Re: [ATM] mirror damage

["Bob May" <bobmay@nethere.com>]

One thing that the others didn't mention is that you need to be more careful
with your glass.  The stuff loves to break apart as you have already found
out.
You can not only get a chip off the glass when you whack it against
something else but you can also get a fracture just from the high pressure
at a single point.
The chamfer of the edge of the grinding surface helps with the compression
fracture (thes chip a clamshell piece off the edge of the glass) by backing
up the edge with a slope of material that supports the glass from fracture.
I'll note that if you accurately measure the glass, you may find that the
chipped area is within the variance of diameter around the glass.  As such,
it is pretty much irrelevant in the end for destruction of the image
quality.
When grinding, don't press hard on the side of the glass or you will
continue getting these chips spalling off of the main glass.  Keep a decent
chamfer on the mirror glass and you won't have any more problems with
spalling.
FWIW, the ancient poeple learned about spalling flint rocks to get their
nice arrowheads and other sharp tools.  You can do the same thing with glass
very nicely and I've seen an arrowhead that a guy did from a broken mirror
piece.
Bob May
bobmay@nethere.com
http://nav.to/bobmay
http://bobmay.astronomy.net

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