Equipment for Astrophotography
The most important element in astrophotography is accurate tracking. You can spend tens of thousands of dollars on great optics, but 1 min of poor tracking ruins the most pristine view. Several elements can influence tracking with any given mount. First, polar alignment helps tracking, but does not have to be much closer than 15 arcmin from the true pole. This will be treated separately in Chap. 14. Second, proper balancing of the mount helps it to work more efficiently. Most German equatorial mounts (GEM) will track even better with a slight overweighting of the east side of the mount, so that the gears are fully engaged without bouncing. Third, blur from guiding errors increases with magnification. Thus, if your mount tracks poorly, try imaging with either a smaller telescope or piggyback your camera with a shorter focal length camera lens. Most mounts, with rough polar alignment, can track a 200-mm lens for a minute or two without motion blur. Finally, if you image without autoguiding (more on that later), you can tweak your tracking with “periodic error correction” (PEC) that is built into many mounts. Generally, amateur-level GEM track better than fork mounts. Mounts are rated by “periodic error” (PE), which is the amount that a perfectly balanced and polar aligned mount will vary during one cycle of its gears, usually between 5 and 10 min. As you might guess, the more expensive GEM mounts track best. The Paramount ME, Astro-Physics 1200, and Mountain MI 250 all have a PE of about 4 arcsec. A more moderately priced mount like the Losmandy G-11 has a PE of about 9 arcsec. Less-expensive GEMs such as the Atlas are marketed by Orion, and both Meade and Celestron have telescopes sold on GEMs. Generally you get what you pay for. Fork mounts, which come with many Schmidt–Cassegrain telescopes, typically have a higher PE than GEMS, typically between 30 and 60 arcsec. Fork mounts usually require an equatorial wedge for tracking during astrophotography. If you have a fork mount, you may choose to begin with piggyback imaging, but with extra effort a fork mount can be effective for higher resolution imaging with careful autoguiding. If you are considering the purchase of a new mount, ask an owner of the mount about how it performs for imaging. You can get excellent advice on many internet forums.