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Telescopes
Astronomical telescopes come in a wide variety,
ranging from the plastic tube sold in toy stores to the famous Hubble
telescope flying in the sky. Here, we only focus on the most basic information
necessary for a backyard astronomer.
An astronomical telescope is used to collect light from a celestial
object and produce a magnified image. There are two basic types of optical
telescopes: refractor and reflector. The former uses lenses
and the latter uses mirrors. The primary lens or mirror that focuses
light is the objective. Both kinds use an eyepiece to
magnify the image formed by the objective. In astronomical observations,
refractors are commonly used for lunar and planetary studies, while
reflectors are preferred for extragalactic studies.
In addition to these two common types, there are a number of other
designs which use a combination of lenses and mirrors to take advantages
of both refractors and reflectors, such as Schmidt-Cassegrain telescopes
and Maksutov telescopes.
reflectors
The reflecting telescope (shown above) was first
designed by Isaac Newton in 1668. Here, a primary parabolic mirror reflects
light rays to an inclined flat mirror, which, in turn, reflects the
rays to an observer located at the side of the tube. This design has
essentially remained the same today in most amateur reflectors.
The most attractive feature of a reflector telescopes is that it provides
the lowest dollar/aperture ratio of any telescope type. Therefore, for
the same money, a reflector gathers most light, which is important in
deep space viewing. Small amateur reflectors, such as a 3" (76
mm) or a 4.5" (114 mm) are also light enough for easy transportation.
Because of the open tube design, a reflector requires alignment and
cleaning once a while. The image from a reflector is usually upside-down
and is therefore not suitable for terrestrial viewing.
refractors
The refracting telescope (shown above) was invented in 1609 by Galileo
Galilei who used it to discover Jovian satellites, lunar mountains,
et al. Galileo's refracting telescope suffered from a number of defects
in image formation, particularly aberrations, due to the use of simple
lenses. Modern optical telescopes use compound objective lenses to minimize
aberrations. The compound lens consists of one convex lens of crown
glass and one concave lens of flint glass. In addition, all lenses are
coated with anti-reflective coatings to maximize transmission. In common
designs for amateur astronomy, light from the objective lens is often
turned 90 degrees by a flat mirror before entering the eye piece.
Refractors provide the best image quality among amateur telescopes,
thanks in particular to the compound objective lens. Images of planets
and the Moon are extremely crispy and sharp. They are also used for
viewing objects on land, when an erecting eye piece is used. In addition
to their high performance, refractors are also maintenance-free and
very portable due to their light weight and compactness. Thus, refractor
has always been one of the most favorable among amateur astronomers.
The disadvantage of a refractor telescope is that it is the most expensive
in terms of dollar/aperture ratio. However, thanks to the web, you can
now directly buy from the factory at very affordable prices.
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