Re: [ATM] meade saturn scope

[Mark Holm <mdholm@telerama.com>]

A sphere with a very mild tde is a closer approximation to a paraboloid
than a plain sphere is.  But, at f/8 on a 4.5" mirror, that has to be a
very, very mild TDE to qualify.  Quantitative Foucault testing at the
very edge is difficult.  Star testing is probably the best way to assess
that.

If the mirror really suffers from a haze of unpolished out pits, then
regrinding, or at least repolishing is the only fix  Before you are
ready to send your other mirrors off for coating, strip the coating off
of this one with circuit board etchant.  If the surface haze goes away
with the coating, then you only have the turned edge/prabolization
problem to worry about.  If the haze doesn't leave with the coating,
then regrind and repolish is the only practical fix.  How far back you
have to take the grinding is a bit difficult to say, it all depends on
which grade of abrasive the incomplete grinding occured at. If the
mirror is plate glass, you might take some small pieces of window glass
and grind them very, very briefly with abrasives of different sizes.
Then, with a strong magnifier, compare the pit sizes.  Back up to the
the next finer grade than the one that seems to match the size of the
remaining pits.

Mark Holm
mdholm@telerama.com


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