Re: [APML]: Coma correctors

[Peter Ceravolo <ceravolo@fox.nstn.ca>]

>Obviously, the question is do you get what you pay for?  The Celestron unit is
>only ~40% of the cost of the others, but is it adequate for photography?

I bought a stack of the Celestron coma correctors a while ago. I took one
apart and reverse engineered the thing and pluged the lens's parameters in
my optical design code. Basically it is a Ross type corrector (Ross back in
the 30's designed one for the 200"), there was an article written about
building one in S&T about 10 years ago or so.

It's a simple design that has one major problem; it intruduces spherical
aberration which blotes the images slightly. However it is not too serious
and it provides uniform spot sizes over the 35mm format. One might see the
difference on tech pan in good seeing. One could get rid of the spherical by
making the primary hyperbolic. In fact, that corrector would probably work
well on a Ritchey-Chretien cass primary working in the Newtonian configuration.

>
>The second question relates to the difference between the Televue and Lumicon
>versions.  With an f/4 primary, which will perform better?  Or will both
>perform well?

I have no knowledge of the Lumicon unit but the TeleVue model uses two
doublets, probably of high index glass, to keep spherical corretion in check
but still correct the coma.

The problem with all of these units is their size. I suspect, please correct
me if I'm wrong, they would introduce more vignetting, especially with an
f/4 beam.

Peter