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

[Guy Brandenburg <gfbrandenburg@yahoo.com>]

The solution is fairly simple, IMO. 
#1 - make sure shte concave surfaces are good.
#2 - make or purchase a monochromatic light source of some kind. (I found it fairly easy to do with some fluorescent light tubes, actually 'black light' ones I found at Home Depot, and some carefully-chosen greenish theatrical lighting gels I picked up at a theatrical supply house).
#3 put the unknown convex on its matching known concave.
Any weirdness in the interference lines will be what you need to fix.
Good luck.
I'm doing the same thing.

Christopher Dalla Piazza <dalchri@hotmail.com> wrote: Well, I'm out of concave surfaces to polish on my Houghton so it's time to 
face up to my next challenge.

How do you test a convex spherical surface?
-or-
How do you test a biconvex spherical lense?

Here is what I will have available:

6.5" diameter 53.95" ROC mirror
6.25" diameter biconcave lens ~43.297" ROC on one surface ~185.137" ROC on 
the other.
One Foucault tester

Here is what I need to test:
6.25" diameter biconvex lens ~43.297" ROC on one surface ~185.137" ROC on 
the other.

Some ideas:
1) Put the Foucault slit on one side of the lens at R1 and a second knife 
edge on the other side at R2.  How do I distinguish errors in one surface 
from the other?  Or, does it not matter so long as the lens surfaces 
combined test null?

2) Use the Foucault tester through the convex surface and use the interior 
reflection off the second surface so that it is tested as a concave surface. 
  Again, how do I distinguish errors in one surface from the other?  Will 
this act as a null test, or will I need to use ray tracing to come up with a 
Couder mask and do zone testing?

3) Test the fully assembled optical path without the secondary.  Would that 
give me normal spherical aberation to test like with a paraboloid or is it 
more complex, also requiring ray tracing?  Sounds a little too complicated 
to be effective.

The surface problems I've been encountering are TDE and raised center 
towards the end of polishing.

BTW, will my problems be reversed?  Will I end up with TUE and a central 
depression on my convex surfaces?

Thank you again for ideas!

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"Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved."
- wrote Charles Darwin.
Guy Brandenburg, Washington, DC
My home page on astronomy, mathematics, education:
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