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are renowned for providing superb views of the planets, Moon, and
double stars. They are ideal scopes for terrestrial observation too.
Refractors have a long, relatively thin tube with a main (objective)
lens at the front that focuses the incoming light, which is then redirected
to a comfortable viewing angle by a diagonal mirror or prism (optional).
The image is then magnified by a small eyepiece lens. Refractors are
rugged, require practically no maintenance, and are highly portable
in smaller apertures.
While more expensive refractors
are prized for their high-contrast, razor-sharp images, the inexpensive
variety commonly sold in department stores should be avoided. They
generally suffer from low optical quality, poorly fitting plastic
parts, and spindly mounts that wobble at the slightest touch, ruining
the view. Claims for high magnification-300x or higher-are deceptive
and a sure sign of an inferior product. The practical limit (for
any telescope) is about 50x or 60x per inch of aperture, or 144x
for a typical 60mm (2.4-inch) scope. Going any higher will provide
only dim, fuzzy images. Magnification is not related to quality
A good-quality refractor
will have at least a two-element (achromatic) objective lens and
sturdy mechanical construction. Fittings for the eyepiece, diagonal
prism, and other accessories should be made of machined metal, not
plastic. The tripod can be wood or aluminum, as long as it's stable.
The Newtonian reflector
(named after Sir Isaac Newton, its inventor) is a popular and economical
astronomical telescope. Its simple high-performance design provides
tremendous light grasp at the lowest cost per unit of aperture of
any type of telescope. It works by reflecting incoming light off a
large, curved "primary" mirror at the base of the optical
tube up to a smaller, flat "secondary" or "diagonal"
mirror near the front end of the tube. The secondary directs the narrowed
light cone out of the tube and into the eyepiece. Small Newtonians
are very portable because the tube detaches from the mount in seconds.
Larger Newtonians can be quite
large and unwieldy, which is part of the reason more compact scope
designs such as the Schmidt-Cassegrain have gained popularity. But
reflectors have much to recommend them. Besides their low cost,
they're pretty easy to make for the do-it-yourselfer. Optical quality
can be very good. The mirrors are adjustable, so you can keep the
optics in perfect alignment (collimation).
Their large apertures make
reflectors ideal for observing galaxies, star clusters, and nebulas.
They also provide sharp, high-contrast planetary and lunar views.
Dew does not readily form on the mirrors because the open tube acts
as a passive dew shield. But because the optical tube is open to
the air, it should be capped when not in use to prevent accumulation
on the mirrors of dust and dirt. Newtonian tubes can be supported
on a variety of different mounts: the standard altazimuth mount
on a tripod, an equatorial mount, or on a cabinet-style "Dobsonian"
mount. Reflectors are not well suited for terrestrial observing,
because the image is often rotated upside-down or sideways.
The compact design and versatility
of the Schmidt - Cassegrain telescope has made it the most popular
telescope type among amateur astronomers. The optical tube is much
shorter and, thus, more lightweight than that of a Newtonian reflector
of similar aperture and focal length because the light path is "folded"
inside the Schmidt - Cassegrain. Light enters through a "corrector"
lens at the front of the telescope and is reflected off the primary
mirror to an adjustable, magnifying secondary mirror on the inside
of the corrector lens. The light beam is then directed out the back
of the tube to an eyepiece.
The Schmidt - Cassegrain
design was made commercially economical in the late 60's
by the production innovations of California telescope maker Tom
Johnson. His techniques for mass-producing the Schmidt corrector
lens were the foundation for Celestron's affordable and wildly successful
8" f/10 "C8," introduced in 1970. This complex-curved
front lens corrects for optical aberrations, and also seals the
telescope tube, which helps to keep the internal optics clean.
For visual observation, the
Schmidt - Cassegrain is an all-around strong performer. Optically
and mechanically, it is ideally suited for astrophotography-from
simple piggyback shots to extended, through-the-scope exposures
of deep-sky objects. Its compactness allows it to be mounted on
a lightweight fork-style equatorial mount, an option not available
to long-tube conventional reflectors. As for price, Schmidt - Cassegrains
fall in the middle between reflectors, which cost less, and refractors,
which cost more, per inch of aperture.
Like the Schmidt - Cassegrain,
the Maksutov - Cassegrain is a catadioptric, or compound, telescope,
combining features of both reflectors and refractors
in a compact body. It uses a combination of mirrors and lenses, most
notably a deeply concave front ("meniscus") corrector lens.
This lens corrects the optical aberrations of the spherical primary
mirror to yield sharp images over a wide field of view. Light passes
through the corrector to the primary mirror and is reflected to an
aluminized spot on the back side of the corrector, then exits out
through an opening in the primary mirror to the eyepiece The Maksutov
design was introduced by the Russian optical scientist D.D. Maksutov
in 1941. "Maks" have a desirable closed-tube design, are
highly portable, and are nicely adaptable for photography.
The most popular Maks are 90mm-diameter
spotting scope models. They're best suited for terrestrial viewing
and photography, but can also be used for casual astronomy. Larger-aperture
models are difficult to manufacture (and thus are very expensive),
and their thick glass corrector lenses take a long time to reach thermal
stability at night.
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