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The AT60ED is a nice small refractor. However, its objective lens simply projects an image at the focal plane. Adding a field flattener mimimizes curvature that degrades sharpness at the edges of the image. The flattener requires a specific back-focus, or distance (about 57 mm for the AT60 FF) between its (T2, 42 mm thread) flange and the image plane where ...


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Large surveys don't usually bother to do anything about bright stars saturating the images. It is more trouble than it is worth to deal with, and these kinds of surveys are designed to study much fainter things. The data from bright stars are just unusable. If an astronomer really wants data on a bright star, they can go to a smaller telescope and/or take ...


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