Testing
A simple way to test a Schmidt corrector plate used in a classical camera is shown below:
Flashlite / 1 / 11 1 / 11 look 1 11 at from > 1 Plate ------1 ronchi 11 150'or 1 ------1 11 Spherical mirror more 1 l 11 1 l 11 1 1 11 ---1-----------------1----------------11 ---------------------------------------- beam \ \ some longitudinal adjustment
A test rig is made of wood. The corrector is at the RC of the mirror. The flashlite is near the focus of the mirror ,adjustable and pointing at the mirror. The flashlite is covered with a ronchi grating. Now look at the corrector from a long distance (150' or more) the more the better. You should see straight lines. If the lines flare at the top or bottom the collimation is off. Unless the bands are grossly out make any correction on the primary. My two plates needed no correction. I could only see some aberration with one ronchi line showing which was probable the primary mirror. Just for fun remove the corrector and take a look. The lines not only bowed but the outer lines went into circles.
For correctors in SCT's or Wrights which are some percentage of a true Schmidt the corrector will be tested in the complete system. A flat (water, mercury or glass) would be ideal. An artificial star as described in Star Testing Astronomial Telescopes by Suiter with a ronchi grating should work. As in the case of the true Schmidt the correction if needed should be done on the primary.
My next and last post will cover a few tips that were missed. Bob Pfaff