Re: [ATM] Aplanatic Schmidt-Cassegrains with aspherical mirrors

["Scott Ewart" <scott.e@quicksurfer.com>]

    Could the corrector in any way help correct field curvature by enabling
a flatter curve somewhere else?  It doesn't sound like it.  It seems to me
that's the one and only strike against an RC designed for photography for
the masses.  What good are pin-point stars at the corner of the field if you
can't focus on them, at least not without another corrector?  Unless they're
coming out with a new curved ccd chip:)

Scott


----- Original Message ----- 
From: "Michael Peck" <mpeck1@ix.netcom.com>
To: <atm@atmlist.net>
Sent: Wednesday, January 12, 2005 10:53 AM
Subject: Re: [ATM] Aplanatic Schmidt-Cassegrains with aspherical mirrors



 The geometry determines field curvature, so you can't do
anything about that and you don't care about distortion.

 On one of
the Yahoo chat groups that sprang up to hype these things a dealer who
probably knows more than he's allowed to say assured me that their new line
of Ritchey-Chretiens *do* have two hyperboloidal mirrors. Since that's a
readily verifiable fact (at least when they become available to people who
aren't constrained by nondisclosure agreements) I have to assume he's
telling the truth. That means they're true RC designs. The one thing that
still puzzles me a bit is that if the mirrors have the conics of an RC the
corrector plate has nothing to do except hold the secondary assembly.
That's fine if you don't mind having a thin piece of glass at the front end
of your telescope, but they also claim to be using it to "minimize"
astigmatism. The only way I see to affect astigmatism given two
hyperboloidal mirrors is to tweak the conics of the mirrors a little bit
and add some correction to the corrector. That will affect the astigmatism
a little bit in the right direction, but only to a very limited extent.

Mike Peck


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