Re: [ATM] Classical Cassegrain or Dall Kirkham ?????????

[Dominic-Luc Webb <dlwebb@canit.se>]

On Thu, 8 Feb 2007, Vimal Mamania wrote:

> One of my friend and I'm also planning to build a catadioptric, preferably around 12" or 14" diameter. Anybody with previous experience on the same, pls. tell us which design to go for out of the three?
>
> 1)      Ritchey Chrétien
> 2)      Dall Kirkham
> 3)      Classical Cassegrain


To me, "catadioptric" means you have a corrector plate in addition to
the primary mirror. In a sense, it is both reflector and refractor. The
designs above are not then catadioptric. Part of the trick in making
these is finding a good combination of a primary you can actually
construct and a secondary with suitable amplification that result in
acceptable performance. I managed to find a lot of bad/pointless
combinations with Oslo LT. Actually making a very fast primary of any
conic other than a sphere is problematic at best. I think what drew
people to the Maksutov in the early days was that it was composed of
nothing but easy spherical surfaces, the first and most basic conic
most ATMs will make. The alternatives would use more complex curves,
like a Schmidt corrector, I guess.

One problem I have with some Cassegrain designs I have considered (not
spherical, BTW) is that I would start with a parabolic primary I knew
I could easily make (like F/5), and then realized I did not want very
high amplification because of limiting atmospheric conditions. Given
that I was considering apertures 12" or higher (already a lot of
focal length), this lead me to skipping the Cassegrain design and
using either a slightly longer focus Newt or a 2X focal expander I
could buy for 40 bucks. You also need to consider that some
combinations won't fold your system much compared to a Newt, so you
may not end up with a very compact design. So after all my fussing
around, my most recent astro image was taken from a camera at prime
focus on a Newt (no diagonal), using an eyepiece in front of the
camera to get adjustable magnification. In retrospect, I ended up with
a real easy & minimal design with fully tunable amplification that is
really no problem to collimate. While brain-dead from optical design
point of view, it suits my imaging needs (albeit, not my intellectual
needs). I ended up building more complicated systems, but not for the
sake of magnification. I went for catadioptric systems to get less
magnification.... wide field cameras!

Dominic-Luc Webb

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