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FULL APERTURE VERSUS OFF-AXIS (REDUCED APERTURE)?

April 26, 2017aperture off-axis reduced versus

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FULL APERTURE VERSUS OFF-AXIS (REDUCED APERTURE)?

1 Answer

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10 Posted

For telescopes up to 5″ aperture, off-axis is not an advantage. Larger apertures suffer more from daytime atmospheric turbulence. This turbulence is magnified by the aperture. Perfect daytime “seeing” only occurs about 1% of the time. Off-axis reduces the aperture and increases the focal length thus reducing the turbulent effects. For average use we recommend off-axis for 10″ and larger telescopes. The solar image is not darker with reduced aperture as we allow for this in the coating density. When viewing through the eyepiece, the field of view is not reduced; it looks the same as full aperture. If full aperture is still preferred, it can be stopped down using a simple mask as conditions dictate.