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A telescope is an instrument that collects and focuses light. The nature of the optical design determines how the light is
focused. Some telescopes, known as refractors, use lenses, .and other telescopes, known as reflectors (Newtonians), use
mirrors.
Developed in the early 1600s, the refractor is the oldest telescope design. It derives its name from the method it uses to
focus incoming light rays. The refractor uses a lens to bend or refract incoming light rays, hence the name (see Figure 3-1).
Early designs used single element lenses. However, the single lens acts like a prism and breaks light down into the colors of
the rainbow, a phenomenon known as chromatic aberration. To get around this problem, a two-element lens, known as an
achromat, was introduced. Each element has a different index of refraction allowing two different wavelengths of light to be
focused at the same point. Most two-element lenses, usually made of crown and flint glasses, are corrected for red and green
light. Blue light may still be focused at a slightly different point.
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A Newtonian reflector uses a single concave mirror as its primary. Light enters the tube traveling to the mirror at the back
end. There light is bent forward in the tube to a single point, its focal point. Since putting your head in front of the telescope
to look at the image with an eyepiece would keep the reflector from working, a flat mirror called a diagonal intercepts the
light and points it out the side of the tube at right angles to the tube. The eyepiece is placed there for easy viewing.
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Newtonian Reflector telescopes replace
heavy lenses with mirrors to collect and
focus the light, providing much more light-
gathering power for the money spent.
Because the light path is intercepted and
reflected out to the side, you can have focal
lengths up to 1000mm and still enjoy a
telescope that is relatively compact and
portable. A Newtonian Reflector telescope
offers
such
impressive
characteristics you can take a serious
interest in deep space astronomy even on a
modest budget.
Newtonian Reflector
telescopes do require more care and
maintenance because the primary mirror is
exposed to air and dust.
small drawback does not hamper this type
of telescope's popularity with those who
want an economical telescope that can still
resolve faint, distant objects.
light-gathering
However, this
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