Manual Celestron AstroMaster LT 76AZ (page 9 of 25) (English)

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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 incom-

ing light rays, hence the name (see Figure 3-1).

Early designs used single element lenses. How-

ever, the single lens acts like a prism and breaks

light down into the colors of the rainbow, a phe-

nomenon 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 differ-

ent 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 mir-

ror 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 diago-

nal 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 inter-

cepted 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 3-1

Cutaway view of the light path of the Newtonian

TELESCOPE BASICS

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Celestron AstroMaster LT 76AZ Manual

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