Re: Picking a Mark IV Lens

[aah@nofs.navy.mil]

Tom gave an informative (as always) description of a comparison between the
Mak 350mm f/4 solution and the 400mm f/4 solution.
  Texereau, in How to Make a Telescope, 2nd ed., page 141, gives a table of
how the relative obscuration of a cassegrain secondary modifies the point
spread function of the telescope.  In particular, for a ratio of 40%
(approx 38mm/90mm), you get about 40 percent of the light in the diffraction
rings instead of the central disk.  This means that the optical system does
not come close to providing the kind of images you would get with the
400mm refractive solution.  I bet the detection ratio is even worse than
what Tom suggested.  I also don't know how well the Mak performs in real
life (scattered light, focal reducer chromatic aberration, etc.), so it will
be *very* instructive to see Mark IV images with this lens.  I'm not saying
that spending $4K per refractive solution is going to be the answer; in fact,
I presume the first Mark IVs will use the Mak just to get out the door in
a reasonable timeframe.  But I am suggesting that we need to understand the
problems with the cheaper compromise and be sure we can live with them.
Arne