University Lowbrow Astronomers

Underwater Polishing.

by Tom Ryan
Printed in Reflections:  November, 1996;
Title Changed: November, 2003.

I’m going to let you in on a little secret.  Its not something that I thought of, nor did I come across it in thirty years of reading everything I could get my hands on about the subject.  Instead, it was discovered (as far as I know) by Karl Mueller.  It’s called underwater polishing, and it’s used to put the final figure on a mirror.

Karl had spent years polishing glass, and encountered the same problems that are described in the writings of Isaac Newton; turned edges and bad control of the surface’s form from the heat and pressure of polishing.  Karl made the simple change of bolting an epoxy painted tub to his polishing post, filling it with room temperature distilled water, and doing his final figuring with the glass completely submerged.

The figure suddenly was sharp to the edge.  It could be controlled by local pressure on the glass.  And no time was lost in waiting for the glass to cool down before testing.  (In the telescope making class of my youth, we waited at least 30 minutes after each five minute figuring spell before testing the Pyrex mirrors.)  This takes final figuring from about three days to about one hour.

Karl further wondered if the better dissipation of heat was the key to his successful method, so he tried working a blank of Zerodur (size invariant with temperature) outside the bath.  The old problems returned in the form of a turned edge.  Back in the bath, the edge turned straight.  Karl thinks these edge effects are due to thermal changes of the pitch lap from evaporative cooling of the polishing slurry.  (For arcane advice on when to polish your mirror, and where to polish your mirror, to minimize these effects when polishing out of a water bath, I refer you to the literature.)

The most amazing thing about this method is that I have never read about it before.  But I’ve tried it, and Karl’s method works.


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