Page 286 - NS-2 Textbook

ASTRONOMY 281

The U.S. Naval Observatory's installation at Flagstaff, Arizona. The building shown houses a sixty-one-inch reflecting telescope.

fracting and the reflecting telescopes. Both types can be fit- the objective inlage. The eyepiece lens then magnifies this
ted with spectrographs to photograph the color spectrum image, making the object appear closer and enlarged.
of incorrUng light. The largest refracting telescope in the world is lo-
TIle magnifying power of a telescope is important cated at the Yerkes Observatory at Williams Bay on Lake
only in observing nearby celestial bodies, since the stars Geneva, Wisconsin. Operated by the University of
cannot be magnified. In observing the stars, the light- Chicago, this refractor has an objective lens with a diam-
gathering power of the telescope becomes all-important. eter of 40 inches (102 centimeters).
The amount of light a telescope can collect depends en- Because a lens can be supported only around its
tirely on the area of its main lens or mirror. The larger the edge, the size and weight of the lens itself produces un-
lens or mirror, the brighter the star will appear. avoidable distortions in the shape, which in turn affect
The refmcting telescope uses two lenses. There is a sin- the image. TIle 40-inch Yerkes refractor thus represents
gle convex (outwardly curved) lens called the objective the practical upper size limit for this kind of telescope. To
lells at the end of the telescope. TIlis lens forms a reduced, get arOlmd this limitation the reflecting telescope was de-
inverted inlage of the celestial body being viewed called veloped. Sir Isaac Newton is credited with developing
the first one in 1672. This type telescope uses an objective
mirror in place of the objective lens. This slightly concave
(inwardly curving) mirror forms an inlage which is then
reflected by a secondary mirror to where the eyepiece
magnifier is mOlmted.
There are two types of reflecting telescope. In the
Newtollian refiectO/; a flat secondary mirror reflects the
A refracting telescope. light and brings it to a focus at the side of the telescope.