David Knisely wrote:

The term "SCT" stands for "Schmidt-Cassegrain Telescope" which is a
noticably-different design from the Classical Cassegrain or the Newtonian.
It falls into the class of "Catadioptric" or "mirror-lens" telescopes, which
use *both* refractive optics *and* reflective optics to form their main images
(ie: lenses *and* mirrors). It is a derrivative of the Schmidt Camera, and,
like its predecessor, the SCT uses a full-aperture 'corrector plate' out in
front of the telescope to introduce an opposite form of spherical aberration
to that produced by the spherical primary mirror. The secondary mirror then
lengthens the focal length and directs the light back through a hole in the
primary mirror. The use of spherical optics and the special corrector plate
is an attempt to deal with off-axis aberrations like Coma. Without the
corrector plate, the images could be nearly useless, so the principle is not
the same as that used for pure reflectors like Newtonians and Classical
Cassegrain telescopes.

Do all or most SCT's use a moving primary mirror for focusing?


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