[ATM] ATM: Knife edge location.

["David Harbour" <scarab2@cox.net>]

My good friend Richard recently sent me some intereseting excerpts from a
treatise on testing. One of the illustrations shows a sort of "top view" of
a set-up that shows the knife edge offset to one side of the light source.

WHAT IS THE ADVANTAGE OF THIS?

It is full of nothing but DISADVANTAGES.

Why has no one passed a law against this?

I remember a neat little article in Sky and Telescope about a fellow who had
figured a very good figure for his RFT mirror; it was either a four, or six
inch f/4. Now, the most remarkable thing about his focographs for this
mirror was that they showed (FALSELY) a KILLER load of astigmatism in the
mirror.

Mount the knife edge above, or below the tester's light source, using one
blade of the jaws of the slit as your knife edge, and you will never see
this false astigmatism. I once figured a very fine 12"  f/2 sphere for a
customer (you all know him), for a Schmidt camera; he was very happy with
it. You would think that I would not have been able to see the true shape of
the mirror- NOT SO! No false astigmatism, no vignetting. Ask me how I did
this in the design of my tester, and I will show you how I did it. Someone
liked my #5 tester so much, that they refused to send it on to the next
person on the list who was to borrow it.

http://www.atmsite.org/contrib/Harbour/Tester.html

I was not surprised.

dave

By the way, without you guys, my life would be a desert. My wife is dying
now. Thanks for being my friends, especially you, RICHARD. Thanks for being
my friend. I am not lonely, because of ALL OF YOU GUYS!!! You all mean the
world to me.


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