The 500-Rule states that to obtain a clear image of stars without trails, take the number 500 and divided it by the focal length to get your exposure time. For example, a 20 mm lens would call for an exposure of about 25 seconds and theoretically, still obtain the stars without trails.
Generally expressed, focal length is the distance (given in millimeters) between the telescope’s primary lens or mirror and the point where the light rays come together in focus. Why is this important? Focal length is the major determining factor of any given telescope’s magnifying power.
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