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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