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There are three basic telescope designs that pertain to the pursuits
of amateur astronomers. They are refractors, Newtonian reflectors,
and catadioptrics.
Refractors
The refractor is the design most people envision when they think
of a telescope. They are, generally, long, thin telescopes that
use a lens at the front of the telescope tube to collect light.
The observer looks through an eyepiece at the back of the telescope,
where the light is focused, forming an image. Refractors are highly
regarded for their sharp, high contrast images. They are best suited
for viewing the moon and planets.
Newtonian Reflectors
Newtonian Reflectors use a highly polished, curved mirror at the
rear of the telescope tube to collect light. This mirror reflects
and converges the light, where it is intercepted by a small mirror
at the top end of the telescope tube. In turn, the light is reflected
into a focuser and eyepiece mounted on the side of the tube. As
such, the observer is positioned at the top of the telescope. They
are generally regarded as the best all-around telescopes, as their
large apertures gather plenty of light, and allow for good planetary
views, all at a moderate price.
Catadioptric
Catadioptric telescopes use compound optical systems. That is,
both mirrors and lenses are employed to collect and focus incoming
light. The observer peers through an eyepiece at the rear of the
telescope tube, where the light is focused. The two most commercially
available catadioptric designs are Schmidt-Cassegrains and Maksutov-Cassegrains.
They offer excellent portability, as the optical tubes are compact
in design, as well as very good optical quality. They are the most
popular type of telescopes for astrophotography. They tend to be
more expensive than reflectors, and less expensive than refractors
of the same size.
Adhere to the American Eyepiece Size Standard
There are two size standards associated with telescope eyepieces:
Japanese and American. Eyepieces built to the Japanese standard
have a barrel diameter of 0.965". Those built to the American standard
have a barrel diameter of 1.25". Generally speaking, inexpensive
beginners' telescopes are usually outfitted with Japanese standard
eyepieces. As such, telescopes in this category have 0.965" focusers,
or eyepiece holders. Much more desirable are telescopes that are
designed to accept American standard eyepieces. These scopes are
generally built to better standards, and are able to utilize much
better quality eyepieces.
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