Re: [ATM] light weight mirrors

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

A 1" thick mirror really doesn't need to have a ribbed structure
at 30" dia.  What you need to do is to slump the face of the
mirror to the approximate ROC of the finished mirror.  This will
keep the grinder from going very deep into the glass to get the
mirror finished.  The technique for grinding is a bit more
complex than that for a thicker/unslumped mirror but that is more
in the direction of not putting a lot of weight on the glass when
grinding and polisheing.  Pushing a large lap on such a mirror
really shouldn't be attempted as the friction is more than a guiy
can do.  From the experience of the list, the max. diameter seems
to be around 16-18" for a lap.  This means that you will be
needing to do subdiameter laps to polish out the mirror and
figure it.
I'll also note that figuring the mirror does become a fair bit
more difficult due to the larger difference between the sphere
and the parabola.
If yo want to do lightweight mirrors thand do ribbing, a thinner
front and back plate of about 1/4" or so will do just fine.
Again, yo need to do slumping, fusinnnnng and so forth to get the
mirror blank into shape.
Richard Schwartz and others have done different designs towards
that end.  I will note that with the ribbed or cellular mirrors,
yo need to take care not to put too much weight on the glass or
put the glass through temp extremes between the parts of the
blank.  Doing so with something other than a good Pyrex blank is
an invitation to sadness as the blank cracks from the stress.
Bob May
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