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Celestron AstroMaster 90 EQ-MD Manual

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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.
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 3-1
A cutaway view of the light path of the Refractor optical design
Figure 3-2
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