FAQ - Catadioptric
A Catadioptric telescope is the generic term for a closed-tube
system where a combination of mirrors and lenses are employed to form an image.
There are two common Catadioptric (otherwise known as "Cat") telescopes on the
market today, the Schmidt-Cassegrain and the Maksutov-Cassegrain. Both are
described in detail below.
Schmidt-Cassegrain
The Schmidt-Cassegrain telescope has a complicated history. In 1672, M.
Cassegrain invented a reflecting telescope which utilized a concave primary
mirror that gathers light from the sky and reflects it up to a convex,
adjustable secondary mirror. The light then reflects down through a hole in the
primary mirror to the "Cassegrain" focus. This design helps to cancel out coma,
a common problem of Newtonian Reflectors. It also places the focus at a more
symmetrical position than Newtonians, allowing for easier installation of
additional equipment such as cameras and their various adapters.
The
"Schmidt" part of the Cassegrain system has to do with a specially-shaped
corrector plate of complex design that "caps" the front end of the tube
assembly. This corrector plate is coated, much as any good camera lens or
binocular optical system is, to allow light to transmit as undisturbed as
possible to it's ultimate destination... your eye.
When you put the
"Schmidt" and the "Cassegrain" together, you get a telescope that is very
compact for it's aperture size but that yields deep-sky and planetary views of
extreme quality. The Schmidt-Cassegrain telescope is among the most popular of
the telescope designs today, simply because of these attributes and the fact
that an amazingly wide variety of accessories is available for this
most-favorite of telescopes. The most common sizes available today are the 5"
spotting scope, which fits on a photographic tripod, and the 5", 8", 10", 11",
12", 14" and 16" astronomical telescope.
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Maksutov-Cassegrain
There are only a
few Maksutov-Cassegrain's to be found on the market today, but since one has
just been introduced by Meade Instruments (a 7" f15) and is getting a lot of ad
space in Sky & Telescope and Astronomy magazines, they are worth a mention
here. Simplistically put, what makes the telescope a "Maksutov" instead of a
"Schmidt" is, you guessed it, the corrector plate. Instead of an aspherical
corrector plate like the Schmidt design, a "meniscus" (meaning
"crescent-shaped") lens is incorporated. This corrector plate is shaped like a
shallow bowl, curving inwards, and has a reflective spot on the corrector itself
which acts as a secondary mirror. Because this corrector lens is slightly
diverging, it can be placed nearer the primary mirror, so a shorter tube can be
used in comparison with the Schmidt-Cassegrain design.
Meade's Maksutov
telescope, for instance, is a 7" f15 model, but the tube is only slightly longer
than the 8" f10 Schmidt-Cassegrain. Because it is f15, the contrast is better,
and so one will notice darker background sky than with a "faster" telescope. In
other words, this Maksutov is like a refractor/Cassegrain combination! The best
of both worlds? Maybe. The jury is still out on that one, but theoretically, it
should come close.
You will notice a very wide price range when shopping
for a Schmidt-Cassegrain or Maksutov-Cassegrain telescope. You may see several
8-inch "SC" models, for instance, ranging in price from $1000 to well over
$2500. These telescopes will all see the same thing, so what are you paying all
that extra money for? I have got two words for you, mounts and electronics.
Let's dissect these money-munching creatures below:
There are two basic
kinds of mounts available for Schmidt-Cassegrain or Maksutov-Cassegrain
astronomical telescopes. Not all of the sizes listed above are available on all
mounts, but with a little creative shopping with a reputable dealer, you can put
together just about anything!
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Fork Mount
The fork mount holds the
optical tube assembly of the telescope suspended between two arms so that it may
swing freely up and down and through the arms for easy movement in declination.
The "arms" resemble the tines of a tuning fork... hence the name. The forks are
mounted in turn on the drive base, a horseshoe-shaped "box" where the
electronics of the telescope are safely kept. The forks rotate in right
ascension from a pivot point in the center of the drive base. The drive base is
then either attached to an equatorial wedge via removable bolts for easy
breakdown of the unit, or it is attached via a central bolt directly to the
tripod base for altazimuth operation (only Meade LX200 telescopes come standard
this way). The wedge on this type of mount is what allows for adjustments in
latitude so that your telescope can be polar-aligned (see Equatorial Mount for
more information). The LX-200 has computer electronics that allow the telescope
to track the movement of stars for a limited time without the use of a wedge...
this is a big breakthrough in telescope design. Tripods come standard with some
telescopes, and are priced optional with others... make sure and read the print
on ads when shopping or ask your dealer for clarification.
German Equatorial
 This is the same mount described
under Refractors (see Equatorial), but I
would like to make a few additional comments here. In order to hold the weight
of a Schmidt-Cassegrain or Maksutov-Cassegrain (or a large refractor or
Newtonain reflector, for that matter), the mount would of course have to be much
more heavy-duty than one that is meant to hold a 60mm, 80mm, or 90mm refractor
or a 4 1/2" reflector. They are generally machined to much higher tolerances,
and the tripods that accompany these mounts are of the same quality as a good
tripod that comes with a fork-mounted system.
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Electronics
Here is the real culprit if
you are looking for something to blame come checkbook-balancing time...
electronics. On fork-mounted Schmidt-Cassegrain or Maksutov-Cassegrain
telescopes, the least you will get electronically is a right ascension motor
drive. This motor will automatically compensate for the earth's rotation, and so
will "track" stars as they move from east to west if the telescope is
polar-aligned correctly. There are also telescopes available with dual-axis
drive, which means that motors compensate not only for east-west movement but
north-south (declination) movement as well. With a dual-axis drive telescope,
you usually get a hand controller as standard equipment. With this little box,
you can move the telescope in all four directions with the touch of a button
(one direction at a time, please!). This comes in handy when you want to take
deep-sky pictures (anything out of our solar system) and need to nudge the
telescope a little bit if wind or less-than-perfect alignment causes the object
you are photographing to move in your camera's viewfinder (this is a very
simplistic statement, but will be covered further in the astrophotography section).
There are
also telescopes on the market that have more sophisticated electronics on board.
Some have a computerized pointing system with a large database of objects to
access. These pointing systems (there are several brands out there... examples
include Celestron's Advanced AstroMaster, Meade's Magellan system, and Orion's
Sky Wizard) literally do what the name implies; they guide you to the object you
have chosen from the database and let you know when you have reached your
destination. These systems, of course, are dependent upon your aligning them
correctly in the first place, but they are easy to use and are a great help in
traversing the Great Wide Open.
Finally, there is a telescope on the
market today that not only guides you to the object, but delivers it to your
"door". It is the Meade LX-200, and has all the bells and whistles you could
hope for (and probably more). A database of 64,360 objects, including all of the
planets and the Moon (!), is accessible through the computer installed in this
wonder. After a guided set-up, which is extremely easy to do, you chose an
object by punching it into the keypad, push "Go To", and viola! The telescope
moves to that object and centers it in the field of view... and beeps when it is
ready for you to look into the eyepiece!
So you see, there really is
something for everyone when it comes to telescopes! Well, except for one thing.
I keep getting asked about that big telescope that is very high in quality for a
low, low price... haven't been able to provide that one yet, but if I find one,
you can be sure it will be an overnight addition to this web page!
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