I would appreciate both theoretical as well as practical(from personal use) thoughts about this type of design scope: feel free to address issues such as focusing,collimation,cool down,visual,photographic and CCD useage and any other comments you'd like to make..Thanks ahead! Rich M.
?pros & cons to Ritchey Chretien design
Started by Rich Michal, 11/13/2002 04:07AM
Posted 11/13/2002 04:07AM
Opening Post
Posted 11/16/2002 07:53PM
#1
When compared to a classical Cassegrain, the only serious disadvantage of the RC is its price. Given systems F# and secondary magnification, there is no considerable differences between the two in their sensitivity to miscollimation (they are both considerably more sensitive to the primary-secondary spacing than systems with less strongly aspherised secondary).
Field-wise, the RC gets ahead with the increase in aperture
and/or lower secondary magnification. For instance, at 200mm f/14 and secondary magnification ~4 they both have aberrated blurs (coma length in the CC and astigmatic blur in the RC) nearly equal to the Airy disc diameter at ~0.4 degrees off-axis. From there, quality angular field diminishes in proportion to 1/D with a classical Cassegrain, and in proportion to 1/sq.rootD with the RC.
Photographic (linear) field is constant in the CC (0.025mm field given by F^2/7.5 in mm), while in the RC increases in proportion to D/sq.rootD, given secondary magnification
(0.025mm field is approx. f*sq.root(1/20Dm) in mm), "f" being the system's f.l. and "m" the secondary magnification.
Fact that the RC gets ahead quality field wise (both visual and photographic) with the increase in aperture is the main reason why the RC was a design of choice for large telescopes. At small apertures, it is not so much of a factor.
Note that even when the blurs are of similar size, the RC still have advantage due to astigmatic blur being, unlike comatic blur, symmetrical.
Field-wise, the RC gets ahead with the increase in aperture
and/or lower secondary magnification. For instance, at 200mm f/14 and secondary magnification ~4 they both have aberrated blurs (coma length in the CC and astigmatic blur in the RC) nearly equal to the Airy disc diameter at ~0.4 degrees off-axis. From there, quality angular field diminishes in proportion to 1/D with a classical Cassegrain, and in proportion to 1/sq.rootD with the RC.
Photographic (linear) field is constant in the CC (0.025mm field given by F^2/7.5 in mm), while in the RC increases in proportion to D/sq.rootD, given secondary magnification
(0.025mm field is approx. f*sq.root(1/20Dm) in mm), "f" being the system's f.l. and "m" the secondary magnification.
Fact that the RC gets ahead quality field wise (both visual and photographic) with the increase in aperture is the main reason why the RC was a design of choice for large telescopes. At small apertures, it is not so much of a factor.
Note that even when the blurs are of similar size, the RC still have advantage due to astigmatic blur being, unlike comatic blur, symmetrical.
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