Thomas M. wrote:

The reflector in the article is not an SCT. It is a simple Cassegrain,
possibly Classical or Gregorian, as the Gregorians were often resorted to
during the era of the "brass" telescope (the tube length argues for a
Gregorian). The SCT is a different and much more recent design (circa

1962)

using spherical optics and a large corrector plate. Clear skies to you.
--



Some of you have a real bad habit of splitting hairs...the concept or
priciple is the same!


No, I'm afraid that it is not really "splitting hairs" at all. Its a matter
of learning and *correctly* using the standard terminology of Astronomical
equipment which has been established at least for the past 40 years if not
longer. The principle behind the SCT is different than that of the purely
reflecting telescopes like Newtonians and Cassegrain instruments, and it was
not the principle used for the telescope noted in the article.
The first reflector telescope design was the Newtonian (invented in
1668), which uses a parabolidal primary and a smaller flat "diagonal"
secondary mirror to send the light collected by the primary mirror out a hole
in the side of the tube. The special parabolidal shape is necessary to form a
fully-corrected image. The second reflecting telescope design was the
Cassegrain (proposed by Guillaume Cassegrain in the year 1677), which uses a
convex secondary mirror to send the light back through a hole in the main
mirror. Like the Newtonian, the design is *purely* a reflector, as it uses
*only* reflective optics (a paraboloidal primary and a hyperboloidal
secondary). The Gregorian is similar to the Classical Cassegrain, but uses a
concave secondary mirror behind the primary focal point to redirect the light
through a hole in the main mirror. These telescopes are *purely* reflective
or "Catoptric" instruments, since they only use mirrors to form their primary
images (except for the eyepiece). They rely only on reflective optics to
create an on-axis image which is corrected for things like spherical aberration.
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. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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