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CCD Technology: A Brief Discussionby Santa Barabara Instruments Group
Raw CCD images are exceptional but not perfect. Due to the digital nature of the data many of the imperfections which appear in the raw image data 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.
BIAS SIGNAL - Initial signal already on the CCD before the exposure is taken.
THERMAL SIGNAL - Signal (Dark Current thermal electrons) due to the thermal activity of the semiconductor.
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 the neighboring photosites and are 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 of the Thermal Signal, this may simply be subtracted from the Raw Image by commands through the control software provided with the camera.
FLAT FIELDING -A record of the photosite to photosite sensitivity variations can be obtained by taking an exposure of a uniformly lit 'flat field". These variations can then be divided out of the Raw Image to produce an image essentially free from this source of error. Any length exposure will do, but ideally one which saturates the pixels to the 50% or 75% level is best.
The Final Processed Image
Final Processed Image = (Raw - Dark)/Flat
All of the digital image processing functions described above can be accomplished by using the SBIG "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. Company Seven and SBIG 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 us to inquire about entering the CCD imaging arena, we try to discuss their existing equipment, and imaging camera application(s). We have found this method is an effective way of insuring that our customers acqurie the proper imaging camera (and accessories as needed) for their purposes. Some of the questions we ask include:
Are you a Macintosh or PC user? Since we offer software to supports either of these platforms then we can insure that you receive the correct software and cabling. We can also answer questions about any unique functions in one or the other computer platform. Demonstration copies of the appropriate software are available in our showoom, and upon request for your review.
Do you have a telescope drive base with a serial port? Do you want to operate from a remote computer? Companies including Cyanongen support the functions of our SBIG products with telescope control and imaging camera oeprations software, and image processing software. We can explain to you 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? We can 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.
M42. 1200 second ModelST-7 CCD image taken through a 7" f/7Astrophysics refractor the self-guiding mode
Do you want to make photometric measurements of variable stars, or determine precise asteroid positions? We can recommend a CCD imaging camera model and explain how to use the specific analysis functions to perform utilizing these tasks. We can help you to characterize your imaging camera by furnishing additional technical data.
Comparative Camera Specifications
Do you want to automatically guide long uninterrupted astrophotographs? As the company with the most experience in CCD autoguiding SBIG and its first distributor Company Seven 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 working properly.
COMPARISON OF SBIG CCD IMAGING CAMERAS
HOW TO GET STARTED USING YOUR CCD IMAGING CAMERA
Every CCD imaging camera that we ship includes CCDOPS Operating Software plus acomprehensive 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.
A PERSONAL TOUCH FROM SBIG AND COMPANY SEVEN
TRACK AND 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 the world's first 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, while the other for collecting the image. They are mounted inclose 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-Guidingfunction.
SBIG is unique in its support of both PC and Macintosh platforms with our hardware and software products; software is available to operate under either MS-DOS, Windows and Macintosh environments. The imaging cameras in this catalog communicate with the host computer through standardserial or parallel ports depending on the specific models. Since there are no externalplug-in boards required with our imaging camera systems we encourage users to operate withthe new family of high resolution graphics laptop computers.
We offer their customer full documentation describing the serial protocol (Command Code Structure) in order for them to write their own control code functions. This allows users to integrate the imaging camera into their system, and control the functions from their own source.
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