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Hi Greg,
My opinion is that a CDK is not good for planetary imaging. There are several reasons for this. The most obvious one is the large size of the secondary obstruction diffracting more light into the diffraction rings and reducing theoretical resolution. A less obvious one is the thermal inertia of the large secondary assembly. It cannot be stressed enough how important good thermal control is in a planetary scope.
There is a subtle difference between an astrograph and a telescope that people tend to overlook: A telescope must be diffraction limited on axis (as a minimum) to be worth of its name, but an astrograph (even the best) does not necessarily have to be. That is why planetary imaging is done with telescopes and not astrographs. Of course, in theory, the CDK is meant to be diffraction limited on axis, but in practice, due to the manufacturing tolerances of the 6 optical surfaces and their spacing, it is highly unlikely to be. And it does not have to be because it is not used at the ridiculous sampling rate that a planetary scope is.
Stefan
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