[ATM] All Spherical Telescopes

[Delyan Toshev <delyan@trirand.com>]

Hello,
Noting the recent increase of topics about all spherical telescope 
designs, it is interesting to see a discussion about viability,
perspectives and performance of such systems for the ATM community.

Although some of these designs were already mentioned recently, the 
information and evaluation about practical implementations is laking. I 
find missing information about problem areas that are not obvious at 
first glance ( for me, at least ) - tolerances for surfaces, tolerances 
for centering and collimation, reflection issues, etc.

It may be useful to make a list of known designs, and discuss the merits 
  and falls of each one. It will be very useful also to make some rating 
of such systems for visual, photographic or combined tasks.

The designs and implementations known to me:

- classic, all spherical Maksutov. The main goal Maksutov had, was to 
design an easy to build, yet high-quality school telescope, for that 
reason he looked at spherical surfaces as a must. I don't know, however 
which of the current production Maks are all spherical.

- Lourie-Houghton variants, recently reappearing on the list :-)
This design is known, at least in Russia, as Volosov-Newton, because of 
the work of russian optician Volosov during the 1940s on the same 
design. It seems he proposed it independently at the same time, as 
Houghton did.
We know few of them were build, and some discussion already made clear 
more or less of their advantages and issues. Russian ATMers build some 
of these too, from 4" to 10" in aperture. One problem that is valid with 
LH is about its many air-to-glass surfaces. Any comments on this? How 
this can be overcome with relevant AR coating?

- spherical catadioptric relay telescope. Sidler and Dilworth are known 
( to me ) were working in this direction. Still, very little information 
about practical implementation. One may conclude some difficulties for 
average ATMer to build some of the components of such telescope, 
particularly Mangin mirror, where there is one mirror-to-glass surface 
which is required to be polished to very low error - about 1/20 of green 
light. Another issue, which such system will present are the tight 
tolerances for centering and linear separation between elements. May be 
someone can bring more information on these issue as well to propose 
some remedy?
Little is known about ATMers successfully implemented any of these. One 
commercial telescope is manufactured, based on same or similar scheme - 
the Clavius 166 - made in France by renowned optics company.

- all spherical catadioptrics with sub-aperture corrector. Although this 
sounds like previous example, with relay lens-corrector, these are not 
quite the same, IMHO. Here I have in mind the design proposed by Klevtsov
(http://www.telescopes.ru/articles/article1.phtml), which is implemented 
by russian company NPZ, the models TAL-150K, TAL-200K, TAL-250K ( 
TAL-300K is planned ). It _appears_, that Vixen implements similar 
design for their VISAC and VMC200 catadioptric cassegrains, too.
The work of Klevtsov is based on previous work of Argunov, another 
russian optician, who quit this direction of work some years before 
Klevtsov made his variant. As one can see, he also uses Mangin mirror 
and small meniscus as a system.
Very controversal system, although it is interesting to separate design 
by given implementation. Still, it is interesting to know how this 
desing is viewed by ATM community.
There are 2 Klevtsov type telescopes build in Spain with aperture about 
0.60m.

As general note of interest is how the practical benefits ( if any ) of 
these designs relate to given aperture? It is my opinion that in case of 
relay telescopes and the Klevtsov ones, the aperture from 10" up is 
starting to look more and more interesting.

Excuse me for the long post and my far from perfect english. Hope, I not 
caused a big suffer to the list :-)

-- 
Best regards,
Delyan Toshev
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