How Much Power?The theoretical limit of useful magnification (power) for a telescope is 50 or 60 times the telescope's diameter (aperture) in inches, or two times the aperture in millimeters. So, for a 60mm refractor, the maximum useful power is 120X (despite what some manufacturers claim!). Higher magnification is pointless, because the image seen in the eyepiece will just be dim and fuzzy. Powers less than the theoretical limit are usually preferred, because sky conditions are rarely ideal. Also, many deep-sky objects need low magnification because of their low surface brightness. For larger telescopes, the highest useful power is about 300X in typical atmospheric conditions. You vary the magnification by using eyepieces of different focal lengths. To calculate the magnification, divide the telescope focal length by the eyepiece focal length, in millimeters. Telescope Focal Length =
Magnification For example, a 40mm eyepiece used with a Celestron 8" SCT (focal length = 2000mm) produces 50X magnification: 2000mm ÷ 40mm = 50X. Replace the 40mm with a 15mm eyepiece and you get 133X. Powers of 10X to 15X per inch of aperture are used most frequently.
Doubling the power gives you one-fourth the image brightness and reduces the sharpness by
one half.
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