Are Aluminum Mirrors in reach of the ATMs?

[Dean Ketelsen <ketelsen@as.arizona.edu>]

Sorry I'm coming in late on this, but in working on several mirrors
up to 1.5 meters, I feel qualified to point out something.  That is
that aluminum, when exposed to air forms an oxide that is very hard.
So after your grinding or polishing run, when you wash and dry off
your metal mirror, the next time you start you have effectively an
aluminum mirror coated with a saphire (ie, very hard) layer that is
difficult to break thru with your next run.  As a result in subsequent
grinding/polishing runs you might get no removal for some time until
you break thru this layer, thus affecting radius and figure changes
you are trying to make.  We found that the way around this was to use
an oil carrier, instead of water, to prevent the oxide from forming.
Between runs the oil was left on the surface.  Unfortunately, the oil
(we used olive oil) reacts with the pitch, but we were able to use
shower curtain material over pitch to keep the two apart for polishing/
figuring.  We used 9 micron diamond to get a polished surface with
shower curtain.  While certainly specular, I would not say you get
nearly as good an optical surface as glass, however.  Our applications
are for illumination mirrors for secondary testing and we only used
aluminum as it was much cheaper than glass substrates in that size.
The application allowed the rougher surface with aluminum.  I just
wanted to point out the oxide problem that I had not seen mentioned
in this round of discussion.

					-Dean