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S.B.I.G. Model ST-237" CCD Imaging Camera System

This model has been discontinued. It remains posted here for information purposes only.

Originally manufactured by SBIG and distributed by Celestron International as the "Pixcel 237" system, this system is now offered only as the "SBIG Model ST-237".

ST-237 CCD Camera with optional Personal Computer
Illustration at Right: "ST-237" Camera Head and CPU with an optional personal portable computer.

The California based "Santa Barbara Instruments Group" (SBIG) company has been a pioneer in developing CCD imaging camera technology for the university, research, and for the amateur astronomy communities. They are known for delivering on their promises, and for treating customers and distributors with an unusually high degree of dignity (for the industry).

SBIG has continuously set new standards in this field such as with their introduction of the unprecedented Model ST-4 system. The ST-4 is not only a CCD camera, but also the first working affordable auto guider for astrophotography; indeed it is used on a 9" f15 refractor telescope to guide the 48" astrographic telescope at Mt. Palomar! The ST-4 continues to dominate its niche as the best stand alone autoguider, and most proven CCD imaging performer of its class. This was followed by numerous innovative systems including the original model ST-6 camera. The ST-6 introduced a unique concept (Patented) for astronomical imaging "Track and Accumulate". The ST-6 provided incredible, here to fore unrealized imaging capabilities at an price attainable by institutions, and the demanding amateur. A magazine review of the ST-6 camera suggested that affordable imaging had finally arrived. This was followed by the introduction of Tricolor CCD imaging (to this day in the undisputed lead), then by the only dual chip cameras (capable of full time imaging and guiding simultaneously) the ST-7 and ST-8 systems. In 1997 their pioneering continued with the unrivaled Adaptive Optics Technology for use with their Patented dual chip technology cameras. SBIG alliances include those companies with the most advanced software writers include the "Software Bisque" company.

In 1996 Celestron and SBIG announced their first joint effort: the "Pixcel 255" CCD Imaging Camera system. For 1998 Celestron and the California based "Santa Barbara Instruments Group" (SBIG) company announced another evolutionary joint effort: the "Pixcel 237" CCD Imaging Camera system. This camera was conceived and then put together with the novice in mind - from the ease of set up and operation, the average beginner can be set up and taking their first image in less than 30 minutes! Now the camera is distributed as the SBIG Model ST-237. It continues to be cmpatible with the innovative Celestron "Fastar" telescopes.

The ST-237 camera is also designed for the experienced CCD user who seeks a broad spectral response and high resolution well suited to imaging the Moon, planets, Sun, and deep sky wonders. With SBIG developed features such as the proven and very capable control and imaging software (which includes the "Track and Accumulate" capability), a fast Parallel port for communication with a controlling PC compatible computer, and the ability to employ the ST-237 as a precision autoguider this system establishes a new benchmark of price and performance. Many capable third parties offer accessories and software support of the industry standard SBIG cameras; these are available for the ST-237 too! The primary improvements over the prior Model (marketed as the Celestron Pixcel 255) are the fact that the v has a detector area 2.17X larger than the 255, and its pixels are 55% smaller (by area) thereby permitting higher resolution imaging of fine detail.

The CCD ("charge coupled device") cameras employ an array made up of thousands of photo sensitive pixels. The array is mounted into a "camera head" which is put in place of an eyepiece on a telescope, or in place of a film camera onto a camera lens. The light striking a pixel is registered in terms of intensity and shade of gray; the first common CCD cameras were 8-bit for 8 shades of gray per pixel, the better cameras of today are 12, 14 or 16-bit. The data is downloaded through an analog to digital converter which in better cameras handles data in a 16 bit stream, and then passed through control circuits in the CCD camera Central Processing Unit (CPU) and out through a cable to either a Serial, Parallel, or SCSI port of a personal computer. For every pixel on the chip common astronomical CCD cameras output a file that can have 8 or up to 16 bits of information. For the Pixcel 237 with 307,200 pixels and 12 bit range this amounts to 3.686 Megabits of data; and so with 8 bits per byte, and 8 bytes per kbyte, this produces a file of 36 kbytes before compression.

The personal computer runs the operating or control software for the CCD camera, and the software may also provide an ability to manipulate an image to better reveal the desired features, or otherwise arrange the image to be more pleasing and or useful to the end user. Because of their ability to "integrate" an image over time (this length of time depends on the camera and environmental conditions) the CCD cameras are the most practical means to image bright objects such as planets or the moon, or very faint extended objects such as galaxies, even from the relatively light polluted regions. Since the imaging can be done and manipulated on a PC, then there is no additional cost or delay in waiting for results from a dark room as there is with film. Furthermore, several images taken through color filters can be combined (registered) and color balanced to reveal much information about the object, or to render film-like aesthetically pleasing images.

The advantages of film remain:

  • a CCD array is relatively small in area compared to that of even a 35mm camera film, and so to image large objects one must employ very short focal length or restrict imaging to smaller objects.

  • a color or black and white print can be made from a personal computer, however the resolution of the CCD array (even if it were of comparable size to a film) is not as good as film. So when making relatively large prints those made from film can be enlarged to produce a larger image.

  • large CCD arrays (such as those presently offered on the Model ST-8, and larger systems) produce images that require more and more personal computer hard disk space as the image sizes can become very large, and

  • larger CCD images require more capable personal computers; typically at least 64 megabytes of Random Access Memory (RAM) and at least a fast "Pentium" or "Power PC" processing power to integrate and manipulate.

The ST-237 is a multi-purpose instrument which features a Texas Instruments made TC-237 detector; this features excellent quantum efficiency and a broad spectral response. The TC-237 is mounted onto a pedestal on top of a thermoelectric cooler within the camera head housing. The head also houses an Analog to Digital converter, cooling electronics and fan, and other readout electronics. With its 12 bit, 22 Khz Pixel Rate A/D converter, and double correlated sampling the images provided by the ST-237 cameras are of photographic quality revealing very good detail and dynamic range (shades of gray, or colors when used with an optional filter wheel/magazine).

The ST-237 camera even when operating with the optional "drop in" color filter wheel magazine is compatible with systems as fast as f2. It will reveal spiral structure in hundreds of galaxies with 1 to 5 minute exposures at the prime focus of an 8" f10 telescope. With such a telescope, stellar photometry down to 18th magnitude is attainable. Planetary Nebulae, Comets, asteroids, become easy targets. Many of these images can be obtained from the comfort of a suburban back yard.

This system permits:

  • 1. When used with a personal computer (MS-DOS or Windows) the camera can function as an integrating, cooled CCD camera to capture monochromatic images of very faint objects. It was carefully designed for easy astronomical and scientific imaging.

Illustration 1. Relative Size of Common Astronomical Imaging CCD's

    To predict actual field of view in degrees (horizontal or vertical) when using the ST-237 CCD system
    with a telescope or a camera lens, then employ the following formula:

      Horizontal: 57.3/Focal Length of Objective (mm) * 4.7

      Vertical: 57.3/Focal Length of Objective (mm) * 3.6

  • 2. When used with the optional Color Filter Wheel Magazine (that easily slips into the camera head) then the camera can function
    to produce tricolor images; these can then be combined (indexed and balanced) to produce color images.

  • 3. When the camera (controlled by a personal computer) is installed on a photoguide telescope,
    or at the guide port of an off axis guiding device can detect any deviations in tracking of a telescope
    (in both axes), and then send correction signals via a relay cable to a telescope drive corrector controller;
    all of this is done with a better frequency, higher accuracy and over a longer period of time than most
    humans can perform. All of this resulting in professional quality astrophotographs limited only
    by the quality of the telescope, and atmosphere.


The camera head is furnished with a female "T-thread" which accepts a variety of adapters to attach the head onto a telescope, microscope, or other system. Furnished with each head are a male nose piece of 1.25 inch diameter (threaded to accept filters). A quiet, no vibration fan vents at the rear of the head to assist with the cooling functions. The camera head incorporate a solenoid vane shutter to facilitate capturing dark/bias frames.

The camera incorporates a thermoelectric cooler. The operating temperature range is user selectable (via the controlling computer keyboard) and it can be regulated to 0.5 degree precision. A regulating thermistor stabilizes the temperature thereby allowing exposure times of up to one hour with very low dark current.

The compact camera head (3.25" x 3" deep with a weight of less than two pounds) has a cable attached that connects into the camera CPU. The CCD detector is carefully mounted into the head with a precession of parallelism to its mechanical axis of 0.001 inches! This cable is attached at the side of the camera head to eliminate any risk of binding the cable against the base of fork mounted systems when imaging near Zenith. The control signals of an optional Color Filter Wheel are transmitted from the CPU to the camera through this cable.

The compact external CPU (approximately 8" x 10" x 2") can be placed up to 15 feet from the head. A PC notebook computer can be placed on top of this CPU. The CPU has an "On-Off" switch", a connector for Power in, and another for Relay outputs (to control a telescope drive corrector). A cable to connect the "TIC" (telescope interface cable) CPU relay control to most common telescope drive correctors is furnished, as is a CPU to PC fast parallel port cable. The A.C. power supply is also furnished; this plugs into 115V and provides 12 V.D.C.. It is possible to operate the camera from a 12 V.D.C. power source in the field too.


The ST-237 requires a personal computer for either guiding or imaging functions. First the furnished Windows compatible operating program is initiated. Optional third party programs that we offer including "CCDSoft" by Software Bisque will also operate the camera functions. These programs automatically establish communications to the camera CPU through the computer's serial port at the highest baud rate possible. For cable lengths of up to 15 feet the speed of communications will be at 115.2 Kbaud.


The camera head CCD detector consists of array elements, called pixels. The smaller array of pixels, arranged in horizontal ("X" axis) and vertical ("Y" axis) rows, convert photons (the light falling on the detector from a star) in to electrons. When a guide star's image is present at a specific pixel the micro controller will note the increased signal intensity from that pixel relative to the others. An indication of the intensity of the signal and the location (X and Y coordinates) of the star on the detector are read on the control software display of a personal computer.

FOCUS:We suggest a parfocal eyepiece (such as the "IFocus" ocular by Software Bisque) be employed to visually find (or use digital or mechanical setting circles), center and focus (or nearly focus) the CCD onto the area to be imaged. Then the eyepiece is removed and replaced by the CCD head. Now select to enter the "Focus" mode. Set the mode to either "Planet", "Dim" or "Full Frame" mode and center the object using the telescope's hand control or from the computer keyboard. As the telescope focuser is adjusted to improve focus, more light from the star will be focused onto a smaller area of the CCD, eventually possibly on to only one pixel thereby increasing the intensity which is indicated on the display as a higher value; this is an aid to obtaining precise focus of the star.

In this mode, the camera will continue to take short exposures and the display screen will automatically refresh and the values adjust accordingly. Once the system is focused onto the star field, the user can select to view the entire field of view of the CCD and then select any particular star to be that which the CCD will use as a reference for guiding.

The CCD camera interface to the telescope consists of a cable from the normally closed/open connector of the camera CPU to the telescope drive corrector or hand control. The camera will produce drive adjustments usually commanded by the telescope drive corrector push button switches. A manual override on the computer keyboard allows the corrections to be made by the user.

By making fine adjustments to the telescope via the computer keyboard, a guide star can be moved onto the detector, or even onto any particular pixel. However, a guide star does not have to be centered onto the CCD detector for the system to function properly. The user can select the length of the star tracker exposure times from 0.01 or more so stars ranging over 10 magnitudes in brightness can be tracked without adding optional filters. The area of pixels to be used (guide box) can also be adjusted to accommodate very long focal length guide telescopes, or mediocre atmospheric seeing conditions.


In operation, the CCD camera can be used as an automatic star guider full time as it will detect a star, and generate the signals which then drive the relays thereby adjusting the telescope tracking. The camera also has a "Track and Accumulate" (TRACCUM) feature pioneered (and then Patented) by SBIG. TRACCUM allows guiding of long integrated images from as short as 10 seconds to 1 hour. The ST-237 will take an exposure, then determine the position of a preselected star, add the image to the image sum building an internal memory buffer, correct the telescope's position and then star the cycle all over again. In this mode, up to 64 images can be co-added. The resulting exposure is almost as good as one long exposure re, depending on the exposure time selected and the actual sky conditions. In another mode, the image is shifted to correct the errors, and added to the image buffer; in this mode the telescope need not be adjusted. With the very good sensitivity of this camera across the spectrum, this virtually assures a selectable guide star will be found within the field of view.

AUTOMATIC DRIVE CALIBRATION: Once an acceptable star is found, and when focus is achieved, the operator will select "Calibrate Drive" function so the CCD system will "learn" the characteristics of the telescope drive. The software sequentially activates relays via the camera head output which send a user selectable 1 to 10 second signal to the telescope drive corrector to move the telescope North-South and back, then East and West and back; the telescope will end up returned to the point at which it started. This function "teaches" the CCD system what commands will cause the telescope to move in what direction, and how fast. This calibration feature is invaluable when using a German equatorial mount, or an off axis guider, where the proper direction can be very difficult to guess.

THE TRACK MODE: After calibration, then the operator will select "Track"; any drifting motion of a star across the CCD causes it to appear at a different pixel at each following exposure. The computer controlling the CCD will then calculate how far the star has drifted and generates a control signal, transmitted through the relays to correct the position. The control signal, and its duration are a function of the star's position error. The camera system can take an exposure (integration), read out all the pixel values, and then calculate and transmit the necessary telescope correction in less than one second.

In the "Track" mode the camera acquires a fresh star image, centers the image on a pixel, and holds that star in position by constantly monitoring it - sending correction signals to the telescope drive immediately after the exposure. The CCD is thermoelectrically cooled to enhance its sensitivity to dim stars. This extreme sensitivity enables guide stars as faint as 8th magnitude to be tracked utilizing as small as a 60mm guide telescope. The rapid calculating power of the computer enables the guide star location to be determined within a fraction of a pixel, enabling better than 1 arc second tracking accuracy.


Capture an image of the desired object by entering the GRAB menu, set the exposure time, and then press the Return key. This will then expose the CCD for the selected exposure time, and transfer the captured image to the buffer memory. With the dynamic range of 12 bit A/D converter there is no need to preset the gain or offset levels to optimize the image.

The image can be immediately displayed on the computer monitor. Auto contrast can be selected, with computers background and range values, or the background and range values may be manually entered to bring out specific features of interest.


An image can be substantially improved by subtracting a Dark Frame of equal exposure, which the camera control software allows. Pixels which have a higher dark current value than the average ("hot pixels"), are greatly suppressed and the displayed image appears smoother. The control programs also support the use of flat field calibration frames to correct for vignetting and pixel to pixel variations. A number of display options are also available to smooth the image or enhance the contrast. Multiple frames can be co-added to achieve even higher sensitivity.

Entering the CROSSHAIR MODE enables functions such as stellar and diffuse magnitude measurement, and photometric measurements of stellar positions and angles. The 12 bit accuracy allows very accurate brightness measurements to be made. With appropriate filters, stellar temperature can be measured!

The potential applications of this technology are still being revealed by innovative operators, sometime with software specifically developed for their tasks!

Imaging Resolution: 640 x 480 Pixels, TI 255 CCD
Imaging Pixel Dimensions: 7.4 x 7.4 microns
Imaging Array Dimensions: 4.7mm x 3.6mm
Comparable Eyepiece Field of View: 6mm
Anti-Blooming Yes

Binning Variable/Software Selectable
Binning Variability 1x1, 2x2, or 3x3
Full Well Unbinned (in Ke-): CCD 30
Full Well Unbinned (in Ke-): A/D 17w Photons/ADU set at 4.1
Readout Noise (in e-): 15 RMS Double Correlated Sampling
Dark Current <1e-/pixel/sec @-5 degrees C

Camera Head Tripod Block: Yes, 1/4-20 tpi
Desiccant: Yes, internal
Standard Control Software: PC DOS, Windows or Win95
Communications Port: Fast Parallel
Analog/Digital Converter: 12 Bit
Download Time, Full Frame: Less than 12 sec.
Shutter: Electro-Mechanical
Fastest Shutter Speed: 0.01 Sec.
Longest Shutter Speed: 1 Hour
Cooling From Ambient: -35 Degrees C
Power Requirements: 115 V.A.C. Provided (12 v.d.c. possible)


  • Company Seven can provide wiring diagrams of most common production telescopes to illustrate
    to the user how to wire the provided relay cable into the telescope.
  • ST-237 Color Filter Wheel Magazine
  • A variety of nosepieces to fit microscopes or other devices
  • Image Processing Software
  • Parfocal Eyepiece to help to find, center, and focus an object
  • Camera Lens Adapter
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