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Section 2 - Introduction to CCD Cameras
Page 12
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. Due to the large size of the
imaging CCD in the ST-1001E, however, a second CCD for tracking cannot be used. For this
model camera we recommend the STV autoguider.
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 ST-7E, ST-8E, ST-9E, ST-10E and ST-1001E 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 ST-1001E. For cameras with
smaller pixels such as the ST-7, ST-8 and ST-10E, 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
observer. In addition to the software provided with the camera, there are a number of
commercial programs available which will process and enhance electronic images. Images
may be made to look sharper, smoother, darker, lighter, etc. Brightness, contrast, size, and
many other aspects of the image may be adjusted in real time while viewing the results on the
computer screen. Two images may be inverted and electronically "blinked" to compare for
differences, such as a new supernova, or a collection of images can be made into a large mosaic.
Advanced techniques such as maximum entropy processing will bring out otherwise hidden
detail.