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CCD Technology: A Brief Discussion

by 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.
      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 (Analog/Digital) 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 the neighboring photosites and are uniform to better than 10% across the entire surface.

Noise Corrections

      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.

      aboutccd1a.gif (26769 bytes)
      aboutccd1b.gif (25675 bytes)
      M27 Planetary Nebula. The top image is a
      narrow band 3600 second CCD image taken
      with a Model ST-7 CCD Imaging Camera
      through a Celestron C-8 with a Ha Filter The
      bottom image is a 1200 second ST-7 CCD
      image taken through a 7" f/7 Astrophysics
      refractor Both images utilized the self-guiding

      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.

      aboutccd2.gif (24226 bytes)
      M42. 1200 second Model ST-7 CCD image
      taken through a 7", f/7 Astrophysics refractor
      utilizing the self-guiding mode.


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 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.


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:

    What type of telescope do you presently own? Having this information allows us to match the CCD imaging Camera's parameters including pixel size and desired fields of view to your telescope. We can also help you to interface the CCD imaging camera's automatic guiding functions to your telescope drive corrector.

    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.

    aboutccd3.gif (38353 bytes)
    M16. Tricolor CCD image taken at the prime
    focus of a Celestron C-S with the Model ST 7
    CCD Imaging Camera and the CFW-8 Color
    Filter Wheel.


    Comparative Camera Specifications

    Camera No. Pixels Pixel Size
    Field of View
    (arcminutes at 80")
    Read Noise
    (eŻ RMS)
    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 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.

    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, 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 specificationsand performance. Figure 1 shown above compares the basic characteristics on each CCDimaging camera in our product line. You will find a more detailed set of specifications with each individual imaging camera description at our Intenret site ("").

    It all starts with the software. If there's any company well known for it's outstanding imaging camera software it is the SBIG CCDOPS Operating Software - world renowned. Our CCDOPS software is available for MS-DOS, Macintosh, and Windows applications. The software includes demonstration CCD images of astronomical objects that you can display and analyze using the image processing and analysis functions of the CCDOPS software. By exploring the software you can become thoroughly familiar with how our imaging cameras work and the capabilities of the software before you purchase an imaging camera.


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    NGC891. 1320 second Model ST-7 CCD
    image taken through a Celestron C-14 at
    f/7 utilizing the self-guiding mode.


    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.

    aboutccd5a.gif (31985 bytes)
    aboutccd5b.gif (32947 bytes)
    Jupiter Comet Shoemaker-Levy Impact Areas.
    One second Model ST-4 CCD images
    taken through a 7" f/7 EDT refractor using
    eyepiece projection at f/5.6.

    Customers who receiving their first 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 parallel cable from the imaging camera to your PC or Mac, initiating the imaging camera, and within 30 minutes or so 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, co-adding images, taking automatic sequences of images, photometric and astrometric measurements, etc.

    aboutccd6.gif (21902 bytes)


    At SBIG and Company Seven we have had much success with a program in which we continually review customer'simages sent to us. We can determine the cause of almost any problem from actual images sent in by a user. We review the images and contact each customer personally. Images displaying poor telescope tracking, improper imaging camera focus, 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 and Company Seven 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.

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    Mosaic Image. Three 20 minute overlapping
    exposures of NGC6992 taken with the Model
    ST-8 CCD Imaging Camera in the high
    resolution mode.


    Using an innovative engineering approach SBIG developed an imaging camera function calledTrack & 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 cumulative telescope periodic error.

    aboutccd8.gif (31020 bytes)
    M3. 120 second Model ST-6 CCD image
    taken at the prime focus of a 20", f/12

    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 ofview. 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.


    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.

    aboutccd9.gif (28940 bytes)
    NGC7635. Twelve hundred second Model
    ST-7 CCD image taken at the prime focus of
    a 12.5", f/6 Ritchey-Chretien telescope
    utilizing the self-guiding mode.


    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.

    aboutccd10.gif (2545 bytes)
    Minor Planet 1996 JG. Sixty second Model
    ST-5 CCD image showing the trail of the
    minor planet.

    We furnish Operating Software for you to install on your host computer. Once the softwareis installed and communication with the imaging camera is set up, then complete control of all imaging camera functions is through the host computer keyboard. The recommended minimum requirements for memory and video graphics are as shown below.

    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

    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.


Contents Copyright 1994-1998 Company Seven and S.B.I.G. All Rights Reserved