Polaroid ST-8E, ST-7E, ST-9E User Manual
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>
Note: This equipment has been tested and found to comply with the limits for a Class B digital
device pursuant to Part 15 of the FCC Rules. These limits are designed to provide reasonable
protection against harmful interference in a residential installation. This equipment generates,
uses, and can radiate radio frequency energy and if not installed and used in accordance with
the instructions, may cause harmful interference to radio communications. However, there is
no guarantee that interference will not occur in a particular installation. If this equipment does
cause harmful interference to radio or television reception, which can be determined by turning
the equipment off and on, the user is encouraged to try to correct the interference by one or
more of the following measures:
• Reorient or relocate the receiving antenna.
• Increase the separation between the receiver and the equipment.
• Connect the equipment into an outlet on a circuit different from that to which the
receiver is connected.
• Consult the dealer or an experienced radio/TV technician for help.
Shielded I/O cables must be used when operating this equipment.
You are also warned, that any changes to this certified device will void your legal right to
operate it.
CCDOPS Manual for ST-7E/ST-8E/ST-9E
Third Revision
December 1999
Table of Contents
1. Introduction.................................................................................................................1
1.1. Road Map of the Documentation............................................................................... 1
1.2. Quick Tour....................................................................................................................1
1.2.1. CCDOPS Software....................................................................................2
1.2.2. CCD Camera.............................................................................................3
2. Introduction to CCD Cameras ...................................................................................5
2.1. Cameras in General ..................................................................................................... 5
2.2. How CCD Detectors Work..........................................................................................5
2.2.1. Full Frame and Frame Transfer CCDs....................................................6
2.3. Camera Hardware Architecture.................................................................................6
2.4. 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
2.5. Electronic Imaging..................................................................................................... 12
2.6. Black and White vs. Color.........................................................................................13
3. At the Telescope with a CCD Camera....................................................................15
3.1. Step by Step with a CCD Camera ............................................................................15
3.2. Attaching the Camera to the Telescope...................................................................15
3.3. Establishing a Communications Link ......................................................................16
3.4. Focusing the CCD Camera........................................................................................16
3.5. Finding and Centering the Object............................................................................18
3.6. Taking an Image ........................................................................................................ 18
3.7. Displaying the Image ................................................................................................18
3.8. Processing the Image................................................................................................. 18
3.9. 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
4. Camera Hardware.....................................................................................................23
4.1. System Components..................................................................................................23
4.2. Connecting the Power...............................................................................................23
4.3. Connecting to the Computer....................................................................................23
4.4. Connecting the Relay Port to the Telescope............................................................23
4.4.1 Using Mechanical Relays ...........................................................................24
i
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
5.1. Lunar and Planetary Imaging...................................................................................33
5.2. Deep Sky Imaging .....................................................................................................33
5.3. Terrestrial Imaging .................................................................................................... 33
5.4. Taking a Good Flat Field...........................................................................................34
5.5. Building a Library of Dark Frames...........................................................................34
5.6. Changing the Camera Resolution ............................................................................ 34
5.7. Flat Fielding Track and Accumulate Images...........................................................35
5.8. Tracking Functions....................................................................................................36
6. Accessories for your CCD Camera..........................................................................39
6.1. Cooling Booster..........................................................................................................39
6.2. Tri-color Imaging.......................................................................................................40
6.3. Camera Lens Adapters and Eyepiece Projection .................................................... 40
6.4. Focal Reducers ...........................................................................................................40
6.5. AO-7 and Lucy-Richardson Software ...................................................................... 40
6.6. SGS - Self-Guided Spectrograph...............................................................................41
6.7. 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
6.8. SBIG Technical Support............................................................................................. 42
7. Common Problems...................................................................................................43
8. Glossary......................................................................................................................47
A. Appendix A - Connector ad Cables........................................................................51
A.1. Appendix A - Connector Pinouts............................................................................. 51
A.2. SBIG Tracking Interface Cable (TIC-78)...................................................................51
B. Appendix C - Maintenance......................................................................................53
B.1. Cleaning the CCD and the Window........................................................................53
B.2. Regenerating the Desiccant .......................................................................................53
C. Appendix C - Capturing a Good Flat Field............................................................55
C.1. Technique 55
ii
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: selfguiding (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.
Page 1
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.
Page 2
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).
Page 3
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
Readout Register
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 offchip and digitally encoded and stored in a computer to be reconstructed and
displayed on a television monitor."
1
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.
Page 6
Section 2 - Introduction to CCD Cameras
Tracking CCD
Imaging CCD
TE Cooler
Micro-
controller
PC
Interface
Telescope
Interface
Desktop
Power
Supply
Parallel Interface
Figure 2.2 - CCD System Block Diagram
16 Bit
A/D
Clock
Drivers
Preamp
Shutter
Host Computer
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.
Camera CCD
Array
Dimensions
Number of
Pixels Pixel Sizes
Tracking CCD TC211 2.6 x 2.6 mm 192 x 164 13.75 x 16 µ
ST-5C TC255 3.2 x 2.4 mm 320 x 240 10 x 10 µ
ST-237 TC237 4.7 x 3.6 mm 640 x 480 7.4 x 7.4 µ
STV TC237 4.7 x 3.0 mm 320 x 200 14.8 x 14.8 µ
ST-6 TC241 8.6 x 6.5 mm 375 x 242 23 x 27 µ
ST-7E KAF0401E 6.9 x 4.6 mm 765 x 510 9 x 9 µ
ST-8E KAF1602E 13.8 x 9.2 mm 1530 x 1020 9 x 9 µ
ST-9E KAF0261E 10.2 x 10.2 mm 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 ST7E 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
Page 9
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 darksubtracted 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
Page 10
Section 2 - Introduction to CCD Cameras
(High = 9µ2 pixels, Medium = 18µ2 pixels, Low = 27µ2 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 Array Dimensions Diagonal Field of View at 8" f/10
Tracking CCD 2.64 x 2.64 mm 3.73 mm 4.5 x 4.5 arcminutes
ST-5C 3.20 x 2.40 mm 4.00 mm 5.6 x 4.2 arcminutes
ST-237 4.74 x 3.55 mm 5.92 mm 8.2 x 6.1 arcminutes
STV 4.74 x 2.96 mm 5.58 mm 8.2 x 5.1 arcminutes
ST-6 8.63 x 6.53 mm 10.8 mm 14.6 x 11 arcminutes
ST-7E 6.89 x 4.59 mm 8.28 mm 11.9 x 7.9 arcminutes
ST-8E 13.8 x 9.18 mm 16.6 mm 23.8 x 15.8 arcminutes
ST-9E 10.2 x 10.2 mm 14.4 mm 17.6 x 17.6 arcminutes
35mm 36 x 24 mm 43 mm 62 x 42 arcminutes
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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