Guidescope or offaxis guider

If your telescope is a Newtonian, classical Cassegrain, or refractor, you can guide with a separate guidescope piggybacked on the main one (or reverse the roles and photograph through the smaller telescope while guiding with the big one). Telescopes of these kinds are not vulnerable to mirror shift, and tube flexure is not likely to be a problem.

With a Schmidt-Cassegrain or Maksutov-Cassegrain, though, the telescope focuses by moving its main mirror, which may shift further during the exposure. This is less of a problem today than during the film era for two reasons. First, many newer telescopes of these types give you a way to lock the mirror in position and do the final focusing with a separate mechanism. Second, and more importantly, significant movement is not likely to occur during a 3-minute DSLR exposure. Mirror movement was more of a problem back in the days of 30- or 60-minute film exposures.

Autoguider or eyepiece

Figure 9.7. Off-axis guider intercepts a small area near the edge of the image and directs it to an eyepiece or autoguider. Alternative is to use a separate guidescope.

Off-axis guider assembly

Figure 9.7. Off-axis guider intercepts a small area near the edge of the image and directs it to an eyepiece or autoguider. Alternative is to use a separate guidescope.

The traditionally recommended guiding setup for a Schmidt-Cassegrain is an off-axis guider like that shown in Figure 9.7. It intercepts a small part of the image in a position that wouldn't have reached the film or sensor anyway. You view the intercepted image with an eyepiece or feed it to an autoguider.

As you might imagine, it's very tricky to get the guiding eyepiece or autogu-ider in focus at the same time as the camera. There may be adjustments on the off-axis guider to help with this; with mine, I ended up having to insert a short extension tube ahead of the camera. When everything is in focus, there probably won't be a star visible in the guider at the same time that the camera is aimed at your chosen object. For these reasons, I am glad to leave the off-axis guider behind and use a guidescope.

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