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Celestron PowerSeeker 60EQ - 21043 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
Cutaway view of the light path of the Newtonian optical design
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