Hallo,
In der Meade RCX Yahoo Gruppe hat Roland Christen (Optiker bei AP) einen schoenen Bericht zur Optik im RCX und LX200R geschrieben.
Clear Skies,
Gert
http://groups.yahoo.com/group/RCX400/message/8196
The RCX is not a reflector, rather it is a Catadioptric system. It has
a lens up front, which is nulled for perfect color error at the 70.7%
zone. For all other zones, the color will be slightly over or under
corrected. Basically the scope has spherochromatism with red being
overcorrected and blue-violet being undercorrected. Because it is
modeled after an RC formula, the corrector has a stronger curve with
the result that the spherochromatism is approximately twice that of a
normal SCT.
Based on measurements made by Paul Jones on a 14" RCX and my own
measurements on the 10" LX200R that I have here, the correction for off-
axis aberrations is quite good. In a classical RC, there are two
optical surfaces, which allows the designer only 2 degrees of freedom
to correct the Sidel aberrations. The two that are chosen are spherical
aberration and coma. What is left is astigmatism and field curvature.
The astigmatism in an RC, combined with its field curvature produces
oval stars in the corners of the format. Reducing these to an
acceptable level requires long primary focal ratios and large central
obstructions. Eliminating them completely requires a dedicated field
flattener.
In the RCX design, there are actually 4 optical surfaces, which allows
the designer extra degrees of freedom to correct the monochromatic
Sidel aberrations. The ones apparently chosen for the Meade RCX and
LX200R designs are shperical aberration, coma, and astigmatism. Yes, if
made right, the RCX will have no astigmatic stars in the corners of the
format. Field curvature will make the stars bigger (because of them
being slightly out-of-focus), but they will be round. Meade could have
chosen to eliminate field curvature also by aspherizing the primary
mirror, but this would have complicated the design greatly and
increased the cost. A simple field flattener will do the same thing.
Bottom line is that without any correctors, the Meade actually has
slightly better off-axis corrections than a classic RC, but it does
have sphero-chromatism which increases the size of stars at the ends of
the spectrum. Only the center green portion of the spectrum will be
sharpest. Assuming that the correction is indeed at the 70.7% zone,
then no amount of refocusing for the other colors will get them any
sharper. However, if the correction zone is off, then refocusing for
best red and best blue can be useful. Therefore I would not recommend
using color cameras for absolutely best results.
Rolando
