Re: [ATM] ritchey cretians

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

If you really want to make a four foot diameter flat, by all
means, go at it! Some people make flats easily while others
stirve and never get there.  The flat is going to want o be a
10:1 or so in order to keep its shape so some 3/4" thick glass
won't do the job.  You're going to be wanting glass that is at
least 5" thick.
My proposal is to just do your testing on Polaris with the KE and
Ronchi Grating as this will let you  know how well your scope is
performing. Do the Primary with a standard Foucault test and then
do a test plate for the secondary to get it close and finish the
secondary in the scope.  Remember that even when unsilvered,
glass does reflect about four percent of the light on it.  This
is more than enough for a scope of that size to get an excellent
image of Polaris.
FWIW, a six inch primary which is unsilvered will give a nice
split of the Double Double for testing purposes. That is less
light than Polaris gives out and your scope will be gathering a
lot more light than a little ol' six inch mirror!
The thing that is nnice about Polaris is that it hardly moves
around in the sky and is usually visible even in a heavily light
polluted area with a scope of the size you are contemplating so
it will be and excellent light source for testing outside your
factory.
Bob May
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