Re: Distortions

[aah@nofs.navy.mil]

Glenn suggested the 90mm f/5.6 Celestron.  This is an interesting idea, and
one that I had not considered.  As Maury mentioned, you do have a central
obscuration which I'd bet takes the effective aperture down to 70-75mm.
Compared to the 100mm lens, you would lose 0.5mag or so.  Maury also mentioned
the small central hole in the primary, and the possible loss of field with
a 2kx2k.  A custom lens would also be easier to design a focus mechanism
(as mentioned by Tom) and for inclusion of temperature monitoring.  My feeling
is that the Celestron is a fallback position, with a custom design the best
choice if it can be done at a reasonable cost.
  Herb mentioned a change in the spot size with wavelength.  This would be
true if you were using a single design for the entire BVRI coverage, but
Tom is proposing 4 separate designs, each specific for a given filter.
  Bob Goff suggested stepping the telescope in a raster pattern to look for
distortion.  This won't work with the current drive since it only guides
to +-4arcsec in RA and steps a few arcmin in dec +-, not to mention the
extra time in multiple exposures.  You have 25 or so Tycho stars per square
degree, which means several hundred in the field of the 400mm, and so have
plenty of reference stars for mapping distortions.
  A lensless Schmidt as suggested by Maury (lots of ideas in that email!)
might work, but you would have a large central obscuration
(the CCD) and probably a longer tube than desired for the mark IV compact
design.  At the same time, one of the problems of refractive optics is the
elimination of coma, and I think that will be the limiting factor in the
4-degree field.  This is not a problem with a Schmidt.  I'd be interested
in what Bob G. has to say about the pros and cons of possible 400mm f/4
systems!  I don't consider myself to be an optics designer in any concievable
way, so input from a real designer is gratefully appreciated.
  Once we start talking about larger apertures, like the C8, then a complete
rethinking of the mechanics would be required.  I'm not saying it couldn't
be done (for instance a bank of 4 C8-like telescopes in a fork mount ala MMT
or the GSU MTT), but it might be more appropriate for the mark V.
Arne