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Petr, for observing the planets, in increasing order of quality:
Short-focus Newtonians (typically f/5 or less) are intended for wide-field imaging, not high magnification. Adding barlows and/or ridiculously short eyepieces is about the worst solution for planetary observing.
(ast Newt = poor choice)
SCT's at f/10 are "all-rounders', able to show pretty much anything but they aren't the optimal solution for anything, either - f/10 is too slow to be much good for imaging, and not long enough to excel at high magnification. Worse still, they have a very big secondary mirror which degrades image quality, and usually have barely adequate optics - most SCT's I have looked through do not produce an acceptable diffraction pattern on a bright star.
(SCT = poor choice)
So... you want long focal length, small secondary obstruction (or none), good optics with the minimum number of surfaces, and without using barlows:
Maksutov's in the range f/15-f/20 are usually the choice for planetary observers wanting high magnification in a compact easy-to-transport scope. Small secondary coupled with optics from a good manufacturer generally results in an excellent image, pretty much the equal of a good refractor of the same aperture and focal ratio.
(Mak = better choice)
Long focal ratio newtonian - at least f/8 and ideally f/10 - these do give excellent high-power images though big and bulky to use. The usual problem is that the mount needed is big, expensive and heavy. I had an f/8 20cm for a while, but its sheer size forced me to part with it at a time when I simply didn't have the space for such a big scope. Another example is the 16" f/7 which ASNSW had at Mt Bowen - this was an exquisite scope for observing the planets, but definitely not portable.
(Long FL newtonian = very good)
Refractors - for a given aperture, a long focus refractor will pretty much outperform everything else, except for one telescope design (below). in this respect I mean an ED APO with a focal ratio not less than f/10, and preferably f/15. Big, heavy, and hideously expensive once you talk about 15 cm apertures or more. A good mount is also a big problem.
(big refractor = excellent choice)
Schiefspieglers = a two or three mirror telescope with folded optics offering long focal ratio (f/20 or more) with an unobstructed light path, folded into a compact box. These look pretty weird but easily give the best images bar none.
Some years ago Barry Adcock (ASV) showed some images of Mars and the moon that he took using a home-made 12" schiefspeigler that looked like they came from the Hubble.
(Schiefspiegler = best choice)
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