Piggybacking

If you have a telescope that tracks the stars (whether on an equatorial wedge or not), mount the camera "piggyback" on it and you can take longer exposures of the sky. For now, don't worry about autoguiding; just set the telescope up carefully and let it track as best it can. Expose no more than 5 minutes or so. As in the previous project, long-exposure noise reduction should be turned on if available. Figure 4.7 shows what you can achieve.

With an equatorial mount (or a fork mount on an equatorial wedge), you can theoretically expose as long as you want, but both hot pixels and tracking errors start catching up with you if the exposure exceeds 5 minutes. If the telescope is on an altazimuth mount (one that goes up-down and left-right, with no wedge), then you have to deal with field rotation (Figure 9.2, p. 101). As a rule of thumb, this won't be a problem if you keep the exposure time under 30 seconds in most parts of the sky. You can expose up to 2 minutes if you're tracking objects fairly low in the east or west.

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