(ATM) Dall-Kirkham optics

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

> Peter, you are sorta on my track. I was looking at a Dahl-Kirkam design
> this after noon and I began changing my mind. I gotta look some more, but
> couldn't I get a lens from Ed Sci and not have to grind and polish that
> part - only the primary? Well, once I saw a picture of Robert Cox (I
> think) with a small foldable telescope like I'm talking about. I'm not
> necessarily wanting a foldable one, just small - and I want to test my
> skills more. So tell me more about my idea of using a plano lens of the 
> correct radii, Please?

> Friendly Bob

Ah!  All these young people who have never heard of Horace DALL !!! 
Horace was (unfortunately, he died a few years ago) an excellent optical
worker who did many things beside make telescopes.  The Dall null test
is one of his, as is the Dall-Kirkham Cassegrainian.  (Kirkham did the
maths after Horace built one of these knowing only that it should be
possible to use a spherical secondary.)

The small telescope that Bob Cox made is a clone of one of Dall's, made
after Dall visited the US many years ago.  The design is more than just a
spherical secondary and elliptical primary.  It uses a pair of relay lens
to re-image and erect the field.  This has many advantages, most noteably
it requires a smaller secondary, and the baffling is more complete than a
classical Cassegrainian can ever be.  He wrote it up in Sky & Telescope
for January 1962, and I believe that Cox wrote about his in Telecope
Making.

The straight Dall-Kirkham Cassegrainian has considerably more coma than
a classical Cass.  F/10 is fast for a D-K, but in such a small size is
probably OK.  Generally, I would recommend a primary of at least f/3,
and probably more like f/4; with a secondary amplification of about 4. 
This should reduce aberrations to a comfortable level.  Obtaining a
commercial lens for a secondary sounds like a good idea, although I would
expect that the chances of finding one of exactly the right curvature to
be extremely unlikely.  The primary, being elliptical, can be null
figured; the light source set up at one conjugate focus, the knife-edge
at the other.  The secondary is probably just as easy in such a small
size as the steep(-ish) curve tends to remain spherical.  I made one and
never checked the figure.  I just polished it then did the final touching
up on the primary by auto-collimation.

Steven Lee