Astronomical Observations by Britannica Educational Publishing

Author:Britannica Educational Publishing
Language: eng
Format: epub
Publisher: Britannica Educational Publishing
Published: 2010-10-15T00:00:00+00:00

The 72-inch reflecting telescope at Birr Castle, County Offaly, Leinster, Ire., was the largest in the world at the time of its construction in the 1840s. Geray Sweeney/Tourism Ireland

Reflecting telescopes have a number of other advantages over refractors. They are not subject to chromatic aberration because reflected light does not disperse according to wavelength. Also, the telescope tube of a reflector is shorter than that of a refractor of the same diameter, which reduces the cost of the tube. Consequently, the dome for housing a reflector is smaller and more economical to construct. So far only the primary mirror for the reflector has been discussed.

One might wonder about the location of the eyepiece. The primary mirror reflects the light of the celestial object to the prime focus near the upper end of the tube. Obviously, if an observer put his eye there to observe with a modest-sized reflector, he would block out the light from the primary mirror with his head. Isaac Newton placed a small plane mirror at an angle of 45° inside the prime focus and thereby brought the focus to the side of the telescope tube. The amount of light lost by this procedure is very small when compared to the total light-gathering power of the primary mirror. The Newtonian reflector is popular among amateur telescope makers.

A contemporary of Newton, Laurent Cassegrain of France, invented another type of reflector. Called the Cassegrainian telescope, this instrument employs a small convex mirror to reflect the light back through a small hole in the primary mirror to a focus located behind the primary. Some large telescopes of this kind do not have a hole in the primary mirror but use a small plane mirror in front of the primary to reflect the light outside the main tube and provide another place for observation. The Cassegrain design usually permits short tubes relative to their mirror diameter.

One more variety of reflector was invented by another of Newton’s contemporaries, the Scottish astronomer James Gregory. Gregory placed a concave secondary mirror outside the prime focus to reflect the light back through a hole in the primary mirror. Notable is the fact that the Gregorian design was adopted for the Earth-orbiting space observatory, the Solar Maximum Mission (SMM), launched in 1980.

Most large reflecting telescopes that are currently in use have a cage at their prime focus that permits the observer to sit inside the telescope tube while operating the instrument. The 5-metre (16-foot) reflector at Palomar Observatory, near San Diego, Calif., is so equipped. While most reflectors have equatorial mounts similar to refractors, the world’s largest reflector, the 6-metre (19-foot) instrument at the Special Astrophysical Observatory in Zelenchukskaya, Rus., has an altitude-azimuth mounting. The significance of the latter design is that the telescope must be moved in both altitude and azimuth as it tracks a celestial object. Equatorial mountings, by contrast, require motion in only one coordinate while tracking, since the declination coordinate is constant. Reflectors, like refractors, usually have small guide telescopes mounted parallel to their main optical axis to facilitate locating the desired object.

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