Re: [ATM] How do I test a convex surface?

["Jerry" <wa4guu@verizon.net>]

You might try doing a raytrace of the refracted center of curve, testing the
back surface through the front.

The raytrace would show the desired spherical aberration (different ROC for
ray height from axis). A zone mask mask could be used. The Ronchi test could
show smoothness.

Reflection off the fist surface would be very small in the center because
the first surface is convex.

Jerry

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Chris Dalla Piazza
Sent: Monday, March 26, 2007 9:06 AM
To: 'ATM Superheros'
Subject: Re: [ATM] How do I test a convex surface?

Do you need anything exotic for the tank glass or would a regular aquarium
tank do the trick?

Even if the aquarium glass compromises the quality of the wavefront, I am
only interested in detecting errors at the edge or center.  I could pretty
much ignore any striatic, rough appearing, or irregular errors since I don't
have a history of that problem.  I never have trouble obtaining a sphere so
even if I munge the solution slightly, I am guessing the result is slight
spherical aberration which I can also ignore.

Am I right in thinking that the only effect of the flat air/glass and
glass/solution interfaces is to alter the apparent ROC of the lens?

This test should also eliminate errors from the surface not being tested.
This sounds pretty neat.  I think I'm going to be googling "Salt water index
of refraction" in the near future.

-----Original Message-----
From: Dominic-Luc Webb [mailto:dlwebb@canit.se] 
Sent: Monday, March 26, 2007 4:36 AM
To: Christopher Dalla Piazza
Cc: ATM Superheros
Subject: Re: [ATM] How do I test a convex surface?


There is one test you did not list, which has actually been
used in ATMing (by me amongst others). There are tests in which
the lens is placed in a tank filled with fluid of same refractive
index as the lens. This fluid is prepared by mixing two solutions
with slightly lower and lightly higher refractive index in
exacting proportions. Then you would test as a concave.

In some cases, since many people now use very high precision
micrometers, I guess you could get pretty close by measuring with
a spherometer from the start and then polishing a little at a
time to see which strokes on which surface improve the test
results. At least for Schmidt cameras, there are tests for the
completely assembled optical system. I could guess there are
similar tests for assembled refractors, but I have not done this.

Dominic-Luc Webb


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