Re: Intermediate ATM project

[Steve Lee <SL@AAOCBN1.AAO.GOV.AU>]

> I don't mean to sound like a goon, but on the Dall-Kirkham are the 
> primary and secondary both concave? If not, is there a type that has both 
> primary and secondary concave?
> And, I don't know very many of the different types of scopes, does 
> everybody else have a book on it or something? If so, which book?

Brand,

The D-K is an ordinary Cassegrain with convex secondary, but it's
spherical instead of a hyperboloid.  The variant you're asking about
is the Gregorian.  In its normal mode, it uses a paraboloidal primary
and concave elliptical secondary.  It is significantly easier to
make than a Cassegrain because the secondary can be null-tested at
its conjugate foci.  It is, unfortunately, not as compact as the
Cassegrain because the secondary lies outside the primary focus.  The
secondary usually has a higher magnification than the Cassegrain, 6
as opposed to 4, but it produces an erect image.  As a sidenote,
it was the first reflecting telescope designed, but unfortunately not
the first built - Newton made his a few years after Gregory designed his.

And now to your next question - what happens if you place a concave
secondary inside the primary focus.  It's called either a Schwartzschild
or Couder camera, depending on the choice of curves for the mirrors.
The secondary acts as a de-magnifier and from a (say) f/6 primary you
get an f/2 focus.  The problem is that the focus is totally inaccessible
inside the tube.  The very few that have been made were done as cameras.
The Schwartzschild was designed to have a flat field, but had significant
astigmatism, whereas Couder (yes, the Couder that Bratislav keeps
going on about because he wrote the book on clever optics - it's just
that he wrote it in French and as never been translated as far as I know)
modified it to remove the astigmatism at the expense of field curvature.
Operating around f/3 they would make a good photographic system

The design pre-dates the Schmidt camera by a year or two, however, and
Couder stopped working on the one he was building (76cm primary I seem
to remember) when Schmidt announced his optical design.  The Schmidt
was clearly superior to the Couder.  Few, if any, were ever made, and
only a few of the Schwartzschild design.  Its design is mention in a
few older books, one of the ATM books mentions it, as does Texereau.

I have a modern design that operates at f/2 and was building one some
10 years ago.  I gave up because the primary is an extreme hyperboloid
(e=3.818) and the secondary is a very steep (f/1.5) ellipse.  Both are
very difficult to make or test and I eventually gave up, although the
main reason was the advent of CCDs that an amateur could afford and not
technical difficulty.

As for required reading - there is no one book to read.  A list would
start with the 3 ATM books, Texereau, Telescope Design by R&vV, but
there are a lot more.  Look through the Willmann-Bell catalogue, it
lists many of them.

Steven Lee
URL:        http://www.aao.gov.au/local/www/sl/
e-mail      SL@aaocbn.aao.gov.au