Raw CCD images are exceptional but not perfect. Due to the digital nature of the data many of the imperfections can be compensated for or calibrated out of the final image through digital image processing.
Composition of a Raw CCD Image.
A raw CCD image consists of the following signal components:
IMAGE SIGNAL - The signal from the source.Electrons are
generated from the
actual source photons.
BIAS SIGNAL - Initial signal already on the CCD before the exposure is taken. This signal is due to biasing the CCD offset slightly above zero A/D counts (ADU).
THERMAL SIGNAL - Signal (Dark Current thermal electrons) due to the thermal activity of the semiconductor. Thermal signal is reduced by cooling of the CCD to low temperature.
Sources of Noise
CCD images are susceptible to the following sources of noise:
PHOTON NOISE - Random fluctuations in the photon signal of the source. The rate at which photons are received is not constant.
THERMAL NOISE - Statistical fluctuations in the generation of Thermal signal. The rate at which electrons are produced in the semiconductor substrate due to thermal effects is not constant.
READOUT NOISE - Errors in reading the signal; generally dominated by the on-chip amplifier.
QUANTIZATION NOISE - Errors introduced in the A/D conversion process.
SENSITIVITY VARIATION - Sensitivity variations from photosite to photosite on the CCD detector or across the detector. Modern CCD's are uniform to better than 1% between neighboring photosites and uniform to better than 10% across the entire surface.
REDUCING NOISE - Readout Noise and Quantization Noise are limited by the construction of the CCD camera and can not be improved upon by the user. Thermal Noise, however, can be reduced by cooling of the CCD (temperature regulation). The Sensitivity Variations can be removed by proper flat fielding.
CORRECTING FOR THE BIAS AND THERMAL SIGNALS - The Bias and Thermal signals can be subtracted out from the Raw Image by taking what is called a Dark Exposure. The dark exposure is a measure of the Bias Signal and Thermal Signal and may simply be subtracted from the Raw Image.
The Final Processed Image
The final Processed Image which removes unwanted signals and reduces noise as best we can is computed as follows:
Final Processed Image = (Raw - Dark)/Flat
All of the digital image processing functions described
above can be
accomplished by using CCDOPS software furnished with each SBIG imaging
camera. The steps
to accomplish them are described in the Operating Manual furnished with
each SBIG imaging
camera. At SBIG we offer our technical support to help you with
questions on how to
improve your images.
HOW TO SELECT THE CORRECT CCD IMAGING CAMERA FOR YOUR TELESCOPE
When new customers contact SBIG we discuss their imaging camera application. We try to get an idea of their interests. We have found this method is an effective way of insuring that our customers get the right imaging camera for their purposes. Some of the questions we ask are as follows:
What type of telescope do you presently own? Having this information allows us to match the CCD imaging Camera's parameters, pixel size and field of view to your telescope. We can also help you interface the CCD imaging camera's automatic guiding functions to your telescope.
Are you a MAC or PC user? Since our software supports both of these platforms we can insure that you receive the correct software. We can also answer questions about any unique functions in one or the other. We can send you a demonstration copy of the appropriate software for your review.
Do you have a telescope drive base with a serial port? Do you want to operate from a remote computer? Companies like Software Bisque fully support our products with telescope control and imaging camera software. We can explain how they work and what they will do for you in your application.
Do you want to take photographic quality images of deep space objects, image planets, or perform wide field searches for near earth asteroids or supernovas? In learning about your interests we can better guide you to the optimum CCD pixel size and imaging area for the application. We can recommend the correct imaging camera and a specific telescope model or a camera lens adapter to support wide field imaging.
Do you want to make photometric measurements of variable stars or determine precise asteroid positions? From this information we can recommend a CCD imaging camera model and explain how to use the specific analysis functions to perform M42. 1200 second Model ST-7 CCD image taken through a 7" f/7Astrophysics refractor utilizing the self-guiding mode these tasks. We can help you characterize your imaging camera by furnishing additional technical data.
Comparative Camera Specifications
|Camera||No. Pixels||Pixel Size
|Field of View
(arcminutes at 80")
|ST-4||192 x 164||13.75 x 16||4.6 x 4.6||8 + 2||150||Stand Alone Autoguider|
|ST-5C||320 x 240||10 x 10||5.6 x 4.2||16||27||Low Cost, Optional Internal Filter|
|ST-237||640 x 480||7.4 x 7.4||8.2 x 6.1||12||15||Low Cost, Optional Internal Filter|
|ST-6B||375 x 241||23 x 27||15 x 11||16||25||Large Pixels, Excellent QE|
|ST-7||765 x 510||9 x 9||12 x 7.9||16||15||Dual CCD Self-Guiding|
|ST-8||1530 x 1020||9 x 9||24 x 16||16||15||Dual CCD Self-Guiding|
|Self Guiding CCD||192 x 162||13.75 x 16||4.6 x 4.6||16||12||Sensitive, High QE|
Do you want to automatically guide long
astrophotographs? As the company with the most experience in CCD
autoguiding we can
help you install and operate a CCD autoguider on your telescope. The
Model ST-4 has a
worldwide reputation for accurate guiding on dim guide stars. No matter
what type of
telescope you own we can help you correctly interface it and get it
COMPARISON OF SBIG CCD IMAGING CAMERAS
The SBIG product line consists of a series of thermoelectrically cooled CCD imaging cameras designed for a wide range of applications ranging from astronomy, tricolor imaging, color photometry, spectroscopy, medical imaging, densitometry, to chemiluminescence and epifluorescence imaging, etc. This catalog includes information on astronomical imaging cameras, scientific imaging cameras, autoguiding, and accessories. We have tried to arrange the catalog so that it is easy to compare products by specifications and performance. Figure 1 shown above compares the basic characteristics on each CCD imaging camera in our product line. You will find a more detailed set of specifications with each individual imaging camera description.
HOW TO GET STARTED USING YOUR CCD IMAGING CAMERA
It all starts with the software. If there's any company well known for it's outstanding imaging camera software it's SBIG. Our CCDOPS Operating Software is world renowned. We send demonstration versions to all prospective customers and urge them to use it and become familiar with the operation of CCD imaging cameras. Our CCDOPS software is available for MS-DOS, Macintosh, and Windows applications. The demonstration software includes CCD images of astronomical objects that you can display and analyze using the image processing and analysis functions of the CCDOPS software. You can become thoroughly familiar with how our imaging cameras work and the capabilities of the software before you purchase an imaging camera.
Every CCD imaging camera that we ship includes CCDOPS Operating Software plus a comprehensive Operating Manual. They are both well organized and easy to use. The software allows you to control all imaging camera functions directly from your computer keyboard. A wide range of image processing functions are included plus photometric and astrometric measurement capability. The Model ST-4 CCD Autoguider comes with a special software package called CCD.
New customers receiving their CCD imaging camera should
first read section 1 in their
CCDOPS Operating Manual. Once you have read that section you should
have no difficulty
installing CCDOPS software on your hard drive, connecting the serial or
from the imaging camera to your PC or Mac, initiating the imaging
camera and within 30
minutes start taking your first CCD images. Many of our customers are
amazed at how easy
it is to start taking images. Additional information can be found by
reading sections 2
and 3 in the CCDOPS Operating Manual. This information allows you to
progress to more
advanced features such as automatic dark frame subtraction of images,
focusing the imaging
camera,viewing, analyzing and processing the images on the monitor,
taking automatic sequences of images, photometric and astrometric
At SBIG we have had much success with a program in
which we continually review customer's
images sent to us on disk or via e-mail. We can determine the cause of
almost any problem
from actual images sent in by a user. We review the images and contact
personally. Images displaying poor telescope tracking, improper imaging
oversaturated images, etc., are typical initial problems. We will help
you quickly learn
how to improve your images. You can be assured of personal technical
support when you need
it. The customer support program has furnished SBIG with a large
collection of remarkable
images. Many customers have had their images published in SBIG
catalogs, ads, and various
astronomy magazines. We welcome the chance to review your images and
hope you will take
advantage of our trained staff to help you improve your images.
Using an innovative engineering approach SBIG developed
an imaging camera function called
Track & Accumulate (TRACCUM) in which multiple images are
automatically registered to
create a single long exposure. Since the long exposure consists of
short images the total
combined exposure significantly improves resolution by reducing the
In the TRACCUM mode each image is shifted to correct guiding errors and added to the image buffer. In this mode the telescope does not need to be adjusted. The great sensitivity of the CCD virtually guarantees that there will be a usable guide star within the field of view. This feature provides dramatic improvement in resolution by reducing the effect of periodic error and allowing unattended hour long exposures. SBIG has been granted U.S. Patent # 5,365,269 for Track & Accumulate.
DUAL CCD SELF-GUIDING
With the introduction of Models ST-7 and ST-8 CCD Imaging Cameras, which incorporate two separate CCD detectors, SBIG was able to accomplish the goal of introducing a truly self-guided CCD imaging camera. The ability to select guide stars with a separate CCD through the full telescope aperture is equivalent to having a thermoelectrically cooled CCD autoguider in your imaging camera.
One CCD is used for guiding and the other for collecting the image. They are mounted in close proximity, both focused at the same plane, allowing the imaging CCD to integrate while the PC uses the guiding CCD to correct the telescope. Using a separate CCD for guiding allows 100% of the primary CCD's active area to be used to collect the image. The telescope correction rate and limiting guide star magnitude can be independently selected. Tests at SBIG indicate that 95% of the time a star bright enough for guiding will be found on the tracking CCD without moving the telescope, using an f/6.3 telescope. Placing both detectors in close proximity at the same focal plane insures the best possible guiding.
The self-guiding function quickly established itself as the easiest and most accurate method for guiding CCD images. Most of the long integrated exposures now being published are taken with this self-guiding method, producing very high resolution images of deep space objects. SBIG has been granted U.S. Patent # 5,525,793 for the dual CCD Self-Guiding function.
SBIG is unique in its support of both PC and Macintosh
platforms with our hardware and
software products. Our software operates under MS-DOS, Windows and
The imaging cameras in this catalog communicate with the host computer
serial or parallel ports depending on the specific models. Since there
are no external
plug-in boards required with our imaging camera systems we encourage
users to operate with
the new family of high resolution graphics laptop computers.
|ST-4||640K||1 MEG||Standard VGA|
|ST-6B||2 MEG||4 MEG||640 x 480, 256 Color or higher|
|ST-7||2 MEG||4 MEG||800 x 600, 256 Color or higher|
|ST-8||5 MEG||8 MEG||800 x 600
1280 x 1024 preferred, 256 Color or higher
|ST-237||2 MEG||4 MEG||640 x 480, 256 Color or higher|
SBIG offers their customers full documentation
describing the serial
protocol (Command Code Structure) to write their own control code
allows users to integrate the imaging camera into their system and
control the functions
from their own source.
September 23, 1998 12:03:49 PM.
Copyright © 1998 Santa Barbara Instrument Group, Inc. All rights reserved.
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