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Chapter 10 The Mounting 1
Everyone is more-or-less familiar with the surveyor's transit, in which two axes at right angles to each other enable the telescope to be aimed at any point in the celestial dome. Its motions are one in azimuth, about the perpendicular axis, and one in altitude, about the horizontal axis.
The names of these motions have been
combined in the term Altazimuth, which is used to describe this type of mounting. When the telescope is swung through 360° in azimuth, the field of view traverses an orbit about the zenith, whereas the stars, in their daily or diurnal motion, describe orbits around the celestial pole.
However, if the instrument is tilted so that its perpendicular axis points to the celestial pole, instead of to the zenith, the former azimuth motion of the telescope will exactly follow the paths of the stars. This is then described as a motion in right ascension; its axis of rotation, parallel to the axis of the earth, is called the polar axis. The former altitude motion has now become one in declination, turning on the declination axis. This is now an equatorial mounting, taking its name from the great circle basic to this system of co-ordinates, the celestial equator.
When the telescope is mounted in this way, objects too faint to be seen with the eye can easily be found. Suppose, for example, we wish to observe the planet Neptune, assuming it to be above the horizon. We find its position (right ascension and declination) in the American Ephemeris. Then we swing the telescope on its
declination axis so that it points the required number of degrees north or south of the celestial equator, and clamp it. Next we turn the telescope on its polar axis until the correct hour angle is read on the hour circle, and Neptune should be in the field of view. Of course, this procedure is possible only on an accurately adjusted telescope, equipped with setting circles, and if the sidereal time is known or computed for the time of observation. (See Chapter XII.)
There is one feature that must be emphasized in the design of a telescope mounting, and that is rigidity. Nothing is as abominable and useless as a telescope which trembles in the slightest breeze. Take a stick from a fire and whirl its glowing end before the eye in all sorts of gyrations — that is what the image of a star
looks like in a weakly mounted telescope. Remember that any shake or vibration is amplified by the magnification being used, and what may appear to be a quite stable instrument may not prove so when you are trying to observe the Cassini division in Saturn's ring with a 1/4" eyepiece. Large size, smoothly operating
bearings, well-fitting parts, and a sturdy support are required in an efficient mounting.
Next- The Mounting part2
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