Re: ATM Modifying a 6" f/12 refractor

[Roger Ceragioli <rogerc@dakotacom.net>]

Glen,

I would avoid the prism (sorry, Fran!) just because the quality may not be
up to snuff.  A good mirror diagonal made, as they usually are, on a
continuous polishing machine will probably be more optically neutral.  In
principle, I believe, a 45 deg. prism should act like a plane parallel
plate combined with a 45 degree mirror: it should not add additional
aberrations to the beam.  But the faces of the prism may not be figured so
well.

Two commonly used fold plans exist for refractors: 1) a "Newtonian"
arrangement, with objective at the front of the tube, a large fold flat at
the bottom, angled so that the return beam is deflected to one side of the
objective, and then a diagonal mirror mounted back up near the top of the
tube, but off to one side out of the way of the incoming beam.  This
preserves your unobstructed aperture and can work very well, if you get
good mirrors.

2) The "periscope" arrangement, with a double fold.  Objective at top,
first fold flat 1/3 of the way along the light path, tilted a few degrees
to send the beam back up the tube to the side of the objective, where it is
again reflected down the tube parallel to its original direction, but
several inches to the side.  This also preserves your unobstructed aperture.

I have used this second arrangement, and it gives you a very short (but
wide) tube.  Unfortunately, you have to hoist the tube up high on your
mounting and you will need still a third diagonal reflection to view near
the zenith.  The "Newtonian" arrangement may be the easier to use, since
your tube can be much lower to the ground and you look horizontally most of
the time, as in a Newt. reflector.

Good luck!

Roger Ceragioli