For testing the mirror the a Foucault tested is used . There are other ways of testing a mirror but the Foucault tester is the probably the best solution for the amateur telescope maker, as it is easy to make and use after a bit of practice.
The Foucault tester.
The the knife edge is attached to the carriage so that it is free to move along the rail .The position of the knife edge is carefully controlled by the micrometer screw that allows readings to be taken.. The knife edge can be made to cut the light cone by using the adjusting knob.
The micrometer and adjusting screw
The micrometer is constructed by photo copying the first 1.5 inches of a ruler that is divided into tenths and sticking the scale onto the brass scale plate. The micrometer knob is attached to a 1/4 x 20 thread that presses up against the carriage plate a spring tensioner is use to ensure geed contact with the screw thread. A scale 0 to 50 is placed around the edge of the knob .
the micrometer shown in fig 3 is capable of reading to .005 inch
A 6 inch mirror set up for testing
To make a test the mirror is set up in these holder and a the Couder screen placed in fount. with the Foucault tester placed at near the center of curvature the exact center of curvature for each of the Couder screen zones can be measured.
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