Optical system for Mark IV

[ALAIN MAURY <maury@ocar01.obs-azur.fr>]

First let me wish you all a happy new year, with a productive
amateur sky survey.

About optical systems. A group a french amateurs have started to use
a 2K chip ( by Thomson ) and have indeed complained about the scarcity
of good enough optical systems. Even a 105 or so astrophysics refractor
was not able to cover correctly the field, and the coma at the edges was
quite poor. They say a CN212 ( by Takahashi ) with field corrector
gave good images...

I don't believe lenses which have a tiny hole in the central mirror
(500mm F/8 and the like ) might be good enough. The diagonal dimension
of a 2K CCD is 43.5mm... I have seen many 1000mm F/10 which people here
in Europe tend to buy directly from former Eastern block countries
like Poland, Slovakia, and the like. They can be found at around
$200 or less. I don't remember seeing any 500 or 350mm lenses however.

In the reputed lenses, which should be able to cover a 2K, or even
a 4K chip is the Pentax SDUF II made for the Pentax 6x7. I know I'll
buy one of these some days. They seem to be quite in use in Japan 
among amateur astronomers. They also are quite expensive, I think
in the $3,000 price range. Some amateurs are using them with small
CCDs and say the images are good, but nobody knows on a large field.

One thing I have read about in a japanese amateur astronomy magazine
called interactive astronomy ( a pendant trimestrial magazine to
the monthly tenmon guide ) is what seem to be a modification of a
regular C8 to a Schmidt telescope. As you may guess I was not able
to read this in the text, but the pictures clearly showed a regular
C8, with an extra piece at the top allowing to replace the C8 schmidt
lens at the radius of curvature. Anybody saw and read this article
on the list ( any japanese amateur astronomer out there ? ).

While you are at it, can anybody make some raytrace of a stopped down
lensless Schmidt camera which could be build with an existing C8 tube
without its lens. What about a huge central obstruction and a diaphragm
taking the outer edge of the mirror ? This would make for a cheap and
good quality telescope, taking into account the 15 micron pixels and
nearly 8 arc second pixels. Maybe a flattening lens would be needed ?
There is a simple formula which gives this plano convex lens with a
radius of curvature something like 2/3 of the focal length of the mirror.
Unless I don't remember and it is the focal length of the correcting
lens and not its radius of curvature.
A mask at the center on the optical path of 175mm of diameter ( letting
the outer 25mm of mirror ) would provide as much light as a 100m
full aperture lens for a fraction of the price of a good quality 100mm
F/4. This mask needs to be placed at the radius of curvature. There
again, from memory, I believe a C8 primary is an F/2.2 mirror. Or the
diaphragm can be a real diaphragm or any combination between taking
just the center or just the edge. In fact if you look at the profile
of a Schmidt lens, there is a zone, known as the Kerber zone where
the two sides of the Schmidt lens is parallel, i.e. can be replaced
by no lens at all at this particular zone. Usually in a classical schmidt
telescope, this zone is at 0.866 the radius. Outside this radius,
the lens behaves as a diverging lens bringing the focus further away,
and inside this zone, the schmidt lens is a converging lens, bringing
the focus inside. The shape of the lens perfectly corrects the mirror
spherical aberration.
If this was possible, it is possible to find used optical tubes ( even
an old orange tube would do ) and build a kind of camera support/Spider.
3 rods could support a cardboard diaphragm of some kind. Focus would be
provided by the C8 focus knob. Could even use the lousy mount for slewing
the telescope.
Alain