Light telescopes are of two basic types distinguished by whether the primary element is a lens or a mirror. Among telescopes of the first type the 40 inch refractor at the Yerkes observatory in Williams Bay, Wisconsin, is the largest. Note that the designation 40 inch refers to the diameter of the lens; the length of the telescope is more like 65 feet. In spite of its great length the 40 inch refractor is so delicately balanced that one person can aim the telescope manually.
Declination of a point on the celestial sphere is like latitude, measured from the celestial equator. Thus, if the declination of a star is +20 degrees, that star must be 20 degrees north of the celestial equator. The right ascension of a point on the celestial sphere is measured eastward from the vernal equinox, the point in the sky at which the Sun is located about March 20th. Thus, a star with a right ascension 14.3 h crosses the meridean (due South) 14.3 hours after the vernal equinox cross the meridean. The star Arcturus has declination +20 degrees, right ascension 14.3 h.
The hour-angle of a star is 0 h when that star is on the meridean. A star whose hour-angle is 1 h is somewhat further to the west, but that star was on the meridean one hour ago. Sidereal time is the R.A. of any object presently crossing the meridean. Thus,
If you look at the digital readouts on the Yerkes 40 inch refractor, the one which reads R.A. changes continually. If one looks through the eyepiece, the stars drift rapidly across the field of view, because of the diurnal rotation of the earth. One can, however, turn on a motor, or clock-drive, which increases the hour-angle at just the right rate in order to keep the R.A. constant. The telescope then tracks the stars toward which it is pointed.
If a sidereal day were precisely 24 of our customary hours long, the same star would always be on the meridean at midnight. Of course, this is not the case, because of the earth's motion about the Sun. One sidereal day, therefore, is only 23 h 56 m long.
Astronomical telescopes have equatorial mountings. One axis is aligned precisely parallel to the earth's axis. A sidereal clock drive rotates the instrument slowly about this polar axis. The other axis is the declination axis, which is locked once the desired star is within the field of view. (Illustrate using 8" Celestron.)
In a reflecting telescope the primary optical component is a paraboloidal mirror, usually made of glass, upon which has been deposited a thin layer of shiny aluminum. Light from a distant star is upon reflection converged to a point. In the case of a large telescope such as the 200 inch at Mount Palomar, an observer can sit right at this prime focus and view the star image with an eyepiece (magnifier). For an 8 inch telescope this is impractical because one's head would block all the starlight from entering the telescope. Therefore, one or another system of mirrors is used in order to bring the focussed image outside the body of the telescope. In the case of a Newtonian reflector, this is accomplished with a single small flat mirror. The Newtonian design, while very simple, is not very practical for huge instruments, because it places the observer at a very precarious position near the top. More practical is the Cassegrainian scheme, which utilizes a small convex mirro to reflect the light back through a small hole in the primary mirror. This design is employed for example in the case of the 200 inch reflector. There is an additional advantage of the Cassegrainian telescope, for it is uncommonly short and compact for its magnifying power.
Many other schemes have been tried during the history of telescope making. Recently Cassegrainian telescopes with correction lenses have become very popular.