Celestron PowerSeeker 21008 Instruction Manual page 11

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 cutaway view of the light path of the Refractor optical design
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.
Cutaway view of the light path of the Newtonian optical design
Figure 3-1
Figure 3-2
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
Newtonian Reflector telescope offers such
impressive light-gathering characteristics you
can take a serious interest in deep space
astronomy
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.
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compact and portable.
even on a modest budget.
A