RE: ATM [Fwd: focal reducers]

["Chalfan, Richard D" <Richard.Chalfan@PSS.boeing.com>]

A focal reducer is a positive lens; as you speculated, it is the
"opposite" of a negative barlow lens.  However, focal reducers need to
be multi-element designs to achieve their optical goals.  So
"plano-convex" is quite a simplification of the actual design.  Be
careful about your assumptions of universality though.  Barlows are
generally "universal" because there is more of a market that way.  It is
possible to design a Barlow to compensate for additional optical
problems in a particular scope type.  For example, the "Barlow" in the
Jones-Byrd spherical-aberration corrected newtonian not only magnifys
the image (so it performs a "Barlow" function) but it also corrects the
spherical aberration of the spherical primary.  So I suppose the
Jones-Byrd corrector is really not properly called a Barlow.  Likewise,
eyepieces can be designed to mate with the rest of the optical system.
However, they would lose their universality (and their marketability) so
eyepieces are rarely computed for the system they will be used in.

The Celestron focal reducer, on the other hand, IS designed to mate with
a particular optical system.  The all-spherical short-tube Schmidt-Cass
design in the commercial Celestron and Meade instruments achieves that
(compact and manufacturable) design at the expense of a very curved
field.  The curvature has been characterized as having the radius of a
basketball.  The Celestron (and I suppose the Meade, though I don't own
one of those) focal reducer includes corrections to improve field
flatness.  (Of course there are so many SCT's in use that there IS a
market for this specialized corrector.  The same is true of the coma
correctors for fast Newtonians that you can buy today, there are lots of
fast Newtonians in the market).

Would it work with your Mak?  Yes, I am pretty sure it would.  Would it
work well?  I would try it.  I don't know what your Mak's field
curvature is, but it is unlikely to be a match for the Celestron focal
reducer.  But your results might be acceptable none-the-less. 

About the glare stop.  Look to see if you have a problem.  Look into the
front of the tube toward the focuser.  If you can see the opening for
the eyepiece, then you have a direct path you want to avoid.  Read
Telescope Optics by Rutten and van Venrooij.  They have an excellent bit
on baffling.  Remember that total baffling may mean unacceptable
vignetting.  You may have to deal with a compromise.

Regards,
 

Richard Chalfan
Kent, WA  USA

> > Gentlemen,
> >
> > I have a 4.5 f22 Maksutov.  I chose that fl with the idea of using
> it as
> > a photographic guide scope.
> > Now my interests have changed and I'd like to lower the f.l.
> >  snip <
>