First, there is the refracting telescope, the kind that takes advantage of the refraction of light through lenses to form images of objects. There are several kinds of refractors, but the simplest consists of two lenses: the objective or main light-gathering lens, and the eye piece, which magnifies the image. This is the kind of telescope Galileo used, and is still in use today. There are several basic kinds of refractors on the market: the achromatic, the apochromatic, and the refractor-style spotting scope.
The achromatic refractor is the least expensive instrument. It is traditionally a
two or three lens system made from crown and flint glass.
While the 60mm altazimuth or equatorial refractor is a popular choice as a first
telescope, you may want to consider an 80mm or 90mm refractor.
The glass is of higher quality, the coatings are better, and the additional light-
gathering will enable you to see brighter images in greater detail.
The extra cost may be worth it.
The apochromatic refractor uses either three or four lenses, each made of a
different material. There is a two element lens, one of which is
flourite, or there is a newer two-element design, made with extra-low
dispersion glass. These newer materials and designs allow for a system
which is virtually free of color fringing or chromatic aberrations. Faster focal
length refractors, which are desirable due to their portabililty,
also have the most stubborn color fringing, so the development of the
apochromatic design made it easier to take the refractor out into the
field. Apochromatic refractors are more expensive, but the views through them
provide a velvety black background and clear, sharp star images.
The Pros and Cons of Refractors
Pros: They provide sharp images. They have longer focal lengths, which
provide better contrast, while shorter APO focal lengths give greater
contrast and portability. As there is no central obstruction, there is no power
that is too low. The rugged construction and the closed tube prevents dust and
moisture from entering the system. The lenses very rarely need to be aligned.
Smaller models are less susceptible to thermal currents. They make a good
spotting scope because the image is right side up. The new apochromatic
design, with shorter focal lengths of four inches or less are extremely portable.
Cons: Over 90mm, they become expensive. All refractors except the more
expensive APO designs have at least some chromatic aberrations, such as
purple fringing around bright objects. There is a narrow field of view in the
longer focal lengths. The long focal length refractors become very cumbersome
in apertures over 90mm, and need a heavy mount.
Altazimuth Mounts
Altazimuth mounts, also called "Alt-Az," are the simplest to operate, and work best for spotting scopes and telescopes used primarily for daytime viewing. These mounts move the optical tube in straight lines, either up and down or back and forth, and include the tripod. Some alt-az mounts have slow motion control cables to help the viewer move the telescope slowly and smoothly in any of these directions. On alt-az mounts, the control cables hang down for easy access, or else exist as knobs or dials on an equatorial mount.
Equatorial mounts
Equatorial mounts allow the viewer to follow objects as the earth rotates, which
causes these objects to seem to move through the sky. The mount is set at the
viewer's latitude, and then is polar-aligned to either the North Celestial Pole in
the Northern Hemisphere or the South Celestial Pole in
the Southern Hemisphere. After that , the viewer needs only to move the
telescope, usually by way of its slow motion control cables or dials, in
either right ascension, or east-to-west movement of the stars or declination, or
north-south movement of the stars to follow a planet or
deep sky object as it slowly moves through the field of vision. This type of
mount is more bulky than the altazimuth design, but it is desirable
because the viewer can usually attach an optional motor to the mount,
counteracting the earth's rotation automatically.