One tried and true method is to just go ahead and try your telescope out as it is being figured. Even before the mirror is coated with aluminum, it reflects somewhere around 5% of the light that hits it back toward the focus. This is enough to examine the image of bright stars. If one is skilled, one can learn a great deal about how good a mirror is by examining the image of stars through a reasonably high powered eyepiece.
Star testing has some advantages over bench testing your telescope:
It also has some distinct disadvantages:
Even so, star testing telescopes is a skill which is worth developing. Probably the best reference on the subject is Dick Suiter's excellent book Star Testing Astronomical Telescopes. Suiter's book is full of very practical hints on how to use star testing to help you diagnose telescope problems, and has excellent diagrams showing various problems and their effects on star images.
Another absolutely excellent resource is the program Aberrator This program allows you to enter the amount of aberration of various kinds, and shows you the effect on images of stars and planets. I just recently ran accross this, and give it my highest recommendation.
While Newton invented the Newtonian telescope in 1668, telescope testing remained essentially hit and miss until Jean=Bernard-Leon Foucault described a method for testing parablolic mirrors in 1859. The mirrors made by the famous astronomer and telescope maker Sir William Herschel were all made without benefit of any kind of testing: Herschel made dozens of mirrors and used the ones that worked the best.
Foucault invented a test that could be carried out easily with only simple equipment, but allows very accurate measurements to be made of the mirrors shape. This test is used with only minor variations by most amateur telescope makers to this day. Despite the simplicity of the test, most telescope makers don't fully understand exactly how it works.
Both tests, the Ronchi and the Foucault test, have a similar setup. A light is placed at the center of curvature for the mirror. The light bounches off the mirror, and is refocussed back at the center of curvature.
The ideal shape for a mirror in a Newtonian reflector is a parabola. Why a parabola, and not a sphere? A spherical mirror takes light rays which emanate from a single point (called the center of curvature and send them back exactly to that point. If stars were that close, a spherical mirror would be ideal, but real stars are a good deal further away, in fact, for all practical purposes, stars are located at an infinite distance.
This is why a parabola is ideal for an astronomical telescope. A parabola takes rays from infinity and sends them towards a single point which is called the focus.
All materials on this website are Copyright 2001, Mark T. VandeWettering. Permission is granted to reproduce and distribute these files for non-profit, personal use.
Mark T. VandeWettering <markv@telescopemaking.org>