Download Celestron Compustar Instruction manual

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during the actual imaging sessions. Planetarium software often has provisions for making such
overlays for use with that program.
Lastly, the image of the guide star through the off-axis guider may not be very good. The offaxis guider acquires stars, as the name implies, quite far off the optical axis of the telescope,
where coma from an SCT‟s optics is more apparent. This gives comma shaped stars on which to
guide. To make matters worse, the optical quality of the small pick-off prism in an off-axis
guider is generally not very high. The alignment of the prism may not be perfectly perpendicular
to the optical axis of the telescope. These factors can result in distorted images of guide stars,
which make it difficult to judge its true position during guiding. These problems are further
accentuated when a focal reducer-corrector, like the Celestron f/6.3 reducer-corrector, is inserted
in the optical train ahead of the off-axis guider. Then, guide stars may be subject to a
considerable amount of distortion and a marked drop in brightness due to vignetting of the
reducer, making it very difficult indeed to track a guide star. Obviously, these are not problems
with separate guide telescopes through which the image of the guide star will be of much higher
quality.
Autoguiding
With knowledge of all the technical difficulties with guiding, one can see other reasons why
astrophotographers were so proud of their images. Each represented a triumph over potentially
dozens of technical problems and stood as a testament to the hours spent in the cold, holding
oneself in some uncomfortable position, yet still competently correcting all the guiding errors.
Autoguiding proved to be a revolution in astrophotography. As the name implies, it is a process
by which the mount guides itself during exposures. It liberated astrophotographers from the
grueling hours described above and even permitted them to sleep while their exposures were
being collected. The first commercially available autoguider was produced by the Santa Barbara
Instrument Group (SBIG) of Santa Barbara, California, in 1989 and was officially known as the
Star Tracker 4, but is more commonly known as the ST-4. The ST-4 consists of a small CCD
camera, inserted in place of the guiding eyepiece, and a computer that sends commands via
cables to the mount to make the guiding corrections. The ST-4 was later discontinued and
followed by the ST-V, which has since been replaced by the ST-G. Currently, many other
autoguiding systems are commercially available. Autoguiders can even be built by
astrophotographers, as is the purpose of this article.
Regardless of their origin, all autoguiding systems consist of the same six components. These
components are: 1. A guide scope or an off-axis guider, 2. A guiding camera to detect and
monitor the guide star, 3. A computer, 4. Autoguiding software, 5. A link between the computer
and the mount, 6. A guide port for the telescope mount to receive input from the computer. The
guide scope or off-axis guider was discussed above; the others are discussed below: