Re: [ATM] Polishing stroke

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Polk, Tom wrote:
> I'm presently polishing a 12.5" f5.4, 0.73" thick. For about 2.5 hours I
> used a 1/4 W TOT & MOT. A preliminary test showed a central hole and an
> TUE. I think the primary cause for the central hole was my lay out of
> polishing pads (first time to use them, heavy central area); the stroke
> was a secondary contributor. The TUE (never had this before) was caused
> by the short 1/4 W stroke. A 1/3 COC TOT & MOT is correcting both.
> Another 2.5 hours and I'll make a pitch lap. The two 8" f/6 and f/5 were
> polished out using a 1/3 COC. After polishing, both were near a perfect
> sphere. I'll post this to the list for feed back (only got two mirrors
> under my belt). Thanks and good luck.

Let's keep in mind that polishing with pads is very different, in 
terms of correcting asphericity, from polishing with pitch, which 
Charles was doing.

A mirror polishing unevenly is a sign of one of two things - either 
more polishing action is occurring in one region due to the technique, 
lap, stroke, etc., or there was asphericity after fine grinding.  For 
the former, the stroke may be adjusted to speed up polishing, but care 
should be used.  For the latter, there is no way of getting around 
polishing away the errors (short of regrinding).

The importance of thorough fine grinding is often underappreciated.

	Mike Lockwood


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