Page 285 - NS-2 Textbook

Astronomical Observations

Until the twentieth century, observations of the heavens the weather affords a maximum number of clear nights
were made visually from Earth's surface, either with the with "steady atmosphere."
naked eye or, after the Middle Ages, with an optical in- The distinguishing feature of an observatory is its
strument called the telescope, a device that magnifies the great revolving dome. Through a slit-like opening in this
image of distant objects. But Earth-based observations of dome, the telescope peers into the night sky. Except for
celestial bodies with an optical telescope, however large the hum of motors and the click of switches, all is quiet as
and refined it might be, are hlndered by Earth's atmos- the astronomers direct the telescope at the desired spot in
phere. Sometimes the atmosphere makes observations the heavens. The whole dome can be made to llll'n to
impossible, as on a cloudy night. Even at the best of point the telescope at stars or planets anywhere in the sky.
times, it will cause distortion of the incoming light from The environment in the dome must be exactly as it is
the body, making it impossible to observe very detailed outside. It must be dark, so that the time-exposlU'e pho-
surface features of even the nearest bodies, like the Moon tography will not be interfered with in any way. TIle tem-
and the planet Mars. perature in the dome must be the same as outdoors, with
Fortunately, twentieth-century technology has pro- no extra heat, since any such warm air would affect the
vided methods of observation of the heavens that are telescope's lenses and mirrors, blurring the photography.
far better than the Earth-based optical telescope. These Thus in winter the astronomer must wear heavy clothlng
include the spectrograph, radiotelescopes, balloon as protection from the cold.
and spacecraft-borne telescopes, and, since the 1960s, Today the telescope and its fine cameras are usually
manned spacecraft. Each of these will be discussed operated by computers. The astronomical photographs
below. are taken on sensitive photographic glass plates instead
of on film. Glass plates do not curl and can be stored and
handled with greater ease. Time exposures are used be-
THE TELESCOPE cause the plate must store up the feeble light received
from the stars, perhaps for hours. Such time exposures
Because of its wide availability and relatively low cost,
reveal the movement of the planets, asteroids, meteors,
the traditional telescope will always have a place in as-
and comets against a background of stationary stars.
tronomy. There are many different sizes and types of op-
The astronomer is much more than a mere Ustar-
tical telescopes. They range from portable models de- gazer." When working with optical equipment in an ob-
signed for the amateur, a few inches in diameter and a
servatory one must be an electronics technician, photog-
couple of feet in length, to giant reflecting telescopes
rapher, and computer operator. During the day the
with computer-driven aiming machinery mOlmted in
astronomer must be a mathematician, physicist, chemist
buildings called observatories. These are used primarily
mechanic, research analyst, and office manager. The li-
by professional astronomers, who make most of their ob-
brary of the observatory maintains a filing system of
servations by means of time-exposure photography,
photographs and written records. Research and labora-
rather than visual sightings.
tory work goes on every working day, and in observa-
If we were to take a trip to an observatory dlU'ing
tOlY shops new ash'onomical instnunents are continually
"working" holU's, it would be at night. We probably
being developed.
would have to drive up a high hill or even a mountain to
get there. The large research obsen'atories are located in
remote places away from the lights, smoke, and smog of TYPES OF TELESCOPES
the cities. On the mountaintop the air is thinner and
clearer, eliminating as much atmospheric haze as possi- The telescope is the most important object in the obser-
ble. Ideally, the observatory is built in a location where vatmy. There are two principal types of telescopes: the I'e-
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