as compared with a large telescope mirror, a large telescope lens?
Wednesday, May 5th, 2010 at
11:54 pm
a. does not disperse light
b. has no advantages
c. can provide higher magnification
d. is easier to support against sag.
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c
to get more magnification, the curvature must be greater. it is difficult to get a perfectly spherically ground surface on a large mirror.
smaller lenses can be ground more spherically, and can be more spherical.
Most modern telescopes use large mirrors to gather light, and small lenses to focus and magnify the image.
If you assume the same aperture mirror and lens, then I would make the following comments…
The mirror would be cheaper because there is only one optical surface to figure and polish. The mirror can be better supported in a purposely designed cell. However care must be taken to ensure that the mirror is losely but securely held otherwise it could cause the shape to become distorted.
Also any reflecting telescope will have a secondary obstruction in the light path which means the very centre of the main mirror will not receive any light. Refractors have a clear light path and therefore provide theoretical diffraction limited performance (meaning the best performance for their aperture).
In the past mirrors have been favoured over lenses because of cost and the absence of chromatic abberation, but in modern optics there really is very little to choose between them.