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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.
Figure 3-1
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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.
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
light-gathering 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.
However, this 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.
Figure 3-2
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