Quote:
Originally Posted by glend
The Cassegrain has a smaller central obstruction ( say 30%), and is slower at f12
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I don't know about the GSO Classical Cassegrains, but a properly designed Classical Cassegrain will usually have a Central Obstruction under 30%, which would be a maximum sized Cassegrain Secondary when an F3 Primary was used. Classical Cassegrains historically have used about an F4 parabolic primary which normally would utilise a Secondary with a 20% to 25% CO. F12 would normally be about as fast as you would find them also, unless specifically designed for imaging. Although things are changing in this regard with more and more scopes being designed faster and around imaging. Many Classical Cassegrains historically were built around F5 primaries with small central obstructions and these made exceptional planetary visual and imaging scopes having about an F20 overall focal ratio.
Parks have sold a series of scopes called "HIT" for years, which are Classical Cassegrains which also have a Newtonian focus. They use an interchangeable secondary mirror to achieve this. You just change out the secondary and you go from a Classical Cassegrain to a Newtonian in a few minutes, so you have 2 telescopes in 1. Optically they are very good, but they are also very heavy. They used to make them in several apertures but only do the 16" now. This is a 16"/F4 Newtonian and a 16" F/18 Classical Cassegrain. The OTA alone weighs 118kg so it needs a serious mount, which is included in the $USD31,999 price tag.
I put a summary together years ago on the different attributes and design parameters of different compound telescope designs:-
The Classical Cassegrain Telescope (CCT) uses a parabolic primary and a hyperbolic secondary. It is an excellent design as it offers very little coma, the same as a Newtonian of the same FINAL f-ratio (so an
f20 CCT has the same coma as an F20 Newtonian). The field curvature
is however dependent on the focal ratio of the primary mirror, and
can be quite bad for designs with very fast primaries (< f3). An F20 system with an F5 primary makes an excellent system, convertable to wide field Newtonian views and high magnification Cassegrain views.
The Dall-Kirkham Cassegrain (DK) telescope has an elliptical primary and a
spherical secondary. The on-axis images are superb, but off-axis they
aren't great. They have about the same coma as a Newtonian with the same focal ratio as the PRIMARY MIRROR, not the FINAL f-ratio, as in a
classical cassegrain. So a DK with an f5 primary and a F20 final
focal ratio has the same coma as a f5 Newtonian. Which isn't bad, but
when you start getting down to f3 primaries you have a telescope better suited for narrow-field work only.
The Ritchey-Chretien Cassegrain (RC) telescope has a hyperbolic primary
and a hyperbolic secondary with the curves of the two mirrors
carefully chosen to simultaneously eliminate spherical aberration
and coma (to third order approximation). They have slightly more
astigmatism and field curvature than the classical cassegrain, but as
these are relatively minor annoyances, you get a wider diffraction
limited field than any reflector design (only the Petzval refractor
significantly betters it).
Another design called the Press Carmichael uses a spherical primary as in (SCT) and an ellipsoidal secondary. These aren't common and exhibit greater aberrations than the 3 forementioned designs.
As a Visual telescope a Classical Cassegrain with an F4 to F5 Primary, a small secondary obstruction and an overall F-Ratio of F16 to F20, is clearly the best choice. As an all round imaging scope an F8 to F12 Ritchey Chretien is the best choice.
Cheers
John B