above the camera to
let all the water drain out of the system. Blowing it out with gently pressure helps clear the
water. You can leave the hoses full of water, but if a leak occurs while you’re not there you
may have a problem.
When packing the camera for a long time, or at the end of summer, disconnect the hoses
and blow out the heat sink to allow the enclosed spaces to dry out and minimize long term
corrosion.
6.2. Tri-color Imaging
You can make splendid color images with your CCD camera by using the optional CFW-8 color
filter wheel. The CFW-8 attaches to the front of the CCD head and allows you to take images in
red, green and blue light of the same object. When these images are aligned and processed a
full color image results.
6.3. Camera Lens Adapters and Eyepiece Projection
Camera Lens adapters are available for the ST-7E/ 8E/ 9E. The camera lens adapter allows you
to mount your camera lens in place of the telescope for very wide field views of the night sky
or for daytime terrestrial imaging. The adapters are available for a variety of the most common
camera lenses in use.
The cameras themselves can be used for eyepiece projection assuming you already have
a T-thread eyepiece projection adapter for. Instead of attaching a 35mm camera to the T-thread
camera adapter you can attach the CCD camera by unscrewing the T-Thread nosepiece fom the
front of the camera.
6.4. Focal Reducers
Several third party vendors, including Celestron and Meade, make focal reducers for their
telescopes that could be used with the ST-7E/ 8E/ 9E cameras. While most will work with the
ST-7 some may have slight amounts of vignetting when used on the larger ST-8E/ 9E.
Note: Many very fast commercial units will not accommodate our filter wheel.
6.5. AO-7 and Lucy-Richardson Software
The AO-7 is the world's only Adaptive Optics accessory for the amateur CCD market and it
works only with the self guided feature of the ST-7E/ 8E/ 9E. The AO-7 is essentially an
electromechanical driven diagonal mirror that goes between the camera and the telescope.
Using the AO-7 allows corrections to be made at rates up to 40 Hz. This yields perfect,
backlash-free guiding and allows removal of some atmospheric effects. When used in
conjunction with the CCDSharp software stellar peak brightness can double and stellar half-
widths can shrink by 25%.
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Section 6 - Accessories for your CCD Camera
6.6. SGS - Self-Guided Spectrograph
The SGS Self Guided Spectrograph takes the tedium out of spectroscopy by allowing you to
image and guide the source on the tracking CCD while acquiring its spectra on the imaging
CCD. No more hunting around to place the object on the slit! With the SGS you can measure
galactic redshifts, stellar classifications and determine nebular constituency. This is another
example of the value of a self-guided camera like the ST-7E/ 8E/ 9E.
6.7. Third Party Products and Services
There are numerous third party products and services available directed to the CCD user. We
mention a few to give you an idea of the ever increasing interest in this new technology.
Scanning through the pages of Sky & Telescope, CCD Astronomy, or Astronomy magazine
reveals many third-party vendors of CCD accessories and software.
6.7.1. Windows Software
If you are a user interested in using Windows software with your camera, SBIG has CCDOPS
for Windows. In addition there are several commercial Windows programs available which
include a stellar database, telescope control for computerized telescopes like the Meade LX200,
and CCD camera functions in an integrated package.
6.7.2. Image Processing Software
There are a host of image processing software packages capable of reading and processing TIFF
files and new packages are being developed which will read and process SBIG image formats
as well. In addition to commercial software, on-line services like CompuServe and GEnie have
many public domain and shareware programs available through their Astronomy interest
groups.
6.7.3. Getting Hardcopy
In our opinion the best method of producing a hard copy of images taken with the CCD
camera is to simply take a photograph of the computer screen while the image is displayed. It
is best to use a long lens and step back from the monitor to reduce the appearance of the
curvature of the edges of the screen, and use an exposure longer than 1/ 30th of a second to
avoid the video refresh rate of your monitor. Darken the room, and use a brighter background
than is visually optimum.
Another way to obtain hard copy is to save the files in TIFF format and send a copy of
the file on a disk to a photo lab which offers printing of digital images. The Windows version
of CCDOPS allows for printing of the images, and there are a number of third party software
programs for the PC such as Pizzaz Plus which will capture and print the display on your
computer screen. These programs, however, do not produce very detailed prints and are
useful only to a very limited degree.
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Section 6 - Accessories for your CCD Camera
6.8. SBIG Technical Support
If you have any unanswered questions about the operation of your CCD camera system or
have suggestions on how to improve it please don't fail to contact us. We appreciate all your
comments and suggestions. Additionally if you are interested in writing software supporting
SBIG cameras, we offer technical support regarding our file formats found in Appendix B, and
Technical Notes regarding the camera command protocol which we will make available upon
request.
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Section 7 - Common Problems
7.
Common Problems
This section discusses some of the more common problems others have encountered while
using our CCD cameras. You should check here if you experience difficulties, and if your
problem still persists please contact us to see if we can work it out together.
Achieving Good Focus - Achieving a good focus is one of the most difficult areas in working
with CCD cameras due to the lack of real time feedback when focusing. Focus can take
a good deal of time, and as with all forms of imaging, focus is critical to getting the most
out of your camera.
Once you have achieved a good focus with your system, it can be very useful for
future observing sessions to scribe an eyepiece or mark down or log positions of each
component so the next time you will at least be close to focus at the start.
If you know where the focal plane lies for your telescope, you can use Table 3.1
to calculate exactly where the CCD is with respect to your system. By placing the CCD
close to the focal plane initially, you can save a lot of time.
The best kind of object to focus on is a star. As you converge towards focus,
more light from the star will be concentrated onto one pixel. Thus, watching your peak
reading while focusing and focusing for a maximum reading is a good way to get best
focus. This is how we do it. It helps to have a dial or indicator on the focus knob so you
can rapidly return to the best point after going through focus.
Elongated Guided Images - When using Track and Accumulate or Self Guiding, if you notice
guiding errors resulting in elongated star images, you are probably using too long a
snapshot time. If the snapshot time is longer than the amount of time your drive can
track unguided with acceptable guiding errors, you will see elongated stars in your final
images. If your snapshot times are getting down to 30 seconds or less you should
improve your drive.
If you are using your camera as an autoguider for film photography and are noticing
unacceptable guiding errors, please check the following before calling SBIG:
1. Can you move the telescope using the Move command? This is an
indicator as to whether or not you are properly connected to your
drive system via the relay cable from the CPU.
2. Be sure that your calibration time gives at least 10 to 50 pixels of
movement for each step of the Calibrate Track command.
3. Check for flexure between the CCD camera head and your system.
Check for flexure between the guide scope or off-axis guider and
your telescope system. This is a very common source of guiding
errors. A very small movement of the CCD head with respect to the
guide scope during an exposure can cause unacceptable streaking.
4. If your mount is stable, try longer exposure times while tracking to
average out the atmospheric effects.
Finding Objects - The size of the CCD used in the ST-7 (roughly 8mm diagonal) can make
finding objects a little difficult. If you experience this problem you might try the
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Section 7 - Common Problems
following suggestions.
The easiest method of finding objects is to use a reticule eyepiece, if the object is
bright enough to see. Pull the CCD optical head from the eyepiece holder and insert a
12-20mm eyepiece, focussing the eyepiece by sliding it in and out of the eyepiece
holder, not by adjusting the telescope's focus mechanism. Center the object carefully (to
within 10% of the total field) and then replace the CCD optical head. Since the head
was fully seated against the eyepiece holder when you started, fully seating it upon
replacement will assure the same focus.
If the object is too dim to see visually you will have to rely on your setting
circles. Go to a nearby star or object that is easily visible and center that object in the
CCD image. Calibrate your RA setting circle on the known object's RA and note any
DEC errors. Reposition the telescope at the intended object, using the correct RA setting
and the same DEC offset noted with the calibration object. Try a ten second or one
minute exposure and hopefully you will have winged the object. If not you will have to
hunt around for the object. You can use the Focus mode in Low resolution mode for
this and hopefully you won't have to search too far. Check in DEC first, as DEC setting
circles are often smaller and less accurate.
Telescope Port doesn't Move Telescope - If you find the camera is not moving the telescope
for Tracking, Track and Accumulate or Self Guiding you should use the Move
Telescope command with a several second period to isolate the problem down to a
specific direction or directions. If you set the Camera Resolution to the Low mode in the
Camera Setup Command, you can move the telescope and Grab an image fairly quickly
to detect movement of the telescope pointed at a moderately bright star. Try each of the
four directions and see which ones move and which ones don't. At this point the most
likely culprit is the hand controller modification. Trace the signals from the camera's
telescope connector back through the hand controller, paying particular attention to the
offending wires.
Can't Reach Low Setpoint Temperatures - If you find that the camera isn't getting as cold as
expected the problem is probably increased ambient temperatures. While these cameras
have temperature regulation, they still can only cool a fixed amount below the ambient
temperature (30 to 40 °C). Lowering the ambient temperature allows the cameras to
achieve lower setpoint temperatures.
CCD Frosts - If your camera starts to frost after a year of use it's time to regenerate the
desiccant as described in Appendix C. This is a simple matter of unscrewing the
desiccant container and baking it (without the little O-ring) in an oven at 350°F for 4
hours.
No Image is Displayed - Try the Auto Contrast setting or use the crosshairs to examine the
image pixel values and pick appropriate values for the Background and Range
parameters.
Horizontal Faint Light Streaks in Image - some PCs apparently have the mouse generate non-
maskable interrupts when moved. These interrupts can slightly brighten the line being
read out. If this occurs, do not move the mouse during read out.
A/D Timeout or Other Errors - Some compters require that you configure the parallel port for
"Standard" mode instead of "ECP" or "EPP" mode. These two enhanced modes can
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Section 7 - Common Problems
cause A/ D Timeout and other parallel errors. Refer to you computer manual for how
to use the BIOS Setup utility to configure your parallel port.
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Section 8 - Glossary
8.
Glossary
Antiblooming Gate - When a CCD pixel has reached its full well capacity, electrons can
effectively spill over into an adjoining pixel. This is referred to as blooming. Kodak
CCDs with the antiblooming option can be used to help stop or at least reduce
blooming when the brighter parts of the image saturate.
Astrometry - Astrometry is the study of stellar positions with respect to a given coordinate
system.
Autoguider - All SBIG CCD cameras have auto guiding or "Star Tracker" functions. This is
accomplished by using the telescope drive motors to force a guide star to stay precisely
centered on a single pixel of the CCD array. The camera has four relays to control the
drive corrector system of the telescope. The CCD camera head is installed at the guide
scope or off axis guider in place of a guiding eyepiece.
CCD - The CCD (Charged Coupled Device) is a flat, two dimensional array of very small light
detectors referred to as pixels. Each pixel acts like a bucket for electrons. The electrons
are created by photons (light) absorbed in the pixel. During an exposure, each pixel fills
up with electrons in proportion to the amount of light entering the pixel. After the
exposure is complete, the electron charge buildup in each pixel is measured. When a
pixel is displayed at the computer screen, its displayed brightness is proportional to the
number of electrons that had accumulated in the pixel during the exposure.
Dark Frame - The user will need to routinely create image files called Dark Frames. A Dark
Frame is an image taken completely in the dark. The shutter covers the CCD. Dark
Frames are subtracted from normal exposures (light frames) to eliminate fixed pattern
and dark current noise from the image. Dark Frames must be of the same integration
time and temperature as the light frame being processed.
Dark Noise - Dark Noise or Dark Current is the result of thermally generated electrons
building up in the CCD pixels during an exposure. The number of electrons due to
Dark Noise is related to just two parameters; integration time and temperature of the
CCD. The longer the integration time, the greater the dark current buildup. Conversely,
the lower the operating temperature, the lower the dark current. This is why the CCD is
cooled for long integration times. Dark noise is a mostly repeatable noise source,
therefore it can be subtracted from the image by taking a "Dark Frame" exposure and
subtracting it from the light image. This can usually be done with very little loss of
dynamic range.
Double Correlated Sampling - Double Correlated Sampling (DCS) is employed to lower the
digitization errors due to residual charge in the readout capacitors. This results in lower
readout noise.
False Color - False Color images are images that have had colors assigned to different
intensities instead of gray levels.
FITS Image File Format - The FITS image file format (which stands for Flexible Image
Transport System) is a common format supported by professional astronomical image
processing programs such as IRAF and PC Vista. CCDOPS can save image files in this
format but can not read them..
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Section 8 - Glossary
Flat Field - A Flat Field is a image with a uniform distribution of light entering the telescope.
An image taken this way is called a flat field image and is used with CCDOPS to correct
images for vignetting.
Focal Reducer - A Focal Reducer reduces the effective focal length of an optical system. It
consists of a lens mounted in a cell and is usually placed in front of an eyepiece or
camera. With the relatively small size of CCDs compared to film, focal reducers are
often used in CCD imaging.
Frame Transfer CCDs - Frame Transfer CCDs are CCDs that have a metal mask over some
portion (usually half) of the pixel array. The unmasked portion is used to collect the
image. After the exposure is complete, the CCD can very quickly shift the image from
the unmasked portion of the CCD to the masked portion, thus protecting the image
from light which may still be impinging on the CCD. This acts as an electronic shutter.
Full Well Capacity - Full Well Capacity refers to the maximum number of electrons a CCD
pixel can hold. This number is usually directly proportional to the area of the pixel.
Histogram - The Histogram is a table of the number of pixels having a given intensity for each
of the possible pixel locations of the image file. Remember that, in the end, the image
file is nothing more than a list of pixel values, one for each CCD pixel. These value
numbers can be displayed in two formats; as a table or plotted as a graph.
Light Frame - The Light Frame is the image of an object before a Dark Frame has been
subtracted.
Path/Filter - For PC users the Path setting tells CCDOPS which directory and disk drive to find
and store the image files. In essence, it defines the current directory. The filter controls
which image files are shown in the Open directory. Only files that match the filter are
shown. For example, a filter setting of "*.ST6" will only show files that have an "ST6"
extension.
Photometry - Photometry is the study of stellar magnitudes at a given wavelength or bandpass.
Pixel Size - The smallest resolution element of a CCD camera is the CCD pixel. The pixel sizes
for each of the SBIG cameras are as follows:
Camera
Pixel Size (microns)
Tracking CCD 13.75 x 16
ST-5C
ST-237
STV
10 x 10
7.4 x 7.4
14.8 x 14.8
23 x 276
ST-6
ST-7E/ ST-8E 9 x 9 (unbinned)
ST-9E 20 x 20 (unbinned)
Planet Mode - Planet Mode is the most useful way to achieve focus. When you select Planet
mode, a full frame is exposed, downloaded, and displayed on the computer monitor. A
small window can be placed anywhere in the image area and the size of the window
6
The TC241 CCD used in the ST-6 actually has 11.5 x 27 micron pixels. The CPU combines two 11.5
micron pixels to make the 23 micron pixel . This gives pixel with a more square aspect ratio.
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Section 8 - Glossary
can be changed. Subsequent downloads will be of the area inside the box resulting in a
much faster update rate.
Quantum Efficiency - Quantum Efficiency refers to the fractional number of electrons formed
in the CCD pixel for a given number of photons. Quantum Efficiency is usually plotted
as a function of wavelength.
Readout Noise - Readout noise is associated with errors generated by the actual interrogation
and readout of each of the CCD pixels at the end of an exposure. This is the result of
fixed pattern noise in the CCD, residual charge in the readout capacitors and to a small
extent the noise from the A/ D converter and preamplifier.
Resolution Mode - The resolution of a CCD camera is determined by pixel size. Pixel size can
be increased by combining or binning more than one pixel and displaying it as one
pixel. Doing so decreases the effective resolution but speeds up the download time of
the image. Maximum resolution is determined by the size of the individual CCD pixel.
The ST-7E/ 8E/ 9E can run in High, Medium, Low and Auto resolution modes.
Response Factor - Response Factor is a multiplier used by CCDOPS to calibrate CCDOPS to a
given telescope for photometric calculations. The Response Factor multiplied by 6700 is
the number of photoelectrons generated in the CCD for a 0th magnitude star per second
per square inch of aperture.
Saturation - Saturation refers to the full well capacity of a CCD pixel as well as the maximum
counts available in the A/ D converter. The pixel is saturated when the number of
electrons accumulated in the pixel reaches its full well capacity. The A/ D is saturated
when the input voltage exceeds the maximum. The saturation values for the various
cameras are shown in the table below.
Full Well
Capacity
Maximum
A/ D Counts
-
e / Count7
Camera
ST-5C
ST-237
ST-6
ST-7E/ ST-8E8
High Res
-
65,535
4,095
1.25
3.4
6.7
2.3
50,000e
-
20,000e
-
65,535
20,000
400,000e
-
50,000e
-
ST-7E/ ST-8E
Med/ Low Res
ST-7E/ 8E/ 9E
Tracking CCD
ST-9E
65,535
65,535
65,535
2.3
1.3
2.8
160,000e
-
150,000e
-
200, 000e
Sky Background - The sky background illumination or brightness is the number of counts in
the image in areas free of stars or nebulosity and is due to city lights and sky glow.
7
8
-
The e / Count shown are for the High Resolution readout mode. These numbers can be different for
other readout modes. Use the Parameters command in the Display menu to see the actual value for
images acquired in other readout modes.
The Non-ABG versions of the ST-7E and ST-8E have twice the full-well capacity of these values
shown.
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Section 8 - Glossary
High levels of sky background can increase the noise in images just like dark current.
For some objects deep sky filters can be used to reduce the sky background level.
Seeing - Seeing refers to the steadiness and the clarity of the atmosphere during an observing
session.
TE Cooler - The TE Cooler is a Thermal Electric cooling device used to cool the CCD down to
operating temperature. The CCD is mounted to the TE Cooler which is mounted to a
heat sink, usually the camera head housing.
TIFF Image File Format - The TIFF image file format (which stands for Tagged Interchange File
Format) was developed jointly by Microsoft and Aldus Corporations to allow easy
interchange of graphics images between programs in areas such as presentation and
desktop publishing. CCDOPS can save image files in this format but it can not read
them.
Track and Accumulate - The Track and Accumulate function is a SBIG patented feature of
CCDOPS that allows the user to automatically co-register and co-add (including dark
frame subtraction) a series of images of an object. These exposures can be taken
unguided as long as the "Snapshot time" does not exceed the length of time before
tracking errors of your clock drive become apparent. This allows you to image and track
without guiding or the need to connect the CCD Relay port to your drive motors.
Track List - The Track List is an ASCII file generated by CCDOPS during a Track and
Accumulate session. The Track List logs all the corrections made by CCDOPS for each of
the images. Track lists are required when flat fielding Track and Accumulate images.
Tri-Color - Tri-Color refers to color images created using three different colors mixed into a
balanced color image using red, green and blue filters. An object is imaged three times,
once with each color filter. The three images are then co-added and color balanced with
the appropriate software.
Vignetting - Vignetting is obstruction of the light paths by parts of the instrument. It results in
an uneven illumination of the image plane. The effects of vignetting can be corrected
using flat field images.
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Appendix A - Connector Pinouts
A.
Appendix A - Connector ad Cables
This appendix describes the various connectors and cables used with the ST-7E/ 8E/ 9E.
A.1. Appendix A - Connector Pinouts
Tables A1 and A2 below show the pin-outs of the Telescope and Power connectors on the ST-
7E/ 8E/ 9E.
Pin Number
Function
1
Chassis Ground
2
3
4
5
CFW-8 Pulse/ AO-7 Data Out
Plus X (Active Low Open Drain)9
Plus Y (Active Low Open Drain)
Signal Ground
6
7
8
9
Minus X (Active Low Open Drain)
Minus Y (Active LowOpen Drain)
+12 Volts Out (100mA max)
+5 Volts Out (300 mA max)
Chassis Ground
Shell
Table A1 Telescope Connector
Pin Number
6, Shell
Function
Earth Ground
DC Ground
-12V DC, 100mA
No contact
+12V DC, 500mA
+5V DC, 2A
5
4
3
2
1
Table A2 Power Connector
+12VDC on outside
Ground in Center
Mating plug is 5.5mm
outside and 2.1mm inside
Figure A1 - Cooling Booster DC Power Jack
A.2. SBIG Tracking Interface Cable (TIC-78)
Many of the newer telescopes have a phone-jack connector on the drive corrector for
connecting directly to the ST-7E/ 8E/ 9E Camera's Telescope Port. These include the Celestron
Ultima, Losmandy CG11 and Meade LX-200. You can interface these telescopes to the
9
The Open Drain outputs can sink 100 mA maximum
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Appendix A - Connector Pinouts
Telescope port with our TIC-78 (Tracking Interface Cable), or you can make your own cable.
Figure A1 below shows the pinouts on these telescopes.
Hand Controller
Button
Modular Phone
Wire Color
Telescope Port Pin
Special
Common
Left
White
Black
Red
Green
Yellow
Blue
None
5
6
7
4
3
Down
Up
Right
Figure A1 - CCD Connector for TIC Mating
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Appendix B - File Formats
B.
Appendix C - Maintenance
This appendix describes the maintenance items you should know about with your CCD camera
system.
B.1. Cleaning the CCD and the Window
The design of SBIG cameras allows for cleaning of the CCD. The optical heads are not
evacuated and are quite easy to open and clean. When opening the CCD chamber, one should
be very careful not to damage the structures contained inside.
To open the CCD Chamber, remove the six screws that hold the 5 inch front cover in
place. Remove the six screws and lift the front cover, exposing the structures inside. There is a
rubber O-Ring that sets in the groove on the top of the Chamber housing.
The CCD array is protected by a thin cover glass that can be cleaned with Q-Tips and
Isopropyl Alcohol. Do not get alcohol on the shutter. Dust on the CCD should be blown off. Use
alcohol only if necessary. The optical window of the chamber housing can be cleaned the same
way. When reinstalling the chamber housing, be very careful to make sure the O-ring is in the
groove when seated.
B.2. Regenerating the Desiccant
This section describes the regeneration procedure for the desiccant used in the ST-7E/ 8E/ 9E.
The desiccant absorbs moisture in the CCD chamber, lowering the dew point below the
operating temperature of the cooled CCD, thus preventing the formation of frost. The
desiccant is contained in a small cylindrical plug that screws into the chamber from the rear. In
normal operation the useful life of the desiccant is over a year. If the CCD chamber is opened
often, the desiccant should be regenerated when frosting is noticed. Follow the procedure
below to regenerate the desiccant:
1. Unscrew the brass desiccant container from the rear of the camera and
remove the O-ring.
2.. Plug the resulting hole by screwing in the supplied bolt or plug 2 or 3 turns.
Finger tight is adequate. Don't put a wrench on it.
2. Heat the dessicant container in an oven at 350°F (175 deg C) for 4 hours.
The solder used to seal the can melts at 460 degrees F, so be sure to stay at
least 50 degrees below this number. Preheating the oven to avoid hot spots
is advised.
3. Replace the desiccant container into the rear of the camera, being careful to
reinstall the O-ring and insure that it does not get pinched.
4. Expect the camera to take an hour or two to reach the frost free state. If it
does seem to frost and you need to capture images, reduce your cooling to
the zero degree C range - the CCD dark current will still be quite low.
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Appendix C - Capturing a Good Flat Field
C.
Appendix C - Capturing a Good Flat Field
This appendix describes how to take a good flat field. A good flat field is essential for
displaying features little brighter than the sky background. The flat field corrects for
pixel non-uniformity, vignetting, dust spots (affectionately called dust doughnuts), and
stray light variations. If the flat field is not good it usually shows up as a variation in
sky brightness from on side of the frame to the other.
C.1. Technique
The first consideration in capturing a flat field is to use the telescope-CCD combination
in exactly the configuration used to collect the image. This means you probably have to
capture the flat field at the telescope. Do not rotate the head between image and flat
field, since the vignetting is usually slightly off center. Do not be tempted to build a
little LED into the telescope or camera for doing flat fields; it doesn't work at all. The
dust debris shadows would be different!
Arrange a light source such as a flashlight, two white cards, the telescope and
CCD as shown in Figure D-1.
Figure D-1: Flat Field Geometry
Telescope
CCD
Flat White
Surface
Flashlight
Flat White
Surface
The key aspects of this geometry are that the reflection off two diffuse surfaces is used,
and the large flat surface is square to the illumination from the small flat surface. When
we do this, the first flat surface is typically a white T-shirt worn by the operator! Take
care that no apparent shadows are cast onto the larger flat white surface. Use an
exposure at the camera that yields an average light level equal to about half of full scale.
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Index
A/ D converter, 8, 27
accessories, 41
Establish COM Link, 16
PC Setup, 16
adaptive optics, 40
antiblooming, 29, 45
Antiblooming Gate (def), 45
AO-7, 40
communications link, 16
connector
Telescope, 49
connector (CPU)
astrometric measurements, 45
Astrometry (def), 45
astrophotography, 12
atmospheric effects, 17
auto contrast, 18
RELAYS, 23, 37
cooling, 8, 48
co-register images, 20, 23
crop, 18
crosshairs, 18
Auto Grab Command, 21
autoguider, 8, 20, 23, 36, 45
Autoguider (def), 45
background parameter, 18
battery operation, 15, 23
binning, 10
Calibrate Track Command, 21, 37
calibration star, 37
camera lens adapter, 33, 40
Camera Setup Commands, 20
CCD, 7, 11
dark current, 8
dark frame, 8, 9, 15, 18
Dark Frame (def), 45
Dark Noise (def), 45
Dark Subtract Command, 19
dark subtracted, 9, 15, 18
DCS, 9, 45
Declination, 15, 23, 37, 44
desiccant, 51
Diffuse Magnitude, 19
diffuser, 16
(def), 45
Dim mode, 19
cameras, 5, 6, 8, 15, 23, 26, 27, 28
cleaning, 51, 53
detector, 5, 7, 23
displaying images, 18
double correlated sampling, 9
Double Correlated Sampling (def), 45
drive corrector, 20
electromechanical shutter, 27
electromechanical vane, 27
electronic images, 12
electronic shutter, 27
Establish COM Link Command, 16
eyepiece, 17, 18
eyepiece holder, 15
eyepiece projection, 40
eyepiece tube, 17
False Color (def), 45
field of view, 11, 28, 29, 37
film, 5, 10, 11
orientation, 15
CCDCOLOR software, 21
CCDOPS software, 8
centering objects, 18
cleaning the CCD, 51, 53
clock drivers, 8
co-add images, 5, 20, 23
cold pixels, 19
color filter wheel, 21, 40
color images, 13, 21
color table, 45
commands
Auto Grab, 21
Calibrate Track, 21, 37
Dark Subtract, 19
film grain, 10
filter wheel, 13
Focus, 17, 19
Grab, 18, 33
Track, 37
filters, 13, 19, 21, 40
finding objects, 18
fine focus, 17
Track and Accumulate, 20, 33, 37
Tracking Parameters, 37
Commands
FITS format (def), 45
flat field, 10, 19, 34, 35
Flat Field (def), 46
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Index
flip, 19
focal length, 30
negative image, 18
nosepiece, 15
focal reducer, 40
Focal Reducer (def), 46
focus
observatory, 23
optical head, 15, 23
optical head orientation, 16
O-ring, 8
Dim mode, 18, 19
fine, 17
path (def), 46
Full frame mode, 17
Full Frame mode, 19
peak, 17
path/ filter (def), 46
PC, 8
PC Setup Command, 16
PEC drive correctors, 20, 37
photometric measurements, 46
Photometry (def), 46
pixel nonuniformity, 19
pixel size, 10, 28
Pixel Size (def), 46
pixel uniformity, 10
planet mode, 19
Planet mode (def), 46
planets, 5, 16, 30, 33
POWER connector, 15
power supply, 8, 15
power supplyr, 23
PPEC drive correctors, 20, 37
preamp, 8, 23
printing images, 12, 41
push to make switch modification, 25
push to make/ break switch modification,
25
Planet mode, 17, 19, 46
Focus Command, 17, 19
focus mode, 17, 19
Focusing, 16
Frame Transfer CCDs, 6
Frame Transfer CCDs (def), 46
frost, 8
Full Frame CCDs, 6
full well capacity, 29
Full Well Capacity (def), 46
Grab Command, 18, 33
graphics, 18
guiding, 11
guiding error, 20, 37
hand controller, 18, 20, 23, 24
hermetic chamber, 8
Histogram (def), 46
host computer, 8, 15, 23
hot pixels, 19
IBM PC, 8
image processing, 8, 12, 18, 41
interference, ii
Quantum Efficiency (def), 47
range parameter, 18
readout
IR blocking filter, 22
joystick, 25
electronics, 23
noise, 29
joystick modification, 26
library of dark frames, 18, 34
light frame, 9, 15, 18
light frame (def), 46
Link, 16
Noise (def), 47
register, 6
regenerating desiccant, 51
RELAYS connector, 23, 37
resolution, 10, 20, 34
Resolution (def), 47
Response factor (def), 47
Right Ascension, 15, 23, 37, 44
Saturation (def), 47
scribing the eyepiece, 17
seeing, 10, 17, 20, 30
Seeing (def), 48
LPT port, 15, 16, 23
magnitude
diffuse, 19
stellar, 19
maintenance, 51, 53
microcontroller, 8
modifying hand controller, 24
moon, 16, 30, 33
self guiding, 23
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Index
separations, 19
zoom, 18
setup, 15
SGS-Self Guided Spectrograph, 41
sharpen, 19
shutter, 8
signal to noise ratio, 22, 29
sky background, 5, 20
smoothing, 19
snapshot, 23
software, 26, 41
spectral range, 5
spectrograph, 41
Status Window, 16
Link field, 16
stellar magnitude, 19
stellar temperature, 19
super pixel, 19
taking images, 18
TE cooler, 8
TE Cooler, 7
TE Cooler (def), 48
Technical Support, 42
telescope, 10, 15, 18, 20
Telescope connector, 49
telescope hand controller, 18, 20, 23, 24
temperature regulation, 9, 18, 27
thermistor, 9
TIC, 49
TIC-78, 24
TIFF format (def), 48
Track and Accumulate (def), 48
Track and Accumulate Command, 20, 33,
37
Track and Accumulate mode, 23, 35
Track Command, 37
track list, 35
Track List (def), 48
track mode, 23
tracking interface cable (TIC), 24, 49
Tracking Parameters Command, 37
Tri-Color (def), 48
tri-color images, 40
T-Thread, 40
vane, 27
vignetting, 10, 19
Vignetting (def), 48
Windows, 41
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