[ATM] Cassegrain vs. Gregorian

["Richard F.L.R. Snashall" <rflrs@rcn.com>]

  Given the same requirements (400 mm f/16, net length 1800 mm, body 
length 1500 mm,
0.36 degree (c. 40 mm) field diameters):

    http://users.rcn.com/rflrs/cass01-04.len
    http://users.rcn.com/rflrs/greg01-04.len

give prescriptions for a Cassegrain and a Gregorian scope, using RMS
wavefront error as the key optimization goal.

At first, it appears that the Gregorian would be a lot easier to build, 
as the
conic constant for the Cassegrain is -4.738 (vs. -0.513) for the secondary,
and slightly larger (than the Gregorian) for the primary.  However, the RoCs
here are quite different, so the actual RMS difference from the best fit
sphere (the measure that is available in ZEMAX) is 0.339 microns
(vs 1.756 microns) for the primary and 0.178 microns (vs 0.590 microns)
for the secondary.

The Cassegrain has RoCs that are longer than the Gregorian, with sagittas of
4.8 mm (vs 8.5 mm) and 1.1 mm (vs 3.3).

In performance, the Cassegrain is somewhat better off-axis than the 
Gregorian.
The field curvature is also about half (RoC is -612 mm vs +311 mm).

Assuming a good method of testing the Cassegrain secondary, does this imply
that the Cassegrain wins hands-down over the corresponding Gregorian, or
is my comparison mixing apples and oranges?

-- 

Rick S.

http://users.rcn.com/rflrs


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