Sbig ST-237 User Manual

Operating Manual

for the ST-237

Advanced CCD Camera

PRICE $10.00

Santa Barbara Instrument Group

Santa Barbara Instrument Group

1482 East Valley Road - Suite 33
PO Box 50437
Santa Barbara, CA 93150
PHN [phone]
FAX [phone]

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.

ST-237 Manual
Second Printing
November 1999

Table of Contents

1. INTRODUCTION TO CCD CAMERAS....................................................................................................3

1.1. HOW CCDS WORK...............................................................................................................................3

1.2. CCDS APPLIED TO ASTRONOMICAL IMAGING ................................................................................... 4

1.2.1. Cooling............................................................................................................................................. 4

1.2.2. Dark Frames..................................................................................................................................... 5

1.3. THE VARIOUS CCD PARAMETERS AND HOW THEY AFFECT IMAGING.............................................5

1.3.1. Pixel Size .......................................................................................................................................... 5

1.3.2. Full Well Capacity............................................................................................................................ 6

1.3.3. Dark Current.....................................................................................................................................6

1.3.4. Read Noise ........................................................................................................................................ 6

1.3.5. Frame Transfer .................................................................................................................................7

1.3.6. Antiblooming Protection ................................................................................................................... 7

1.3.7. A/D Bits and Digitization Rate.......................................................................................................... 8

1.3.8. Binning .............................................................................................................................................9

1.3.9. Spectral Response............................................................................................................................. 9

1.4. CAMERA HARDWARE ARCHITECTURE ............................................................................................. 10

2. THE FIRST DAY WITH THE CAMERA ............................................................................................... 13

2.1. SETTING UP THE SYSTEM..................................................................................................................13

2.1.1 Installing the CCDOPS Software ..................................................................................................... 13

2.1.2. Getting Acquainted with CCDOPS Software.................................................................................... 13

2.1.3. Connecting the Camera to the Computer......................................................................................... 16

2.1.4. Establishing a Communications Link with the Camera.................................................................... 17

2.1.5. Operating your Camera with CCDOPS - a Daytime Orientation.................................................... 17

2.2. THE FIRST NIGHT WITH THE CAMERA.............................................................................................. 19

2.2.1. Focusing the Camera ...................................................................................................................... 20

2.2.2. Finding and Centering the Object ................................................................................................... 21

2.2.3. Taking an Image ............................................................................................................................. 21

3. SOFTWARE FLOW..................................................................................................................................23

3.1. IMAGE ACQUISITION ROUTINES........................................................................................................23

3.1.1 Grab ............................................................................................................................................... 23

3.1.2 Focus.............................................................................................................................................. 23

3.1.3 Track and Accumulate..................................................................................................................... 24

3.1.4 Eyepiece Mode................................................................................................................................ 24

3.1.5 Auto Grab....................................................................................................................................... 24

3.1.6 Color Grab...................................................................................................................................... 25

3.2. FILE OPERATIONS .............................................................................................................................. 25

3.2.1 Naming Conventions....................................................................................................................... 25

3.2.2 File Path and Filter......................................................................................................................... 26

3.2.3 Saving Images................................................................................................................................. 26

3.2.4 Loading Images............................................................................................................................... 27

3.3. IMAGE DISPLAY ROUTINES ............................................................................................................... 27

3.3.1 Photo vs. Analysis Mode ................................................................................................................. 27

3.3.2 Image Parameters........................................................................................................................... 27

3.3.3 Slide Shows..................................................................................................................................... 28

3.3.4 Printing Images............................................................................................................................... 28

3.3.5 Background and Range................................................................................................................... 28

3.4. IMAGE PROCESSING ROUTINES......................................................................................................... 28

3.4.1 Depixelize and Reduce.................................................................................................................... 29

3.4.2 Filtering..........................................................................................................................................29

3.4.3 Image Corrections........................................................................................................................... 30

i

3.4.4 General Purpose ............................................................................................................................. 30

3.5. IMAGE ANALYSIS ROUTINES.............................................................................................................30

3.5.1 Histogram....................................................................................................................................... 31

3.5.2 Cross Hairs..................................................................................................................................... 31

3.6. SETUP FUNCTIONS.............................................................................................................................31

3.6.1 Camera Setup.................................................................................................................................. 31

3.6.2 Graphics Setup................................................................................................................................ 32

3.6.3 Communications Setup.................................................................................................................... 32

3.7. GENERAL PURPOSE CAMERA FUNCTIONS........................................................................................32

3.7.1 Establish Link ................................................................................................................................. 32

3.7.2 Information..................................................................................................................................... 32

3.7.3 Shutdown......................................................................................................................................... 32

3.7.4 Autoguiding .................................................................................................................................... 33

3.7.5 Filter Wheel.................................................................................................................................... 33

4. ADVANCED IMAGING TECHNIQUES.................................................................................................35

4.1. TAKING A GOOD FLAT FIELD ............................................................................................................ 35

4.2. TRACK AND ACCUMULATE............................................................................................................... 35

4.3. COLOR IMAGING................................................................................................................................35

4.4. AUTOGUIDING.................................................................................................................................... 36

4.5. FIELD OPERATION.............................................................................................................................. 37

5. GLOSSARY...............................................................................................................................................39

6. HINTS AND TIPS......................................................................................................................................43

6.1 QUESTION AND ANSWER....................................................................................................................... 43

6.2. CCDOPS USE TIPS ............................................................................................................................. 45

6.3. HINTS AND TIPS FOR LAPTOP USERS................................................................................................ 45

6.4. TELESCOPE TIPS ................................................................................................................................ 46

A. APPENDIX A - SPECIFICATIONS ......................................................................................................... 49

A.1 DC POWER JACK................................................................................................................................ 49

A.1 COMMUNICATIONS PORT .................................................................................................................. 49

A.1 TELESCOPE PORT...............................................................................................................................50

B. APPENDIX B - FILE FORMATS ............................................................................................................. 51

B.1 SBIG COMPRESSED AND UNCOMPRESSED FORMATS...................................................................... 51

B.2. TIFF FORMAT .................................................................................................................................... 56

B.3. FITS FORMAT .................................................................................................................................... 56

C. APPENDIX C - MAINTENANCE ............................................................................................................ 57

C.1. REPLACING THE FUSE........................................................................................................................ 57

C.2. DISASSEMBLING/REASSEMBLING THE OPTICAL HEAD....................................................................57

C.3. CLEANING THE OPTICAL WINDOWS..................................................................................................58

C.4. REPLACING THE DESICCANT.............................................................................................................58

D. APPENDIX D - ADVANCED IMAGE PROCESSING TECHNIQUES ................................................. 59

D.1. ASTROMETRY AND PHOTOMETRY.................................................................................................... 59

D.1.1 Astrometric Measurements..........................................................................................................59

D.1.2 Photometric Measurements......................................................................................................... 60

D.1.3 Calculation of Centroids............................................................................................................. 62

D.1.4 Calculation of Separation ........................................................................................................... 63

D.1.5 Calculation of Magnitude ........................................................................................................... 63

D.1.6 Calculation of Diffuse Magnitude.................................................................................................... 64

D.2. FLAT FIELDING TRACK AND ACCUMULATE IMAGES....................................................................... 64

ii

D.3. FURTHER READING............................................................................................................................ 65

iii

Introduction

Congratulations and thank you for buying the SBIG ST-237 Advanced CCD Camera. This
camera offers incredible performance in a small package for a moderate cost. Using the camera
will expand your astronomical experience by allowing you to easily take images like the ones
you've seen in books and magazines, but never seen when peeking through the eyepiece. CCD
cameras offer convenience, high sensitivity (a typical deep-sky image is several minutes), and
advanced image processing techniques that film just can't match. While CCDs will probably
never replace film in its large format, CCDs offer ease of use and allow a wide range of
scientific measurements. Their use has established a whole new field of Astronomy. Some of
the features you'll discover about your camera include:

• Based on the Texas Instrument TC37 CCD with 640 x 480 pixels that are 7.4
microns square.

• Double Correlated Sampling readout with 12 Bit A/D for the lowest possible
noise.

• Convenient and fast parallel interface offers full frame download times under 4
seconds.

• Thermoelectric cooling and vibrationless fan give sky background limited
performance.

• Integral Shutter Wheel that can be replaced with a Color Filter Wheel for Tricolor
imaging.

• Telescope port for use as an Autoguider.

• Advanced CCDOPS software for data acquisition, display and processing.

• Track and Accumulate1for hassle-free long duration exposures.

• 12 VDC wall transformer or use with battery in the field.

• Standard T-Thread allows use with a variety of telescope adapters including the
standard 1.25 inch nosepiece and eyepiece projection units.

This manual is organized for two types of use. Some sections have been designed to be read
through from the start while you're learning about the camera and the software whereas other
sections are to be used for reference. Briefly the manual consists of the following sections:

• Section 1 describes CCD cameras and how they work. While it is the first section
of the manual, it is a bit technical, and some users may wish to skip ahead to the
second section then come back to this section once they have had a little hand's
on experience.

• Section 2 tells you how to install the camera and camera software and takes you
step by step through the process of taking your first images. Even if you have
experience with other cameras you should browse through this section and read
any sections that are new to you.

• Section 3 describes the CCDOPS imaging software that comes with your camera.
It presents the software from the standpoint: "What commands do I use to take

1

Track and Accumulate covered by SBIG US Patent 5,365,269.

Page 1

Introduction

images?", etc. While it doesn't discuss every nuance of each command (that's
reserved for Section 4) it will give you a good feel for the flow of the software.

• Section 4 presents some more detailed information about how you use the
camera for some slightly more advanced tasks. The discussion of even more
advanced topics is continued in Appendix D. Once you have become familiar
with the basic operations of the camera you will want to read these sections.

• A separate manual describes each and every command in the CCDOPS software
and is meant more as a reference section than a narrative one. If you're
wondering what the "Dark interval" setting in the Track and Accumulate
command does this section is for you.

• Sections 5 and 6 provide a Glossary of common CCD imaging terms, a Question
and Answer section on the most common questions you'll have and a section of
useful hints and tips for using the camera. Again, this is a good section to read
once you have had a little time with the camera.

• Finally, the Appendices provide a wealth of technical information about the
camera.

Page 2

Section 1 - Introduction to CCD Cameras

1. Introduction to CCD Cameras

The CCD (charge coupled device) 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, one-step color, and independence of the wall plug (the ST-237 camera 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 the camera to support both
efforts, as an imaging camera and as an auto-guider to aid astrophotography.

1.1. How CCDs Work

The CCD is a solid-state imaging detector that is quite commonly used in video tape cameras
and is starting to find acceptance in still frame cameras. It has been used for Astronomical
Imaging for over twenty years. The CCD is arranged as a rectangular array of imaging
elements called pixels. An image is formed by reading the intensity of these pixels.

The basic function of the CCD detector is to convert an incoming photon of light to an

electron which is stored in the detector array until it is read out, thus producing data which
your computer can display as an image. 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

4) charge detection

The first operation relies on a physical process known as the photoelectric effect ­when photons or particles strike 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

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Section 1 - Introduction to CCD Cameras

Readout Register

pixels. This register collects a line at a time and then transports the charge
packets in a serial manner to an on-chip amplifier. The final operating step,
charge detection, is when individual charge packets are converted to an output
voltage. The voltage for each pixel can be amplified off-chip and digitally
encoded and stored in a computer to be reconstructed and displayed on a
television monitor."

2

Output

Y=1

Amplifier

Y=N

X=1 X=M

Figure 1.1 - CCD Structure

1.2. CCDs Applied to Astronomical Imaging

When CCDs are applied to astronomy, with the relatively long exposure times (compared to
the 30 frames per second used in video camera), special considerations need to be applied to
the system design to achieve the best performance. This section discusses the cooling and dark
frame requirements of astronomical imaging.

1.2.1. 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. 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. The goal 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.

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,

2

"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 1 - Introduction to CCD Cameras

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. In your camera for example, cooling
the CCD from room temperature (25°C) down to 0°C results in an eight-fold reduction in dark
current.

The ST-237 uses a thermoelectric (TE) cooler to cool the CCD. The TE cooler is a solid-

state device that acts like a heat pump. By running electrical current through the TE cooler,
heat is pumped out of the CCD through the TE cooler. The camera also has a temperature
sensing thermistor attached to the CCD to monitor the temperature, and the camera electronics
control the temperature at a user determined value for long periods. As a result, exposures up
to an hour long are possible, and saturation of the CCD by the sky background typically limits
the exposure time.

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, and can be used to advantage with the
CCD camera.

1.2.2. 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 an "image" of the dark. This dark frame will contain an image
of the noise caused by dark current (thermal noise) and other fixed pattern noise When the
dark frame is subtracted from the light frame, this pattern noise is removed from the resulting
image. The CCDOPS software can automatically perform this.

1.3. The Various CCD Parameters and How they Affect Imaging

If you scan the CCD related literature you will see a slew of new terms describing CCDs and
their performance. In this section we will discuss the more common CCD parameters and their
effects in an imaging application.

1.3.1. Pixel Size

Every CCD, independent of the manufacturer, is divided into a relatively large number of
small pixels. In your CCD camera the imaging area of the CCD is 4.7 mm x 3.6 mm and the
pixels are 7.4 microns square (1 micron is one thousandth of a millimeter or roughly 0.0004"). If
you looked at the CCDs available from the various manufacturers you would see that their
pixels typically vary from 7 microns on the small end to 25 microns on the large end. There are
advantages and disadvantages associated with the size of pixels in a CCD.

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Section 1 - Introduction to CCD Cameras

While having small pixels may seem advantageous in terms of offering "higher

resolution", large pixels gather more light and are thus "more sensitive". You can also adjust
your telescope configuration to accommodate various size pixels, using faster telescopes to
increase the speed of small pixel CCDs or longer focal lengths to increase the resolution of
larger pixel CCDs.

Often times the basic goal is to match the CCD resolution to the telescope resolution and

to the overall seeing. It would be a waste to use a pixel size of 7 microns on a telescope with a
spot size of 25 microns or to configure the CCD/telescope to produce an image scale of 10 arc­seconds per pixel when you're looking for fine planetary detail.

1.3.2. Full Well Capacity

The full well capacity of a CCD is the number of electrons each pixel can hold before the pixels
are full. While this may seem like an important consideration in choosing a camera, you need
to think about how the camera is used.

The typical CCD astronomer is taking images of faint galaxies and nebulae. While

exposures are long, you very rarely will expose the CCD to more than a small fraction of its full
well capacity on these dim objects. Some stars in the image will expose to the full well capacity,
but not much of the nebulosity. So even though small pixel size CCDs have lower full well
capacities than large pixel CCDs, most applications do not stress this CCD parameter.

1.3.3. Dark Current

Every CCD, independent of manufacturer, will suffer from dark current. One manufacturer's
CCD may have lower dark current than another manufacturer's, but they all have dark current.
The only way to reduce dark current in a CCD is by cooling it, and in general the more cooling,
the better. But there is a limit.

In astronomical imaging, you're looking at objects against the sky background, and that

sky background isn't perfectly dark. City light and sky glow itself cause the sky to actually
have some brightness. As far as the CCD is concerned from a noise standpoint, it can't tell the
difference between electrons generated by dark current and those generated by sky
background. Because of this, cooling the CCD beyond the point where the dark current is less
than the sky background will not result in any further improvement in image quality.

As a matter of interest, your CCD camera produces roughly 1 e-/p/s

(electron/pixel/second) of dark current when cooled to -5°C and a typical dark sky through an
F/10 telescope produces 2 e-/p/s. Sky background scales inversely with the square of the

telescope's F-Ratio and for example is 25 times higher at F/2 than it is at F/10.

1.3.4. Read Noise

The read noise of a CCD is the noise inherent in the CCD's amplifier and charge detection
circuitry. This read noise forms a noise floor below which the CCD will not detect weak
signals. Something you're imaging must rise above the read noise level before you'll be able to
see it.

Since the read noise is the noise floor, it is really only important for very short exposures

of dim objects. As the signal from your target or the signal from the sky background builds up,
it will cross the noise floor and the noise in the image will be determined only by the brightness

Page 6

Section 1 - Introduction to CCD Cameras

of the target and the exposure time, not the CCD's read noise. For example, the read noise in
your CCD camera is 15 electrons RMS, and when your signal has built up to 225 electrons (15
electrons RMS squared) the read noise is no longer the dominant noise in the final image. At
F/10 the sky background alone achieves this level in 2 minutes and at F/2 it occurs in 4
seconds!

There are techniques CCD manufacturers can use to reduce the read noise. One of the

techniques is Double Correlated Sampling where some of the noise in the amplifier is
subtracted out by making two measurements per pixel rather than one. Your Advanced CCD
camera uses such a technique.

1.3.5. Frame Transfer

There are two basic types of CCDs available: Frame Transfer CCDs like the one used in your
camera and Full Frame CCDs. A Frame Transfer CCD is divided into 2 separate areas on the
CCD. One area, called the image area, is sensitive to light and that is where the image builds
up when exposed to light. But CCDs can't be "turned off" and they continue to build up signal,
even throughout the readout phase. Remember that during readout, the rows of charge are
shifted up to the readout register. It's sort of like advancing the film in a camera with the
shutter open. Unless the CCD is covered with a shutter during the readout, the continued
buildup of signal throughout the readout phase will cause streaking.

The second area in the Frame Transfer CCD, called the storage area, is shielded from

light by an aluminum layer on the CCD. This storage area is used as an "electronic shutter"
whereby data from the image area, after completing the exposure, is rapidly shifted into the
storage area where it is then digitized. A fast shift from the imaging area to the storage area
insures minimal streaking. Once the image is in the storage area, it can read out by the camera
electronics without causing streaking.

The simple answer to streaking you might say is to use a mechanical shutter, and in fact

your camera does have a shutter but the accurate timing of exposures is not limited by the
speed of that shutter but by how rapidly the imaging area can be moved into the storage area.
In this way the mechanical shutter is used to cover the CCD chip for taking dark frames while
short exposure images can be achieved electronically, without the limitations of mechanical
shutters.

1.3.6. Antiblooming Protection

As described above, the individual pixels in the CCD have a limited full well capacity. When a
pixel fills up with charge, the excess charge generated has to go somewhere. Again, there are
two basic types of CCDs available.

Standard CCDs, when reaching the saturation point, will spill the charge into

neighboring pixels, typically up and down the column in a line that is called blooming. If for
example you had a pixel that was exposed to 10 times its full well capacity, it would bloom
until a column of ten pixels was saturated, causing streaks in the image. The second type of
CCD offers Antiblooming protection.

In an Antiblooming protected CCD, when the charge in the pixel gets above some

threshold, typically one-half the full well capacity, the majority of the excess charge gets bled
off into a drain on the CCD. For example, a CCD with a 100X Antiblooming protection will

Page 7

Section 1 - Introduction to CCD Cameras

drain off 99% of the excess charge, allowing a pixel to overexpose to 100-fold before blooming
occurs.

There is a price to pay however with Antiblooming protection and that's why

manufacturers produce both protected and unprotected CCDs. First off, the process of
Antiblooming protection causes a nonlinearity in the response of a CCD. If you were trying to
make accurate Photometric measurements, you would want the integrated star brightness kept
below the knee where the Antiblooming kicks in. The second detriment to some Antiblooming
protected CCDs is that at the integrated circuit level, the Antiblooming structures can reduce
the sensitive area of the individual pixels, causing a slight reduction in overall sensitivity.

The Texas Instruments (TI) TC237 CCD used in your camera has Antiblooming

protection and, according to TI, the structures required to implement the Antiblooming
protection do not cause any reduction in sensitivity

1.3.7. A/D Bits and Digitization Rate

If you browse through the literature on specifications of the various CCD cameras, you see
some of them are 8 bits, some are 12 bits and some are 16 bits. While in general an A/D
(Analog to Digital) converter with greater precision is desired, there is a point where the extra
precision doesn't get you any increased performance. In most CCD cameras it's actually the
CCD that limits the performance, not the A/D converter.

As a starting point, you can take the CCD's full well capacity and divide by the CCD's

read noise to come up with a figure for the CCD's dynamic range. In this way the dynamic
range is the ratio of the brightest object you could image without saturating to the dimmest
object you could detect. You could see that a 16 bit A/D with a dynamic range of 65,000 is
overkill for a CCD with a dynamic range of 4000 for example. Let's look at your camera. The
CCD has a full well capacity of 30,000 electrons and a read noise of 15 electrons RMS giving a
dynamic range of roughly 2000. A 12 bit A/D offers a dynamic range of 4096 and covers the
CCD fairly well.

One thing you do want to do with the A/D is make sure that the A/D's noise (typically

a fraction of a count) is lower than the CCD's noise so that you are truly CCD limited. Setting
the A/D to have a fraction of the noise of the CCD allows averaging several images to improve
noise.

Finally, the CCDs can be used in a mode where the pixels are combined in a process

called binning which is described in detail below. Binning reduces the CCD's spatial resolution
like increasing grain size in film, but increases the CCD's sensitivity and dynamic range. With
your camera you can bin the CCD 2 by 2 or 3 by 3 resulting in increased dynamic range.

The final consideration regarding the A/D is how fast the data is digitized and

downloaded from the CCD to the computer. The A/D is not the only contributor to that time.
The actual transmission of the data to the computer is a significant portion of it. In your
camera an entire image can be digitized by the A/D converter and sent to the PC in under 15
seconds. This is done using the PC's standard Parallel port without requiring the addition of
expensive (and difficult to configure) SCSI adapter cards.

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Section 1 - Introduction to CCD Cameras

1.3.8. Binning

Binning is a process where multiple pixels in the CCD are combined to form a single larger
pixel. This reduces the CCD's resolution but increases the sensitivity. Different CCDs from
various manufacturers support different types of binning.

Some CCDs support on-chip binning, where all the pixels in the group are combined in

the CCD itself. This has the advantages of lower noise (a single read noise is generated for the
group of pixels) and higher speed digitization since fewer pixels are involved. In imaging
applications you tend to bin the images in both directions to preserve a "square" aspect ratio.
The one final advantage of binning is that it increases the overall image throughput, reducing
the digitize and download times due to the reduced number of pixels involved.

The camera software allows you to select a High resolution 7.4 micron square mode

where the pixels are unbinned, a Medium resolution mode resulting in 14.8 micron square
pixels and a Low resolution mode with 22.2 micron pixels. The latter is useful for fast
acquisition of faint objects.

1.3.9. Spectral Response

Like film, CCDs have a varying response to differing wavelengths. The basic fabrication
techniques used in manufacturing the CCDs greatly affect their spectral response. At the
extreme Red end of the spectrum, the CCDs loose their sensitivity because the photons simply
do not have enough energy to generate electrons in the CCD wells. At the Blue end of the
spectrum, the photons do not penetrate deep enough into the CCDs to get into the wells and
are stopped by the top layers of the CCD. Between the Red and the Blue, interference effects in
the top layers of the CCD can also cause peaks and valleys in the response.

This affects you in several ways. The most obvious is the overall effect that causes you

to take longer exposures with CCDs for various colors. For example, when taking color
images, your Blue exposure is typically several times longer than the Red exposure to give an
image with similar quality or Signal/Noise. One last interesting note about CCD's spectral
response is that they have much more response in the near infrared than film.

The TC237 CCD used in your camera is made using Texas Instrument's Virtual Phase

technology that gives excellent Blue response compared to other CCDs. This is achieved by
reducing the number of photon-absorbing clocking gates in the CCD.

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Section 1 - Introduction to CCD Cameras

Figure 1.2 - CCD Quantum Efficiency

1.4. Camera Hardware Architecture

This section describes the ST-237 Advanced CCD Camera from a systems standpoint. It
describes the elements that comprise a CCD camera and the functions they provide. Please
refer to the figure below as you read through this section.

CCD

Shutter/

Filter Wheel

Micro-

controller

Gate

Array

CPU

Clock

Drivers

Postamp/

A/D

Converter

Host Computer

Optical

Head

Preamp

TE Cooler

Figure 1.3 - CCD System Block Diagram

The ST-237 camera is a two piece system consisting of an Optical Head and a CPU. The Optical
Head houses the CCD and Preamplifier and the CPU contains the Readout and Control

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Section 1 - Introduction to CCD Cameras

Electronics. The CPU is powered by an external 12 VDC wall transformer and can be easily run
off a 12 volt battery in the field. Finally, the CPU interfaces to the PC through the Parallel Port
and is controlled by the software.

Examining the Optical Head in more detail shows the "front end" of any CCD camera

which is the CCD sensor itself. In this case the CCD is the Texas Instruments TC237. The CCD
is cooled by mounting it on a thermoelectric (TE) cooler. The TE cooler pumps heat out of the
CCD and dissipates it into the camera body where it is removed by a small fan in the rear of the
Optical Head.

Since the CCD is cooled below 0°C (32 °F), some provision must be made to prevent

frost from forming on the CCD. The camera has the CCD/TE Cooler mounted in a windowed
hermetic chamber sealed with an O-Ring. The hermetic chamber does not need to be a
vacuum, and contains a desiccant packet to absorb the small amount of moisture that might
condense.

Other elements contained in the optical head include the preamplifier and a rotating

shutter wheel. The shutter wheel makes taking dark frames a simple matter of pushing a
button on the computer. Remember that the shutter wheel 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 is based upon the electronic clocking scheme applied to the CCD. Also, the
shutter wheel can be replaced by a Color Filter Wheel which allows taking Tricolor images and
dark shuttering.

The Microcontroller acts as the "brains" of the camera electronics. It is responsible for

starting and stopping exposures in the CCD, clearing the CCD at the start of the exposure and
transferring the image into the CCD's storage area at the end of the exposure. The
microcontroller also regulates the CCD's temperature by monitoring the temperature sensor
and adjusting the power to the TE cooler. Finally, the microcontroller provides control of the
Shutter/Filter Wheel and the Telescope Port that is used when Autoguiding.

The final element of the camera system is the host computer and operating software.

The CCDOPS software runs under MS-DOS and is used to acquire, display and process images
from the camera. As you will learn, CCDOPS is a powerful package with a user-friendly
interface. It is unequaled in the industry. Also, third party software packages such as Software
Bisque's SkyPro for Windows will support the ST-237 Advanced CCD Camera and open up a
whole realm of capabilities including integrated Telescope and Camera control.

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Section 2 - The First Day with the Camera

2. The First Day with the Camera

This section takes you step-by-step through your first use of the software and camera.

2.1. Setting up the System

This section tells you how to install the CCDOPS software and establish a communications link
with the camera.

2.1.1 Installing the CCDOPS Software

The CCDOPS software is provided on floppy diskette, and should be copied to your system's
hard disk prior to use. SBIG's CCDOPS software which supports all cameras except the
original ST-4 is available for Windows, DOS and the Macintosh (Mac users need the optional
Mac/SCSI Adapter). Follow the instructions below to install your software:

Windows - The software comes as a 2 diskette set. Insert the first floppy disk and run

the Setup.exe program to install the software.

DOS, Macintosh - Create a CCDOPS directory on your hard disk and copy the contents

of the floppy disk to your hard drive.

After you have finished installing the software, place the floppy disk in a safe place in case you
need to reinstall it later.

2.1.2. Getting Acquainted with CCDOPS Software

Upon entering CCDOPS a warning (referred to as an Alert) states no camera is hooked up yet.
To proceed, hit any key and you are presented with our user friendly menu based interface
shown in the figure below:

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Section 2 - The First Day with the Camera

* File Camera Display Utility Misc Track Filter

O

pen Alt-O

S

ave Alt-s

Menu

Display

Display mode: Analysis Photo
Auto contrast: Yes No
Background: 100

Menu Bar

[ Enter ]

[ Esc ]

Dialog

Status

Welcome to the CCDOPS Software

Status Box

Data Buffer

Name:M27.237

Camera

Link:
Res:

The menu bar starts with an * at the top left and extends horizontally to the Filter menu item on
the right. Navigating between these menus can be accomplished with a mouse or left/right
cursor arrows on your keyboard. Go ahead now and move between these items.

The File menu is the most frequently used since it navigates you toward image retrieval

(Open command) and the image Save command. It also is the way to the off switch, via the
Exit command. To pull down the File menu, highlight it and simply hit Enter or click on it
when highlighted. To leave it, hit Esc key or click on another menu.

Camera Status

Figure 2.1 - The CCDOPS User Interface

Camera Menu

Move to the right and Camera menu is highlighted. Pull it down to see the 3 sections making
7 commands. None of these function yet since a camera is not hooked up or powered. Notice
that you are alerted to this fact by the software when you hit Enter on a highlighted command.

Display Menu

Moving right again to the Display menu, you see 2 sections of 5 items. You can use your
up/down cursors or mouse to navigate between these. Similarly, when selected, error messages
come up in all but the Slide Show command. Select it. Now you are prompted by a new
window (referred to as a Dialog) to select a slide show script. Move the highlighted box to the
DEMO.SLD script which we created for you and hit Enter. Now you are presented with more
choices. Temporarily ignore these and hit Enter again. When in doubt, just hit Enter to use the

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Section 2 - The First Day with the Camera

standard defaults. You are now viewing a library of images created with the TC-237 chip in
your camera. These will run until you hit Esc.

You can adjust your monitor's brightness or contrast for best view. If you find an image

you wish to freeze, hit the Space bar. Hit the R key to resume slide show or hit any other key
to fast forward through the images.

File Menu

To call up specific images stored in image files, Esc the Slide Show command. In order to
display an image, it is necessary to first select its image file. Move the cursor to the File menu,
hit Enter, select the Open command and again hit Enter. Now you see the File Open dialog
presenting you with many choices. First are file directories shown in <>. These are created by
the software and later by you to organize and store images. The current directory you are in is
noted at the top center of list of files. Move the highlighted cursor to the MOON.237 file and
click on it twice in short succession (double-click) or hit Enter.

You are first shown with the image header data pertaining to that image. After viewing

the data, hit Enter. Highlight the Display menu and hit Enter to bring it down. Execute the
Image command and the software brings up the Display Settings dialog. Use the mouse or the
cursor keys to select the Photo display mode and then hit Enter one last time and your selected
image is now displayed.

If you have a VESA based SVGA display adapter, depress the up/down arrows to

adjust the image brightness and right/left arrows to adjust its contrast. This feature allows real
time display optimizing. If you are not getting a satisfactory image display, refer to the
Graphics Setup command in Section 5, the Software Reference section for help. Note that
images will almost always look better on a video monitor than from a laptop LCD. If you wish
to increase image size, again refer to Graphics Setup command in Section 5 and change the
graphics card to VGA or MCGA. Your selected image will still be held in the image buffer,
ready for redisplay when selected. When you're done with looking at the image hit Esc to
return to main menu.

Utility Menu

The next menu to the right, Utility, allows you to modify an image. Pull down the Utility
menu. These commands can be used without permanently modifying the selected image until
you save it. Move back to File menu.

Open and display the Jupiter image using the File Open and Display Image commands

and study the image briefly. Esc back to the main menu. Under the Misc menu use the
Graphics Setup command and make sure the graphics card is set to Auto. Press Enter and then
redisplay the image. Adjust the image with arrow keys for the best detail. Hit Esc and move to
Utility menu and execute the Sharpen command. In the Sharpen dialog select
Lunar/Planetary and Medium settings. Hit Enter. A sharpened image of Jupiter comes up
and notice how the red spot (light oval) now shows on Jupiter's right side by one of its moons.
Also, cloud band detail has been enhanced. At this point, you could save this image, but be
cautious and make sure you have read the manual and understand what the save function can
do to the original unsharpened Jupiter image. You are only a few Enter keystrokes away from
modifying your original data permanently unless you heed the warning box displayed while

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Section 2 - The First Day with the Camera

executing the Save command. No problem, just hit Esc or be sure to rename the modified
Jupiter file before saving it so as not to modify the original file.

Display the sharpened Jupiter image again using the Display Image command and hit

Esc. We will go to the Utility menu and do something else to this image. Select the Depixelize
Image command and hit Enter. You have magnified the number of pixels in this image four­fold. Be aware that this would increase the size of the image on disk four-fold as well if you
saved it. Notice that even more detail becomes visible. Vary the contrast and brightness with
cursor arrow keys for the best view.

You have just successfully retrieved an image and optimized it for viewing. Since this is

any easily repeated example, let's get rid of the modified image by hitting Esc and moving to
Misc menu on the main menu bar.

Misc Menu

The Misc menu contains one section with 3 commands. You previously used this to verify the
graphics setup. These commands are covered in detail in Section 5 if you want to refer to it,
otherwise let's move right over to the Track Menu.

Track Menu

In the Track menu you see 3 sections containing 6 commands. Except for the Track and
Accumulate command, these all pertain to using your CCD camera as a separate auto guider
for astrophotography with accessories such as a Radial Guider, guide scope or to guide a
separate piggyback camera from your main scope. It is for advanced users with appropriate
equatorial tracking mounting systems.

Filter Menu

The last menu on the top right is the Filter menu. The commands in this menu are used for
controlling the Color Filter Wheel accessory and are discussed in Section 5 and in the manual
that comes with the Color Filter Wheel.

This basic preview should make you comfortable with the ease of moving around in

CCDOPS and what it entails. You can exit through the File menu now. Notice how a warning
box still alerts you that you are leaving an unsaved image that will be lost when you exit.
Specify "Quit:Yes" and hit Enter to leave CCDOPS and exit back to a DOS command. You can
now shut down your computer or type in WIN and hit Enter to go back into your Windows
software.

2.1.3. Connecting the Camera to the Computer

Place the camera's CPU midway between the computer and the telescope or some other
convenient place. With the power to the CPU disconnected, plug the Optical Head into the
CPU at the marked location and plug the beige parallel cable provided with the system into the
CPU. Connect the other end of the parallel cable into your computer's Parallel port (printer
port). Finally plug the power supply into the CPU and turn on the power switch. The CPU's
power switch should light up and the small fan on the back of the optical head will begin to
spin.

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Section 2 - The First Day with the Camera

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.

2.1.4. Establishing a Communications Link with the Camera

Run the CCDOPS software and when it starts up, it will automatically attempt to establish a
link to the camera. If the camera installation is successful, the "Link" field in the Status Window
is updated to show the Link status. If the camera is not connected or the LPT port setting has
not yet been properly set, a message will be displayed indicating that the software failed to
establish a link. If this happens, use the Communications Setup command in the Misc menu to
configure the CCDOPS software for the parallel port you are using. Then use the Establish
COM Link command in the Camera menu to re-link.

2.1.5. Operating your Camera with CCDOPS - a Daytime Orientation

With the Camera menu highlighted, select the Setup command. Notice the choices for
temperature regulation, etc. Ignore these for the time being and hit Esc to get rid of the dialog.
Again select the Camera menu and then execute the Grab command. Note the exposure time,
etc. Verify that the exposure time is set to :1.00 second and other settings are Dark frame:Only,
Auto display:Analysis, Exposure delay:0 and Special processing:None. With the CCD camera
nosepiece uncovered, hit Enter. A sequence of events occurs and you will notice a spotted
image. This represents a 1 second exposure dark frame at your room temperature with the
CCD chip still covered by the internal mechanical shutter wheel to keep out light. At the upper
left on your current screen is a menu named Display. Under the image is a box with
information pertaining to this image. Of interest is Back (background) and Range. These are
pixel values that can theoretically go from a low of 0 to a high of 4,095. These will register
numbers in the low thousand range. Using your mouse or Enter key, pull down the Display
menu. Notice 8 choices. Select the X-Hairs command.

You are presented with a lot of data pertaining to the small crosshair cursor (+) now

located in your dark frame image. The cross placement is now in your control and is used to
obtain pixel information. Move it around with the arrow cursors or rapidly by mouse point
and click. Notice 9 lines of data in the X-Hair box that are updated with each pixel position
movement. A convenient zoomed box located below and left of the entire main image
magnifies the immediate area surrounding the crosshair cursor. The very center of this zoomed
box represents the cursor position. Move the crosshair toward the top of the frame. Now
slowly move it, an arrow stroke at a time, to find a dark pixel (low value). A reading of 400 to
1400 is typical. Now move it to the bottom of the image and note the brightest pixel value. It
might be in the range 1200-1300. A single very hot pixel could give a value of 3000. Hitting
Enter again brings up more advanced crosshair commands. Hit Esc twice to view just the
image. This is what you will typically view after you capture an image.

Next, Esc back to the main menu and hit the Grab command again. Select Dark

frame:None. With the camera nosepiece uncovered, hit Enter to begin another exposure. This
time you are taking a light frame. Notice how smooth the image looks. Repeat the above steps
used in evaluating pixels in the dark frame. Notice that they all saturated with 4,095 counts.
This is the maximum amount of light entering as photons and converted to electrons that the
CCD can read.

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Section 2 - The First Day with the Camera

Next, cover the nosepiece of the camera and use the Grab command again, only this

time select Dark frame:Also and hit Enter. This image has a much lower background and
range, and represents a dark subtracted image. Still, pixel intensity variations fall between 50
and 250. Some individual hot and cold pixels may show at 30 to 900 counts, but their Average
Value is 100 or so. This "Avg" value (displayed in the crosshairs data) represents the low end
intensity of your pixel image data, while 4,095 represents the maximum intensity value. All
your CCD image data is represented as shades of grey between 100 and 4,095 counts. This is a
very large range, far beyond the photographic process capability.

Let's Esc to the main menu and use the Setup command in the Camera menu. We will

cool the camera to 0 degrees C. with a new Setpoint setting of 0. Also, set Temperature
regulation to Active. Nothing happens yet until you hit enter. Notice the lower corner of the
Camera Status that shows Temperature and that it is rapidly dropping. When it stabilizes at
around 0, you can see the percentage cooler capacity you are using shown in parentheses.
Grab another dark subtracted image(Dark frame:Also) and study the display. First notice the
much more uniform look the pixels show. The benefits of CCD cooling are very evident.
Notice that the average count remains consistently around 100 but that the pixels are much
more closely clustered around 100 counts.

3

If your room temperature is about 70 degrees F., try further cooling to -8 degrees C.

Nearly 100% cooler capacity may be indicated. For good astroimages, don't run above 90%.
Grab an image once the temperature stabilizes in the -8 degree range. Notice that the image is
even more uniform appearing now. The average pixel count is still around 100, but "hot" pixels
have greatly diminished except for an errant few that may always read high. No amount of
cooling can help these. They are inherent in the manufacture of most CCDs and typical.

Summary

Even at a 1 second exposure, the value of cooling is demonstrated to be important as well as the
dark taking and subtraction procedure. These steps are absolutely necessary to get the most
from your CCD camera. If you were at the telescope, all that would be left to do is to save this
image. Let's do that now but first we will need to use the Create Directory command in the File
menu to make a new directory for our images. Execute the command and when it asks you for
a directory name type CCD.4 Hit Enter to create the directory.

Finally, get back to the main menu and use the Save command in the File menu. Type a

name for the image (that follows standard DOS naming convention) with 8 legal characters,
e.g., the first eight letters of your name and hit Enter (don't put spaces in the file name). Then
use the Open command in the File menu and look for your name there. Highlight it and hit
Enter to open it. Notice the displayed header data has the file name you gave it, the date and
time setting of your computer as well as exposure time and temperature, etc. Some other
parameters were not yet set by you and may be incorrect. See Section 5 and the Telescope Setup
and Edit Parameters commands for more on this. This data will always be connected with the

3

You might think the pixel values would be clustered around 0 counts but instead they are clustered
around 100. That's because the CCDOPS software adds 100 counts to the dark subtracted image to
stop pixels from going below zero when the two images are subtracted. Just remember that 100
counts represents zero signal.

4

As a convenience the software makes up directory names for you by adding the current month and
day to DATA. You can use this name or type in your own name.

Page 18