[ATM] Tests of Surface Quality, Laser test for polish

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Francis J. O'Reilly wrote:
> Thank you for your comments. I did not do any surface tests between grades 
> of abrasives, none whatsoever. I relied solely on the clock. 45 minutes to 
> an hour of each grade then clean up and move on.

While it sounds like in your case you ended up with a good polish (and 
I'm glad you dit), in general, especially for first-timers, this is 
quite risky, and not advisable unless the change in abrasive sizes is 
very small between each grade, and unless your process is very well 
characterized.  It's easy for larger pits to slip through if the 
difference in size of two abrasive grades is sizeable or if you change 
types of abrasive (from carbo to AlOx), such as going from 320 grit to 
25u.

This risk is greatest if the radius changes while grinding (and it 
usually will with hand work), as either the outer or inner part of the 
mirror will be "left behind" and will harbor larger pits.  A 
spherometer can help one monitor the radius.

> My surface was pretty good before I began polishing. I've had good luck with 
> 5 micron and 3 micron CeO before. I was not dissapointed this time either. 
> I've decided to call it good after seven and a half hours of polishing. The 
> surface is very clear. I cannot see any grey when I shine a high power 
> American Optical light ion the surface, even when I amplify the light with a 
> magnifying glass. I'm done.
> I'm not concerned about the laser results at all based on what I am told by 
> others whom I respect such as Dick Parker. The test is too sensitive for 
> telescope purposes. I'm not even sure that what I am seeing are pits. But in 
> any event they are uniform from the center to the edge.

There will always be a very faint laser return, but it will be uniform 
across the optical surface if it is clean.  Observing the brightness 
of the laser spot during the first few hours of polishing will reveal 
any zones created by fine grinding.  By observing this, one can learn 
a lot about how to get a good sphere at the very end of fine grinding 
in order to make polishing as expedient as possible.

	Mike Lockwood

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