Forgive me, this is an extreme stupid question

Slone,

One more. Go to # This site will help you
immensely.

Don.

"SCT" is short for "Schmidt-Cassegrain Telescope", and refers to a
family of optical designs which feature (1) a concave primary mirror,
(2) a convex secondary mirror positioned a little way toward the
primary from where the primary comes to focus, and (3) a Schmidt-type
corrector plate, essentially the same diameter as the primary, located
somewhere beyond the convex secondary.

(1) and (2) together define the "Cassegrain" configuration. A
Schmidt-type corrector plate resembles a flat piece of glass to the
naked eye, but at least one surface departs from flatness by enough
to matter, to correct spherical aberration produced in the rest of
the system.

Commercially available Schmidt-Cassegrains have rather short, stubby
tubes. Celestron and Meade are the big manufacturers.

"ETX" is short for nothing, it is Meade's label for a small but
growing family of Maksutov-Cassegrain telescopes. Maksutov-Cassegrains
are like Schmidt-Cassegrains except that for them: ... (3) a Maksutov-type
corrector, essentially the same diameter as the primary, located at or
beyond the convex secondary. A Maksutov corrector has essentially the
same design function as a Schmidt corrector, but a different optical
principle. It resembles a quite thick glass salad bowl -- a "deep dish"
at the front of the tube. Occasionally, the convex secondary mirror will
be a metallized spot on the back (convex) surface of the corrector.

--

Jay Reynolds Freeman -- freeman at netcom dot com -- I speak only for myself.

Sct is a compact design that allows you to have large aperture in a scope
perhaps less than half the lenghth of the same aperture newtonian. The
Celestar 8 and Meade LX-50 are examples.

ETX is a slightly different compact design that came out recently. It is
essentially a maksutov - cassegrain ( same as above but with a longer focal
lenghth and slightly smaller secondary mirror). It is very popular, because it
is small enough to carry easily by hand, and has very sophisticated
computerized features for aiming the scope automatically at whatever you want
to see. The present one is somewhat limited because because it has relatively
small apeture of 3.5 inches, but now they have introduced a 5 inch model with
the same computer capabilities.

Hope this helps
Mark

Slone:

I don't remember of the FAQ lists all the acronyms used on this newsgroup,
but I encourage you to check it out. This is good advice for any "newbie"
on any newsgroup. Usually the "extreme stupid questions" are answered in
the FAQ for the newsgroup, if you can find it.

SCT = Schmidt Cassegrain Telescope.
ETX = Specific model of a Meade small aperture (3.5 inch) Maksutov
Cassegrain telescope. Currenty the most popular amateur telescope on the
market and perhaps the best value available today. Really a nice little
scope, especially for beginners.

Telescopes are currently generally broken down into three categories:

1. Refractors - The usual long tube with a lens at one end and an eyepiece
at the other. This is what most people think of when you say "I have a
telescope." It is perhaps the least common type of telescope actually used
by amateur astronomers for a variety of reasons. Galileo is the first
person generally credited with using this type of telescope to examine
astronomical targets.

2. Newtonian Reflectors - These are long tubes with a parabolic mirror at
the "bottom" and a small "secondary" mirror up the tube which reflects the
light perpendicular to the tube so you can peer into the side of the tube
with your eyepiece. As the name implies, Sir Isaac Newton invented this
type of telescope. This used to be the most common type of telescope used
by amateur astronomers, again for a variety of reasons. There are some
variations of Newtonian designs, but they are relatively rare. Perhaps the
most common modification of the basic Newtonian is the "Cassegrainian"
design, where the parabolic mirror has a hole drilled in its center and the
light is reflected back through that hole so you can aim the telescope much
like a refractor.

3. Catadioptrics - These are a combination of the Refractor and the
Reflector and there are many varieties of these. The two most common
variations are the "Schmidt-Cassegrain" and the "Maksutov-Cassegrain"
models. Both of these use spherical mirrors with a "correcting lens" in
front of the tube, with the mirror drilled in the center and the light
reflected back through the mirror (thus the "Cassegrain" in the title).
The difference between the Schmidt and the Maksutov is the design of the
correcting lens. The Schmidt uses a specially ground "flat" lens in the
front. The Maksutov uses a deeply curved lens. You can look in an optics
book if you want to examine the differences. In general the Maksutov
design allows for a smaller central secondary mirror, which allows for
slightly greater contrast. In general the Schmidt designs are larger in
aperture than the Maksutovs allowing for greater detail. Catadioptrics are
now the most commonly sold variety of telescope for amateurs.

The beauty (or horror) of telescopes is that ALL of them are basically
compromises each trying to balance a variety of limitations (called
"aberrations") and portability issues. Here is a quick rundown of the
compromises that is SURE to irritate the heck out of virtually everyone on
this newsgroup.

Refractors come in two varieties (usually), the "achromatic" and the
"apochromatic" although this has been blurred a bit lately with some
"semi-apochromatic" designs using Extremely low Dispersion" glass. Without
getting into too much detail, the basic problem with refractors is
something called "chromatic aberration." Designing that away is very hard
and very expensive. Basically you have two choices. Pay a reasonable
amount (say less than $500) for a refractor that shows significant violet
fringes around bright objects (an "achromatic"), or pay an exorbitant
amount (say well over $1,000) for a refractor that corrects the problem (an
apochromatic) design. Or go for a compromise that has smaller violet
fringes for about $1,000. The major limitation of refractors is size. A
7" refractor is a BIG refractor. A 7" SCT is a SMALL SCT. In general the
strength of a refractor is contrast. Since there is no secondary mirror
obstructing the light path, the images can be exquisitely sharp. For
planetary viewing (Saturn, Mars, Venus, etc.) contrast RULES, so many
amateurs accept the light-gathering limitations of refractors because
planets are bright enough that you can see fine detail with a 4" scope.

Newtonian Reflectors are usually considered to be the best value in pure
telescope viewing. You can get a very nice 8" Newtonian reflector for
$500. This would give you a pretty nice view of Saturn, for example,
probably blowing away a refractor costing perhaps ten times as much. You
sacrifice a little contrast due to the secondary mirror in the light path,
and a LOT of portability. The loss of contrast may make it difficult to
make out detail in the cloud belts of Jupiter (compared to an apochromatic
refractor), but the increased detail (due to larger aperture) might make up
for it. This is rich ground for debate on this newsgroup and at star
parties everywhere. The major problem with Newtonian Reflectors is
portability and mounting. A nice 8" scope is usually 48" long and close to
a foot in diameter. To mount this on a standard tripod is pretty hard, so
most Newtonians are on a "Dobsonian" mount which is nice, but difficult to
automate for star tracking, forcing you to manually move the scope as the
celestial objects scoot across your viewing field. Believe me, they scoot
pretty fast at 200 magnification...

Finally Schmidt-Cassegrains or Maksutov-Cassegrains are usually considered
to be the best compromise. They have virtually no color aberration since
almost all of the magnifications of the image is done by the mirror and
mirrors don't suffer from chromatic aberration. (Yes, I know there is
sphero-chromatism and possible chromatic aberration from the correcting
lens, cut me some slack here). This means that for general use you give up
some additional contrast, even compared to a Newtonian, but you get much
more portable packages and easier mounts.

There, that's about all I know about telescopes. I hope it helps.

-sdg

Hi Slone.
Well, an SCT is a *type* of telescope. It is the acronym for
"Schmidt-Cassegrain Telescope".
The ETX on the other hand, is a particular brand name scope made by Meade
Instruments. The ETX uses a design called the "Maksutov-Cassegrain",
similar to the SCT, but it uses a different type of corrector lens.

Hope this helps,

Phillip Hosey
LaGrange, GA

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Auburn Astronomical Society
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