[ATM] Conic Constant

[spoon at PageAmerica.net (Mike Spooner)]

For 10" Pyrex mirrors the temp change barely matters. For 20" Pyrex mirrors
the temp change really matters.

I believe Ceravolo once wrote that slight undercorrection is acceptable but
any overcorrection is undesirable. Somewhere else I read that Alika Herring
stated that mirror made for use in the mountains should be figured
differently than one figured for sea level (I don't recall his exact
wording) but he didn't really explain what he meant.

My own experience in "Adventures in ambient" show that 20" Pyrex mirrors
I've figured are not exactly uniform when it comes to correction change with
respect to temperature. Since I learned from Richard to absolutely,
implicitly trust everything written (NOT <g>) I checked it out myself. I'll
just pass it on that if you're making a 20" class mirror, don't test and
final figure it in a shop that is warming up (say with morning sun) or
you'll be disappointed with it's performance in evening use. Since most on
this group star test before coating, this probably isn't a big issue as
you'll certainly catch it then.

Best,
--Mike Spooner