Guiding; Electronic Imaging - Santa Barbara Instrument Group STL-1001E Operating Manual

2.4.6. Guiding

Any time you are taking exposures longer than several seconds, whether you are using a film
camera or a CCD camera, the telescope needs to be guided to prevent streaking. While modern
telescope drives are excellent with PEC or PPEC, they will not produce streak-free images
without adjustment every 30 to 60 seconds. The Research Series cameras allow simultaneous
guiding and imaging, called self-guiding (US Patent 5,525,793). This is possible because of the
unique design employing 2 CCDs. One CCD guides the telescope while the other takes the
image. This resolves the conflicting requirements of short exposures for guiding accuracy and
long exposures for dim objects to be met, something that is impossible with single CCD
cameras. Up to now the user either had to set up a separate guider or use Track and
Accumulate to co-add several shorter images. The dual CCD design allows the guiding CCD
access to the large aperture of the main telescope without the inconvenience of off-axis radial
guiders. Not only are guide stars easily found, but the problems of differential deflection
between guide scope and main scope eliminated.
Track and Accumulate is another SBIG patented process (US #5,365,269) whereby short
exposures are taken and added together with appropriate image shifts to align the images. It is
supported by the camera software, but will generally not produce as good as results as self
guiding, where the corrections are more frequent and the accumulated readout noise less. It is
handy when no connection to the telescope drive is possible and also works best on cameras
with larger pixels like the STL-1001E or STL-1301E or for cameras with smaller pixels in binned
mode. For cameras with smaller pixels imaging in high resolution mode, SBIG is proud to
make self-guiding available to the amateur, making those long exposures required by the small
pixel geometry easy to achieve!
2.5.

Electronic Imaging

Electronic images resemble photographic images in many ways. Photographic images are
made up of many small particles or grains of photo sensitive compounds which change color or
become a darker shade of gray when exposed to light. Electronic images are made up of many
small pixels which are displayed on your computer screen to form an image. Each pixel is
displayed as a shade of gray, or in some cases a color, corresponding to a number which is
produced by the electronics and photo sensitive nature of the CCD camera. However,
electronic images differ from photographic images in several important aspects. In their most
basic form, electronic images are simply groups of numbers arranged in a computer file in a
particular format. This makes electronic images particularly well suited for handling and
manipulation in the same fashion as any other computer file.
An important aspect of electronic imaging is that the results are available immediately.
Once the data from the camera is received by the computer, the resulting image may be
displayed on the screen at once. While Polaroid cameras also produce immediate results,
serious astrophotography ordinarily requires hypersensitized or cooled film, a good quality
camera, and good darkroom work to produce satisfying results. The time lag between
exposure of the film and production of the print is usually measured in days. With electronic
imaging, the time between exposure of the chip and production of the image is usually
measured in seconds.
Another very important aspect of electronic imaging is that the resulting data are
uniquely suited to manipulation by a computer to bring out specific details of interest to the
Section 2 - Introduction to CCD Cameras
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