A few notes on making Schmidt corrector by the vacuum method.
Why the vacuum method? It is possible to generate a zone free plate with almost no testing. The process is somewhat like making a spherical mirror (no aspherizing to do). If the plate is a little under or overcorrected you can put the correction on the primary mirror. Most any good glass can be used (I used BK7 and plates glass). I have made two plates (11 inch and a 16 inch) and found them to be interesting projects. Any advanced ATM with access to a metal lathe or knows a machinist friend that can make the vacuum pan should have no major problems.
Post 1 Literature and Formulae
The formulae and math needed to make a corrector plate which is to much to describe here can all be found in an article, The Vacuum Method of Making Corrector Plates (Sky and Tel. June 1972 p388-393) Most of the information is by Dr. Edgar Everhart from his article in Applied Optics May 1966 "Making Corrector Plates by Schmidt"s Vacuum method". The math is here to do the design. I used the partial vacuum method with 1/2 correction each side of the plate. No need calculating Young"s modulus because the plate deflection will be measured with a dial indecator (see page 389). Also do not add extra deflection for the weight of tool as the pan is filled with water preventing extra deflection. Be sure to do the example on p392 to check your math.
Other articles: Construction of a 12 inch Schmidt- Cassegrain Sky and tel. Nov. 1976 p382. (some info on vacuum pan design with water) Wright Schmidt Construction Notes RTMC 90 proceedings (Special data on plate thickness , Should be 1/30 to 1/40 of plate dia. 1/34 is best and write up an the pan) Amateur Telescope Making Book 3 p371-372
Some other articles on SCT's and Wright's but not on vacuum plate making: Australian 11.5 inch Wright Ski and Tel. May 1972 p320. Diffraction-Limited Schmidt-Cassegrain Telescope Sky and Tel. Apr. 1966. Phoenix Amatuer's 12.5 Schmidt Cassegrain Sky and Tel Apr. 1970 p254. Schmidt-Cassegrain Optical System with Flat Field-I Sky and Tel May 1965. Schmidt-Cassegrain Optical System with Flat Field-II Sky and Tel. Jun 1965. Telescope Optics Evaluation and Design (you need this book)
In my next post I will talk about the tools to make a corrector plate. One little scratch can ruin day Bob Pfaff