Sbig ST-6, ST-5, ST-4X User Manual
CCD Camera Operating Manual
for the
Model ST-4X, ST-5 and ST-6
Santa Barbara Instrument Group
1482 East Valley Road
Santa Barbara, CA 93108
Phn (805) 969-1851
Fax (805) 969-4069
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.
Changes or modifications not expressly approved by the party responsible for compliance could
void the user's authority to operate the equipment.
Also note that user must use shielded interface cables in order to maintain product within FCC
compliance.
CCDOPS Manual
Revision 2A
January 1996
Table of Contents
1. Introduction ............................................................................................................1
1.1. Road Map of the Documentation ............................................................................1
1.2. Quick Tour...............................................................................................................1
1.2.1. CCDOPS Software.................................................................................1
1.2.2. CCD Camera..........................................................................................2
2. Introduction to CCD Cameras................................................................................3
2.1. Cameras in General..................................................................................................3
2.2. How CCD Detectors Work......................................................................................3
2.2.1. The ST-4X CCD and Frame Transfer CCDs...........................................4
2.3. Camera Hardware Architecture..............................................................................5
2.4. CCD Special Requirements......................................................................................7
2.4.1. Cooling...................................................................................................7
2.4.2. Readout Types.......................................................................................8
2.4.3. Dark Frames...........................................................................................8
2.4.4. Flat Field Images....................................................................................8
2.4.5. Pixels vs. Film Grains.............................................................................9
2.5. Electronic Imaging...................................................................................................9
2.6. Black and White vs. Color......................................................................................10
3. At the Telescope with a CCD Camera.................................................................13
3.1. Step by Step with a CCD Camera..........................................................................13
3.2. Attaching the Camera to the Telescope.................................................................13
3.3. Establishing a Communications Link....................................................................15
3.4. Focusing the CCD Camera ....................................................................................15
3.5. Finding and Centering the Object..........................................................................17
3.6. Taking an Image ....................................................................................................17
3.7. Displaying the Image.............................................................................................17
3.8. Processing the Image.............................................................................................17
3.9. Advanced Capabilities...........................................................................................18
3.9.1. Crosshairs Mode (Photometry and Astrometry).................................18
3.9.2. Sub-Frame Readout in Focus...............................................................18
3.9.3. Track and Accumulate.........................................................................19
3.9.4. Autoguiding.........................................................................................19
3.9.5. Auto Grab ............................................................................................20
3.9.6. Color Imaging......................................................................................20
4. Camera Hardware.................................................................................................21
4.1. System Components..............................................................................................21
4.2. Connecting the Power............................................................................................21
4.3. Connecting to the Computer.................................................................................22
4.4. Connecting the Relay Port to the Telescope..........................................................23
4.5. Modular Family of CCD Cameras.........................................................................26
5. Camera Software Reference.................................................................................31
5.1. Different Host Computers.....................................................................................31
5.1.1. Installing the Software.........................................................................31
5.1.2. The CCDOPS User Interface................................................................32
5.1.3. CCDOPS for IBM PCs..........................................................................35
i
Command Mode..................................................................................35
Graphics Mode.....................................................................................36
5.1.4. CCDOPS on Macintosh Computers ....................................................37
5.2. CCDOPS Menus and Commands..........................................................................37
5.2.1. Command Tree....................................................................................37
5.2.2. File Menu on the Macintosh................................................................39
5.2.3. File Menu on the PC ............................................................................40
Save FITS Command............................................................................41
Save TIFF Command ...........................................................................42
5.2.4. Edit Menu on the Macintosh ...............................................................43
5.2.5. Camera Menu ......................................................................................44
Grab Command ...................................................................................45
Auto Grab Command..........................................................................46
Focus Command..................................................................................47
Macintosh Focus Window...................................................................49
PC Focus Mode Menus........................................................................50
ST-4X Camera Setup Command..........................................................51
ST-5 Camera Setup Command ............................................................52
ST-6 Camera Setup Command ............................................................53
5.2.6. Display Menu on the Macintosh..........................................................54
5.2.7. Display Menu on the PC......................................................................55
Display Image Command....................................................................56
PC Display Mode Menus.....................................................................57
Macintosh Contrast Window...............................................................59
Macintosh Color Table Editor..............................................................60
5.2.8. Utility Menu.........................................................................................61
Edit Parameters Command..................................................................63
5.2.9. Misc Menu ...........................................................................................64
Mac Setup Command..........................................................................65
PC Setup Command ............................................................................66
Telescope Setup Command.................................................................67
5.2.10. Track Menu..........................................................................................68
Track and Accumulate Command.......................................................69
Calibrate Track Command...................................................................70
Tracking Parameters Command..........................................................71
5.2.11. Filter Menu...........................................................................................73
Filter Setup Command.........................................................................74
6. Advanced Imaging Techniques...........................................................................75
6.1. Lunar and Planetary Imaging................................................................................75
6.2. Deep Sky Imaging..................................................................................................75
6.3. Terrestrial Imaging ................................................................................................76
6.4. Taking a Good Flat Field .......................................................................................76
6.5. Building a Library of Dark Frames........................................................................76
6.6. Changing the Camera Resolution..........................................................................77
6.7. Making Astrometric and Photometric Measurements..........................................77
6.7.1. Astrometric Measurements..................................................................77
6.7.2. Photometric Measurements.................................................................79
6.7.3. Calculation of Centroids......................................................................80
6.7.4. Calculation of Separation.....................................................................81
6.7.5. Calculation of Magnitude....................................................................81
6.7.6. Calculation of Diffuse Magnitude.......................................................82
6.8. Flat Fielding Track and Accumulate Images.........................................................82
ii
6.9. Tracking Functions ................................................................................................84
6.10. PC COM Port Compatibility Testing.....................................................................85
7. Accessories for your CCD Camera.......................................................................87
7.1. Tri-color Imaging...................................................................................................87
7.2. Camera Lens Adapters and Eyepiece Projection...................................................87
7.3. Focal Reducers.......................................................................................................87
7.4. Third Party Products and Services........................................................................87
7.4.1. Windows Software...............................................................................87
7.4.2. Image Processing Software..................................................................88
7.4.3. Getting Hardcopy................................................................................88
7.5. SBIG Technical Support.........................................................................................88
8. Common Problems ...............................................................................................89
9. Glossary.................................................................................................................93
A.1. Appendix A - Connector Pinouts.........................................................................97
A.2. Using RS422 on the PC ..........................................................................................98
A.3. SBIG Tracking Interface Cable (TIC) .....................................................................98
B. Appendix B - SBIG File Formats..........................................................................99
B.1. SBIG Image Formats..............................................................................................99
B.1.1 Type 3 Format......................................................................................99
B.1.2. Image Compression...........................................................................101
B.2. PC Color Table Formats.......................................................................................102
B.3. TIFF Format .........................................................................................................103
B.4. FITS Format..........................................................................................................103
C. Appendix C - Maintenance.................................................................................105
C.1. Replacing the Fuse...............................................................................................105
C.2. Cleaning the CCD and the Window....................................................................105
C.2.1 Cleaning the ST-4X and ST-5.............................................................105
C.2.2 Cleaning the ST-6...............................................................................105
C.3. Replacing the Desiccant in the ST-6.....................................................................106
D. Appendix D - Cross Platform Compatibility ....................................................107
iii
Section 1 - Introduction
1. Introduction
Congratulations and thank you for buying one of Santa Barbara Instrument Group's CCD
cameras. The model ST-4X, ST-5 and ST-6 represent the state of the art in CCD camera systems
with their low noise and advanced capabilities. 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, but never seen when peeking through the eyepiece. SBIG CCD cameras offer
convenience, high sensitivity (a typical deep-sky image is only two to ten minutes), 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 Astronomy that is growing by leaps and bounds.
1.1. Road Map of the Documentation
This manual describes the ST-4X, ST-5, and ST-6 CCD Camera Systems from Santa Barbara
Instrument Group. For new users to the field of CCD Astronomy, Sections 2, 3 and 4 offer
introductory material about CCD Cameras and their uses 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 into section 5 which
gives detailed and specific information about the SBIG software. This section is written more as
a reference than as general reading. Sections 6 and 7 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 8 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.
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.
• For this quick tour you can run the CCDOPS software from the distribution
floppy disk or you can install the software on our hard disk (refer to Section
5.1.1).
• On the Macintosh you can double-click on the CCDOPS icon. On the PC 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.
• On the Macintosh the image is displayed automatically. On the PC you then
use the Image command in the Display menu to display the image.
Page 1
Section 1 - Introduction
• Try using the crosshairs. On the Macintosh use the Show Crosshairs
command in the Display menu. On the PC 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. On the Macintosh you check
the Invert checkbox and then click the Do It button in the Contrast window.
On the PC you hit the Esc key to quit the Crosshairs mode, then hit the 'N'
key.
• Try the photo display mode. On the Macintosh use the Photo Mode
command in the Display Menu. On the PC you 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.
• Load up the other sample images and display them using the photo display
mode.
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 2 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 pin-hole 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 2
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). 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.
It is for this reason that we designed our cameras to support both efforts, as a stand-alone
tracker, in the case of the ST-4, and as a tracker/imaging camera in the case of the other SBIG
CCD products.
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
horizontal register of pixels. This register collects a line at a time and then
Page 3
Section 2 - Introduction to CCD Cameras
X=1
X=M
Y=1
Y=N
Output
Readout Register
Amplifier
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
Figure 2.1 - CCD Structure
2.2.1. The ST-4X CCD and Frame Transfer CCDs
In the ST-4X, the CCD is read out electronically by shifting each row of pixels into a readout
register at the Y=0 position of the CCD, 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 Y=164 position. Note that the CCD elements are still
collecting light as they step up to the readout register.
The ST-5 and ST-6 CCD cameras use a more advanced 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. The ST-4X CCD minimizes the
effect of not having a frame transfer buffer by reading out the array relatively quickly, reading
30,000 pixels in about 0.7 seconds. As long as the CCD exposure is greater than about one
second this technique will reduce streaking of the stars to acceptable levels.
Planets pose a particular problem to the ST-4X CCD since they are so bright that
exposures of 1 second at f/10 are badly overexposed. The ST-4X has a "Half Frame" mode for
planets and bright stars to solve this problem. In the Half Frame mode the upper half of the
CCD is used as a frame buffer for a bright object positioned in the lower half of the CCD as
shown below in Figure 2.2. A short exposure can be taken and the bottom half of the array
shifted rapidly up to the upper half. The 82 lines of short exposure data can then be readout at
the normal rate. This method works quite well, and uses enough pixels such that 0.5 arcsecond
per pixel scale factors can be achieved while viewing an entire planet.
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 4
Section 2 - Introduction to CCD Cameras
CPU
Power
Supply
CCD
TE Cooler
Host Computer
Optical
Head
RS232
Preamp
Clock
Drivers
Postamp/
A/D
Converter
Microcontroller
Frame
Store
Figure 2.2 - ST-4X Half Frame Positioning
When using the original ST-4 for a long exposure, a glow was present in the upper left corner of
the image, near pixel (1,1). This was due to an electrical luminescence in the readout electronics
that could saturate the array in the corner in exposures several minutes long. This glow has
been essentially eliminated in the ST-4X.
2.3. Camera Hardware Architecture
This section describes the SBIG CCD camera from a systems standpoint. It describes the
elements that comprise a CCD camera and the functions they provide. Please refer to Figure 2.3
below as you read through this section.
Figure 2.3 - CCD System Block Diagram
At the "front end" of any CCD camera is the CCD sensor itself. As we have already learned,
CCD detectors are a solid state image sensor organized in a rectangular array of regularly
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Section 2 - Introduction to CCD Cameras
spaced rows and columns. Table 2.1 below lists some interesting aspects of the various CCDs
used in the ST-4X, ST-5 and ST-6 cameras.
Array
Camera CCD
ST-4X TC211 2.6 x 2.6 mm 192 x 164 13.75 x 16 µ
ST-5 TC255 3.2 x 2.4 mm 320 x 240 10 x 10 µ
ST-6 TC241 8.6 x 6.5 mm 375 x 242 23 x 27 µ
The CCD is cooled by mounting it on the cold side of a thermoelectric (TE) cooler. The TE
cooler pumps heat out of the CCD with its own internally generated heat and dissipates it into a
heat sink which forms part of the optical head's mechanical housing. In SBIG cameras this
waste heat is dumped into the air using passive radiators, making the design and operation of
the heads simple and not inconvenienced by requirements for liquid recirculation cooling.
Since the CCD is cooled below 0°C, some provision must be made to prevent frost from
forming on the CCD. SBIG 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 Optical heads. Keeping the size of
the hermetic chamber small (in the case of the ST-4X and ST-5) or using desiccant in the
chamber (ST-6) keeps the total amount of moisture that can condense small.
Other elements contained in the optical head include the preamplifier and an
electromechanical vane (in the case of the ST-6). The vane makes taking dark frames a simple
matter of pushing a button on the computer. We refer to this as a vane rather than a shutter
because it does not perform the task of timing the exposure, it merely blocks the light from the
CCD to facilitate taking dark images. Timing of exposures in SBIG cameras is based upon the
clocking scheme applied to the CCDs.
As far as the ST-4X and ST-5 are concerned, that's all of the system components
contained in the optical head unit, owing to its small size. The Clock Drivers and the PostAmp/Analog to Digital Converter reside in different places in the ST-4X/ST-5 and the ST-6.
The Clock Drivers adapt the logic-level signals from the CPU's 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 Postamp further amplifies and conditions the
CCD's output signal for digitization by the Analog to Digital Converter (A/D). In the ST-6 both
the clock drivers and the A/D are contained on a board in the optical head. In the ST-4X and
ST-5 these electronics are on a board in the lower half of the CPU.
This leaves to be discussed only the elements in the CPU, namely the Microcontroller,
the Frame Store, and the Power Supply. The microcontroller is a 9.2 MHz 80188
microcontroller, based upon the 8088 microprocessor used in the IBM PC. It controls the
operation of the CCD camera at the lowest level, receiving commands from the Host Computer
and executing sequences of instructions to control and acquire images. The frame store allows
holding three images in the CPU (a light image, a dark image, and a double-precision
accumulation image). Finally, the power supply takes the supplied 12 Volts and produces the
various regulated voltages required by the CPU in addition to a variable output supply for
powering the TE Cooler.
Although not part of the CCD Camera itself, the Host Computer and Software are an
integral part of the system. SBIG provides software for its cameras that support both the IBM
Dimensions
Table 2.1 - Camera CCD Configurations
Number of
Pixels Pixel Sizes
Page 6
Section 2 - Introduction to CCD Cameras
PC (and Compatible) and Macintosh computers. The software allows image acquisition, image
processing, and auto guiding with ease of use and professional quality. Many man-years and
much customer feedback has 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 noise and dark current combine to place a lower limit on the ability of the CCD to
detect faint light sources. If the CCD is producing more electrons from its own internal
processes than is produced by photons from a distant object, the signal from the object is said to
be "lost in the noise", and will be impossible to display without sophisticated image processing
software. The same is true if the immediate environment is producing the noise. There are
several sources of noise, both internal and external which can contribute to this problem. Noise
here refers to the "gritty" look of short exposure images.
Internally, the CCD generates thermal noise and readout noise caused by the operation
of the electronics on the chip. In unusual circumstances, radio frequency interference can
contaminate the CCD just as it can affect your television set or radio, but this is rarely a problem
in normal operating environments. Power lines, switches turning on and off, spark plugs, even
cosmic rays will register if conditions are right. Of course, there is one external source of "noise"
you do not want to eliminate - the photons coming from the object you are imaging! So the trick
is to eliminate unwanted sources of electron production in the chip and thus make the detector
more sensitive to the remaining source of electron production by incoming photons. As you
can imagine, the reduction of unwanted noise is important for the best performance of the CCD.
The user will naturally have to do his or her best to reduce external sources of noise in the
environment. The internal noise of SBIG cameras is kept to an absolute minimum by using state
of the art technology.
Dark current is thermally generated electrons in the device itself. 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, this source of noise is reduced,
the sensitivity increased, and longer exposures are possible. In fact, for every 8°C of additional
cooling, the dark current in the CCD is reduced to half. All SBIG cameras use a thermoelectric
(TE) cooler to cool the CCD. The ST-4X and ST-5 have a single stage cooler whereas the larger
format ST-6 utilizes a two stage cooler. The ST-5 and ST-6 have temperature sensing
thermistors on the CCD mount to monitor the temperature, and the CPU 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-5 and ST-6 also means that one dark frame can be
used for similar exposures on several nights.
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
Page 7
Section 2 - Introduction to CCD Cameras
is why deep sky filters are so popular with astrophotography. They reduce the sky background
level, increasing the contrast of dim objects.
2.4.2. Readout Types
In order to read out the values of the charge stored in the pixels as they are shifted to the
readout amplifiers, the charge 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 longer read out times (two measurements are made instead of one). If the
accuracy of the data is not critical, as in finding objects or focusing, the extra time spent in
double correlated sampling is not necessary. In this case a rapid readout mode may be
available which ignores the small residual charge.
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.
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. It is 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 however always
recommended for images that are intended for accurate photometric measurements.
Page 8
Section 2 - Introduction to CCD Cameras
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-4X is 13.75 x 16 microns (1 micron = 0.001mm ≈ 0.04 mil). In the ST-5 the pixels are 10 x 10
microns. In the ST-6 the pixels are 23 x 27 microns. Film grain ranges from several hundredths
of a micron to several microns depending on the film speed and quality of the emulsion.
However, the effects of seeing are probably the limiting factor in any good photograph or
electronic image. For example, on a perfect night with excellent optics an observer might hope
to achieve sub-arcsecond seeing. More often, however, with the average night sky and even
very good optics, seeing may be limited to several arcseconds and you would probably be very
satisfied with seeing of one or two arcseconds.
Using an ST-5 camera with 10 micron pixels, an 8" f/10 telescope will produce a single
pixel angular subtense of one arcsecond, seeing permitting. A 10" f/3 telescope will produce
images of 2.6 arcseconds per pixel. If, however, seeing affects the image by limiting resolution
to 3 arcseconds, then you would be hard pressed to see any resolution difference between the
larger and smaller pixels as you are mostly limited by the sky conditions.
Another important consideration is the field of view of the camera. For instance, while
the ST-5 has smaller pixels than the ST-6, its area is also smaller, resulting in a corresponding
smaller field of view at a given focal length. For comparison, the diagonal measurement of a
frame of 35mm film is approximately 40mm, whereas the diagonal dimension of the ST-5 chip is
approximately 4mm. The relative CCD sizes for each camera 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
ST-4X 2.64 x 2.64 mm 3.73 mm 4.5 x 4.5 arcminutes
ST-5 3.20 x 2.40 mm 4.00 mm 5.6 x 4.2 arcminutes
ST-6 8.63 x 6.53 mm 10.8 mm 14.6 x 11 arcminutes
35mm 36 x 24 mm 43 mm 62 x 42 arcminutes
Table 2.2 - CCD Array Dimensions
A subtle 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.
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.
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Section 2 - Introduction to CCD Cameras
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 the best 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
purpose is simply to make a record or catalog the image file for easy identification, a dot matrix
or laser printer should be fine.
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 is color blind and the pixel values
that the electronics read out to a digital file are only numbers representing 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
Page 10
Section 2 - Introduction to CCD Cameras
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 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 turned until the green filter is in place and another
image is 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 surroundings to learns 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 you computer. Then you will want to get in
focus, 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
All SBIG cameras are similar in configuration. The CCD head attaches to the telescope by
slipping into the eyepiece holder. A fifteen foot cable runs from the head to the CPU box, which
is usually on the ground near the telescope. A ten foot cable connects the host computer's serial
port to the CPU box. The CPU is powered by a wall transformer although operation from a car
battery is possible.
Important Note: Never connect or disconnect the CCD head from the CPU box unless the power
switch on the rear side of the CPU box is turned OFF. Damage to the CCD head,
or the CPU could occur.
Referring to Figure 3.1 below, connect the CCD head to the CPU connector marked "CCD
HEAD" with the supplied cable. Next, connect the serial cable to the COM connector along the
same front panel of the CPU box and connect the opposite end to the serial port of your
computer.
Insert the CCD Camera's nosepiece into your telescope's eyepiece holder. You should
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. You should orient the camera
so that the CCD's axes are aligned in Right Ascension and Declination. Use Figure 3.2 below
showing the back of the optical heads as a guide for the proper orientation.
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Section 3 - At the Telescope with a CCD Camera
POWER
RELAYS
COM
AUX
CCD HEAD
JUMPER
CCD HEAD
To Host
Computer
ST-4X/ST-5 Connections
Wall
Transformer
Wall
Transformer
POWER
RELAYS
COM
AUX
CCD HEAD
ST-6 Connections
To Host
Computer
Next connect the power cable and plug in the transformer. Turn on the CPU by pressing the
power switch on the back panel of the CPU box. The red LED in the power switch should glow
indicating power has been applied to the unit. When power is applied to the unit, the CPU
section is automatically reset. You can manually reset the CPU at any time by pushing the
RESET button on the front panel of the CPU, next to the power connector. Pushing reset is only
desirable if the camera fails to talk to the host computer. You can hear the relays momentarily
engage inside the CPU box with a 'click' when you push in the RESET button.
Figure 3.1 - CPU Connections
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Section 3 - At the Telescope with a CCD Camera
ST-6 Orientation
RA
DEC
ST-4X/ST-5 Orientation
RA
DEC
Figure 3.2 Orientation of the Optical Head Viewed from Back
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, initializing the offset adjustment in the
case of an ST-6 head, and seeking the highest Baud rate. If the software is successful the "Link"
field in the Status Window is updated to show the communications parameters achieved. If the
camera is not connected or the COM 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 PC or Mac Setup command in the Misc menu to configure the CCDOPS
software for the serial port you are using. Then use the Establish COM Link command in the
Camera Menu to establish communications with the CPU.
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-4X powers up with the TE cooling turned on which will
be adequate. The ST-5 and ST-6 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 40°C below the ambient temperature.
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. 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:
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Section 3 - At the Telescope with a CCD Camera
Back Focus Distance
from Table 3.1
Diffuser
Camera Distance
ST-4X 0.040 inch
ST-5 0.050 inch
ST-6 0.560 inch
Table 3.1 - Camera Back Focus
To find the 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 5th to 7th magnitude star (you don't want to focus on bright stars
like Sirius because the CCD saturates quickly unless you're way out of focus). If you want, you
can center the image on the computer monitor by fine adjusting the telescope position although
this is not necessary. As long as the star is in the field of view and not so close to the edge that it
will drift out while you are focusing, the positioning is not critical.
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 the 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 cameras 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.
Camera Saturation Counts
ST-4X 16384
ST-5 16384
ST-6
65535
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
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.
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Section 3 - At the Telescope with a CCD Camera
3.5. Finding and Centering the Object
Once best focus is achieved, we suggest using the Focus command in "Dim" mode to help
center objects. This mode gives a full field of view, but reduces resolution in order to increase
the digitization and download time. 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 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, reinsert the CCD head and set an exposure time of about ten seconds. 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.
3.6. Taking an Image
Take a CCD image of the object by selecting the Grab command and setting the exposure time.
Star 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. In the case of
the ST-4X and ST-5 you are reminded to cover the telescope for the dark frame. In the case of
the ST-6 the dark frame is taken automatically.
Because the ST-5 and ST-6 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 host
computer. 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 dramatically improved by subtracting off a dark frame of
equal exposure for an ST-6. On the ST-4X and ST-5 this effect isn't very dramatic for exposures
shorter than a few seconds. You will typically do this as part of the Grab command although it
can also be done manually 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.
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Section 3 - At the Telescope with a CCD Camera
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 14
to 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. Section 6.7 gives detailed information about how you use the crosshair mode to
make astrometric and photometric measurements.
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 11x11 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
neighboring block of pixels into a "super pixel".3 This reduces the download and display times
proportionately, as well improving sensitivity. While you would not want to use the Dim mode
for critical focus adjustments due to the large effective pixel, 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.
One final Focus readout mode is the Spot mode. In Spot mode the entire image is
digitized at high resolution, and the image is scanned for the brightest pixel. A small box of
pixels surrounding the brightest pixel is then downloaded and displayed. The increase in
2
On the PC the Crosshairs mode is accessed through the Display Image command in the Analysis
mode. On the Macintosh you use the Show Crosshair command in the Display Menu.
3
The Dim mode combines pixels after they are digitized which is referred to as off-chip binning.
Page 18
Section 3 - At the Telescope with a CCD Camera
throughput is dependent on the camera being used and can range from 2:1 for an ST-6 which
has a slower 16 bit digitization rate to 4:1 on an ST-5 with its faster 14 bit readout. Spot mode is
probably most handy for telescopes that suffer from a lot of image shift during focus. Where as
Planet mode shows the same area image after image, Spot mode tracks the brightest object
around in the field of view.
Another aspect of the Focus command and its various modes is the Camera Resolution
4
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 all support High, 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.
3.9.3. Track and Accumulate
An automatic Track and Accumulate mode (patent pending) 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 don't show up and the accumulated image has enough integrated exposure to
yield a good signal to noise ratio.
Procedureally the camera will take an exposure, determine the position of a preselected
star, co-register and co-add the image to the previous image in the CPU, 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 field of view. This new feature provides dramatic performance for the
amateur, enabling unattended hour long exposures!
3.9.4. Autoguiding
The CCDOPS software allows the ST-4X, ST-5 and ST-6 cameras to be used as autoguiders
through the commands in the Track menu. While these systems are not stand-alone, requiring a
host computer, they can accurately guide long duration astrophotographs.
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 an end all cure to telescope
drive corrector 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
4
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.
Page 19
Section 3 - At the Telescope with a CCD Camera
and a moderately 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.
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.
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-6A, which provides this capability.
The color filter wheel allows conveniently 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.
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 places an IR
blocking filter in series 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. In black
and white images your eye is capable of pulling out 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.
Page 20
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-4X, ST-5 and ST-6 CCD cameras consist of three major components: the CCD Sensor
and Preamplifier, the Readout/Clocking Electronics, and the CPU. Where each of these
functions resides varies across the product line.
The CCD and Preamplifier are always mounted in the optical head which usually
interfaces to the telescope through a 1.25 inch draw tube, sliding into the telescope's focus
mechanism. The placement of the physically small preamplifier close to the CCD is necessary
to achieve good noise performance.
The Readout and Clocking Electronics are housed in the Optical Head in the case of the
ST-6, and housed in a second sub-chassis of the CPU in the ST-4X and ST-5. With the ST-6's
requirements for larger cooling fin area and the miniaturization of the electronics we were able
to fit all the Readout and Control Electronics into the Optical Head of the ST-6. The desire to
stay with the smaller format Optical Head in the ST-4X and ST-5 made placing the Readout and
Control Electronics with the CPU a necessity.
As a side note it's interesting to understand "Why does the ST-6 Optical Head need to be
so big"? The reason is heat dissipation. The CCD used in the ST-6 has roughly 6 times the
package area of the CCDs used in the ST-4X and the ST-5. A larger package requires a larger
amount of cooling to achieve the same operating temperature. In the case of the ST-6 we must
pump roughly 1.5 Watts of heat out of the CCD to cool it to -20°C which requires us to supply
almost 10 Watts to the two-stage TE cooler. That 10 Watts has to be dissipated into the air,
requiring the large fin area found in the ST-6 head. The ST-4X and ST-5 only have to dissipate 2
Watts for the same amount of cooling.
The CPU is the master controller of the CCD camera system. Housed within the CPU
chassis are a flexible power supply, allowing the unit to run off 12 Volts from a wall transformer
or a car battery, a microcontroller, and a frame storage buffer. The microcontroller receives
high level commands from the host computer and translates them into sequences of actions.
For example when the host computer wants to acquire an image it sends the CPU a "take image"
command. The CPU starts by clearing the CCD with the necessary clocking, times the
exposure, and at the end of the exposure clocks the CCD again with a readout sequence, storing
the digitized data in the frame storage buffer. All this happens while the CCD's temperature is
being regulated and communications with the host computer are being maintained.
4.2. Connecting the Power
The power supply in the CPU is designed to run off 12 Volts AC or DC. Most users will find
using the wall transformer supplied with the systems to be the most convenient way to power
the system. In the field however, battery operation is the most logical choice. In that case you
can simply unscrew the power cable from the wall transformer and attach it to the battery. AC
systems (like the wall transformer) do not have a fixed polarity: swapping the leads at the
transformer does not make a difference as far as the operation of the CPU is concerned since the
output of the transformer is isolated from any other grounds in the system. When powering the
Page 21
Section 4 - Camera Hardware
CPU with a DC supply the polarity of the applied voltage is important, mainly because it is
common in DC systems to connect the negative lead to ground. When powering the CPU from
a battery or DC power supply observe the polarities shown in Table 4.1 below for the Power
Connector on the CPU:
Voltage Power Connection
+12V to +15V Pin 9
0V or Ground Pin 4
Table 4.1 - CPU Power Connections
One interesting item about the design of the power supply in the CPU bears mentioning. The
power supply, which is a switching power supply, offers very high efficiency but has an
unusual characteristic. As the input voltage to the power supply drops (due to brownouts or
overall low voltage) the current increases to accommodate the voltage drop. At some point the
CPU will detect that the input voltage has dropped too much and will shut down the TE Cooler
(the largest current drain) in an attempt to not over current the transformer. If this occurs the
host software will tell you about it. If it happens repeatedly you might suspect that the voltage
supply to the wall transformer is low, possibly due to voltage drops in a long extension cord,
etc.
Foreign users of SBIG systems may need to obtain a local version of the wall transformer
as SBIG does not supply them. A trip to your local "Radio Shack" may be necessary to find a
12V, 20VA transformer for the ST-4X and ST-5 or a 12V, 50VA transformer for the ST-6.
4.3. Connecting to the Computer
The ST-4X, ST-5 and ST-6 CCD Cameras are supplied with a 15 foot cable to connect the system
to the host computer. The connection is between the CPU's COM connector and the Host
Computer's serial COM port. This cable is available in several varieties to support the various
host platforms and we try to query users about their systems to insure they receive the correct
cable. PC based systems have either a 9 or a 25 pin male D type connector at the rear of the
computer for their COM ports. Macintosh computers mostly have a round 8 pin female DIN
connector for their Modem and Printer ports.
For PC systems we recommend using COM 1 or COM 2 for connecting to the camera.
While CCDOPS supports COM 3 and COM 4, often times there are conflicts between these two
COM ports and COM 1 and 2. If the cable we supply you does not have the connector that
mates with your COM ports then a quick trip to Radio Shack for an adapter will solve the
problem. On PC based systems there can also be conflicts between other add-ons and the COM
ports: most commonly Modem cards, Scanner cards, and Mice. If you experience
communications problems with your CCD camera we recommend starting from zero by
removing all possibly conflicting cards from the PC and removing all drivers from your
CONFIG.SYS and AUTOEXEC.BAT files, adding these items back one at a time to find the
source of the conflict. The other common problem that PC users experience in communicating
with the cameras is when running under Windows. Windows can severely limit the speed
CCDOPS uses in communicating with the CCD Cameras and we highly recommend you only
run CCDOPS from DOS when capturing images.
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Section 4 - Camera Hardware
For Macintosh users we recommend connecting the
camera to the Modem port . The printer port can be used but you
will have to turn off AppleTalk and any Background Printing
options you may have enabled.
While the serial cables supplied with the cameras run 15 feet, the length can be extended
to over 1000 feet if done properly. For Macintosh systems this is merely a matter of obtaining or
making a longer cable (refer to the table of COM pinouts in Appendix A). For PC systems the
problem is a bit more involved.
The serial drivers in PC based computers are RS-232 drivers that are not intended to
drive long cables at the high Baud rates used in communicating with the CCD cameras. While
these drivers will typically drive 30 to 50 feet of cable they will rarely drive 100 feet. The
problem is that the RS-232 drivers will not drive the cable capacitance, a problem that is
exacerbated by the requirement to use shielded cable to prevent emissions of radio frequency
interference. What we recommend is a two step approach to tackling the problem.
Try a simple three-conductor shielded cable if your cable run is less than 100 feet. You
may get lucky and be spared the additional expense of a more extensive solution. If you have
problems with your longer cable, and you're sure the problems are related to the length of the
cable (always try our standard cable first to make sure there isn't some other problem) then we
recommend you use a five-conductor shielded cable and an RS-232 to RS-422 converter. RS-422
uses two drivers per signal, driving a pair of wires differentially, and can drive much higher
cable capacitances over longer distances. The CPU has RS-422 drivers so you need only get a
converter for the PC end of the communications link. For specific information about RS-422
converters refer to Appendix A.
Modem Printer
4.4. Connecting the Relay Port to the Telescope
The ST-4X, ST-5 and ST-6 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 two modes: the Track mode and the Track and
Accumulate mode.
In the Track mode the host software corrects the telescope as often a 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 buttons on the
telescope's hand 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
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 coregistration of images that accounts for the streakless images.
Page 23
Section 4 - Camera Hardware
A: Unmodified Push to Make Switch
switch
B: Modified Push to Make Switch
normally open
common
switch
relay
cnonc
The host software and the CCD camera control the telescope through the Relay port on
the CPU. By interfacing the CPU to the telescope's hand 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 CPU's Relay port to your telescope if you are planning on
using the camera system as an autoguider 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 CPU Relay port. All that's required is a
simple cable to attach the CPU's 15 pin Relay port to the telescope's telephone-jack type CCD
connector. SBIG offers its TIC (Tracking Interface Cable) for this express purpose although it
would take only one-half hour to modify a standard 6-pin telephone cable to interface to the
Relay port (see Appendix A for specific pin outs, etc.).
Older telescopes generally require modifying the hand controller to accept input from
the CPU's Relay port. The difficulty of this task varies with the drive corrector model and 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.
In general, the Camera has five internal relays that are used in tracking applications.
There is one relay for each of the four correction directions on the hand controller (North, South,
East and West) plus an additional relay for an alarm should the CPU be unable to continue
guiding for some reason. Each of the relays has a Common, a Normally Open, and a Normally
Closed contact. For example, when 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. These relay
contacts are brought out the CPU's Relay port and the standard cable supplied by SBIG has
twelve colored wires with tinned flying leads (see Appendix A for a pinout of the Relay port
and the standard Relay Cable) that you solder into your hand controller.
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.
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
Figure 4.1 - Push to Make Switch Modification
Page 24
Section 4 - Camera Hardware
cnonc
switch
common
normally open
normally closed
A: Unmodified Push to Make/Break Switch
B: Modified Push to Make/Break Switch
normally open
normally closed
cnonc
relay
common
cnonc
189
15
15 Pin Male D Connector
(back view)
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 CPU 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. One caveat about this type of switch
configuration is that the CPU must be plugged into the hand controller (although the CPU
needn't be powered up) in order for the hand controller to function on its own. This is due to
the necessity of keeping the relay's Normally Closed contact intact since as previously
mentioned the Relay and the Hand Controller switch are in series. This need for the continued
presence of the CPU can be alleviated by making a CPU eliminator as shown in Figure 4.3 to
plug into the end of the relay cable. The CPU eliminator essentially makes the four Normally
Closed contacts.
Figure 4.2- Push to Make/Brake Modification
Figure 4.3 - CPU Eliminator Plug
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 in the CPU can be interfaced to
the joystick as shown in Figure 4.4 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.
Page 25
Section 4 - Camera Hardware
potentiometer
ABC
A: Unmodified Joystick
wiper
B: Modified Joystick
nccno
+ relay
ABCncc
no
- relay
potentiometer
ABC
A: Unmodified Joystick
R
wiper
B: Joystick Eliminator
nccno
+ relay
BCAncc
no
- relay
R/2
R/2
Figure 4.4 - Joystick Modification
A slight variation on the joystick modification is to build a complete joystick eliminator as
shown in Figure 4.5 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.
Figure 4.5- 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. This line is being expanded by the introduction of the ST-4X and
ST-5. All three of these cameras share a common CPU referred to as the Universal CPU. A
single CPU can support the three different cameras through the use of an integrated Optical
Head and Readout/Clocking Electronics in the case of the ST-6 or with the separate Optical
Head and Readout/Clocking Electronics in the case of the ST-4X and ST-5.
The benefits of a modular line of CCD Cameras are many fold. 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 single CPU supporting all three systems, camera
control software like CCDOPS can easily support all three 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 three different packages for each of the cameras.
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