ATM Troublsome Focault Testers

[David Sleeter <sleete19@idt.net>]

When dealing in used optics a few years ago, I bought a Focault Tester 
for checking the quality of questionable mirrors, and I encountered the 
following problem. 

After playing with the tester for a few days and having difficulties 
getting a clean edged shadow, I noticed that the "knife edge" was stamped 
from aluminum sheet metal, maybe .040" thick. Not quite sure what effect 
the knife's thickness might have, I replaced the aluminum edge with a 
single edged razor blade. 

VOILLAH!, I got a nice sharp edged shadow. With a little more experiment 
I verified that to get a sharp edged shadow, you need a very THIN and 
sharp edged knife; a razor blade works perfectly. 

I can't remember WHO I bought the tester from, but I do remember that it 
was one of the places that takes out big ads in Sky & Telescope. If any 
ATMs have purchased commercially made Focault Testers and are having 
trouble getting them to work properly, look at the knife edge. If it's 
made from sheet metal, replace it with a razor blade, and that might 
solve your problem.

Dave Sleeter


-- 
Life is short and the path is often unclear.
It is a great sin to knowingly contribute to someone's confusion.