- Key People:
- Lewis Morris Rutherfurd
- Giuseppe Campani
- On the Web:
- OpenStax - Astronomy 2e - Telescopes (May 01, 2025)
The most important of all the powers of an optical telescope is its light-gathering power. This capacity is strictly a function of the diameter of the clear objective—that is, the aperture—of the telescope. Comparisons of different-sized apertures for their light-gathering power are calculated by the ratio of their diameters squared; for example, a 25-cm (10-inch) objective will collect four times the light of a 12.5-cm (5-inch) objective ([25 × 25] ÷ [12.5 × 12.5] = 4). The advantage of collecting more light with a larger-aperture telescope is that one can observe fainter stars, nebulae, and very distant galaxies.
Resolving power is another important feature of a telescope. This is the ability of the instrument to distinguish clearly between two points whose angular separation is less than the smallest angle that the observer’s eye can resolve. The resolving power of a telescope can be calculated by the following formula: resolving power = 11.25 seconds of arc/d, where d is the diameter of the objective expressed in centimetres. Thus, a 25-cm-diameter objective has a theoretical resolution of 0.45 second of arc and a 250-cm (100-inch) telescope has one of 0.045 second of arc. An important application of resolving power is in the observation of visual binary stars. There, one star is routinely observed as it revolves around a second star. Many observatories conduct extensive visual binary observing programs and publish catalogs of their observational results. One of the major contributors in this field is the United States Naval Observatory in Washington, D.C.
Most refractors currently in use at observatories have equatorial mountings. The mounting describes the orientation of the physical bearings and structure that permits a telescope to be pointed at a celestial object for viewing. In the equatorial mounting, the polar axis of the telescope is constructed parallel to Earth’s axis. The polar axis supports the declination axis of the instrument. Declination is measured on the celestial sky north or south from the celestial equator. The declination axis makes it possible for the telescope to be pointed at various declination angles as the instrument is rotated about the polar axis with respect to right ascension. Right ascension is measured along the celestial equator from the vernal equinox (i.e., the position on the celestial sphere where the Sun crosses the celestial equator from south to north on the first day of spring). Declination and right ascension are the two coordinates that define a celestial object on the celestial sphere. Declination is analogous to latitude, and right ascension is analogous to longitude. Graduated dials are mounted on the axis to permit the observer to point the telescope precisely. To track an object, the telescope’s polar axis is driven smoothly by an electric motor at a sidereal rate—namely, at a rate equal to the rate of rotation of Earth with respect to the stars. Thus, one can track or observe with a telescope for long periods of time if the sidereal rate of the motor is very accurate. High-accuracy motor-driven systems have become readily available with the rapid advancement of quartz-clock technology. Most major observatories now rely on either quartz or atomic clocks to provide accurate sidereal time for observations as well as to drive telescopes at an extremely uniform rate.
A notable example of a refracting telescope is the 66-cm (26-inch) refractor of the U.S. Naval Observatory. This instrument was used by the astronomer Asaph Hall to discover the two moons of Mars, Phobos and Deimos, in 1877. Today, the telescope is used primarily for observing binary stars. The 91-cm (36-inch) refractor at Lick Observatory on Mount Hamilton, California, U.S., is the largest refracting system currently in operation. (The 1-metre [40-inch] instrument at Yerkes Observatory in Williams Bay, Wisconsin, U.S., has been inactive since 2018 [see table].)
| name | aperture (metres) | type | observatory | location | date observations began |
|---|---|---|---|---|---|
| Gran Telescopio Canarias | 10.4 | reflector | Roque de los Muchachos Observatory | La Palma, Canary Islands, Spain | 2007 |
| Keck I, Keck II | 10, 10 | reflector | Keck Observatory | Mauna Kea, Hawaii | 1993, 1996 |
| Southern African Large Telescope | 11.1 × 9.8 | reflector | Sutherland, South Africa | 2005 | |
| Hobby-Eberly Telescope | 11.1 × 9.8 | reflector | McDonald Observatory | Fort Davis, Texas | 1999 |
| Large Binocular Telescope | 2 mirrors, each 8.4 | reflector | Mount Graham, Arizona | 2008 | |
| Subaru | 8.3 | reflector | Mauna Kea, Hawaii | 1999 | |
| Antu, Kueyen, Melipal, Yepun | 8.2, 8.2, 8.2, 8.2 | reflector | Very Large Telescope | Cerro Paranal, Chile | 1998, 1999, 2000, 2000 |
| Frederick C. Gillett Gemini North Telescope | 8.1 | reflector | International Gemini Observatory | Mauna Kea, Hawaii | 2000 |
| Gemini South Telescope | 8.1 | reflector | International Gemini Observatory | Cerro Pachon, Chile | 2000 |
| MMT | 6.5 | reflector | MMT Observatory | Mount Hopkins, Arizona | 2000 |
| Walter Baade, Landon Clay | 6.5, 6.5 | reflector | Magellan Telescopes | Cerro Las Campanas, Chile | 2000, 2002 |
| Bolshoi Teleskop | 6 | reflector | Special Astrophysical Observatory | Zelenchukskaya, Russia | 1976 |
| Hale Telescope | 5 | reflector | Palomar Observatory | Mount Palomar, California | 1948 |
| William Herschel Telescope | 4.2 | reflector | Roque de los Muchachos Observatory | La Palma, Canary Islands, Spain | 1987 |
| Victor M. Blanco Telescope | 4 | reflector | Cerro Tololo Inter-American Observatory | Cerro Tololo, Chile | 1974 |
| Anglo-Australian Telescope | 3.9 | reflector | Siding Spring Observatory | Siding Spring Mountain, New South Wales, Austl. | 1974 |
| Nicholas U. Mayall Telescope | 3.8 | reflector | Kitt Peak National Observatory | Kitt Peak, Arizona | 1970 |
| Canada-France-Hawaii Telescope | 3.6 | reflector | Mauna Kea, Hawaii | 1979 | |
| 3.6 | reflector | La Silla Observatory | La Silla, Chile | 1977 | |
| Hooker Telescope | 2.5 | reflector | Mount Wilson Observatory | Mount Wilson, California | 1918 |
| Samuel Oschin Telescope | 1.2 | reflector | Palomar Observatory | Mount Palomar, California | 1948 |
| 1 | refractor | Yerkes Observatory | Williams Bay, Wisconsin | 1897 | |
| Lick Refractor | 0.9 | refractor | Lick Observatory | Mount Hamilton, California | 1888 |
Another type of refracting telescope is the astrograph, which usually has an objective diameter of approximately 20 cm (8 inches). The astrograph has a photographic plateholder mounted in the focal plane of the objective so that photographs of the celestial sphere can be taken. The photographs are usually taken on glass plates. The principal application of the astrograph is to determine the positions of a large number of faint stars. These positions are then published in catalogs such as the AGK3 and serve as reference points for deep-space photography.
Reflecting telescopes
Reflectors are used not only to examine the visible region of the electromagnetic spectrum but also to explore both the shorter- and longer-wavelength regions adjacent to it (i.e., the ultraviolet and the infrared). The name of this type of instrument is derived from the fact that the primary mirror reflects the light back to a focus instead of refracting it. The primary mirror usually has a concave spherical or parabolic shape, and, as it reflects the light, it inverts the image at the focal plane. illustrates the principle of a concave reflecting mirror. The formulas for resolving power, magnifying power, and light-gathering power, as discussed for refractors, apply to reflectors as well.
The primary mirror is located at the lower end of the telescope tube in a reflector and has its front surface coated with an extremely thin film of metal, such as aluminum. The back of the mirror is usually made of glass, although other materials have been used from time to time. Pyrex was the principal glass of choice for many of the older large telescopes, but new technology has led to the development and widespread use of a number of glasses with very low coefficients of expansion. A low coefficient of expansion means that the shape of the mirror will not change significantly as the temperature of the telescope changes during the night. Since the back of the mirror serves only to provide the desired form and physical support, it does not have to meet the high optical quality standards required for a lens.
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.
Laurent Cassegrain of France, a contemporary of Newton, invented another type of reflector. Called the Cassegrain 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. illustrates a typical Cassegrain reflector. 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 (200-inch) 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 10.4-metre (34.1-foot) instrument at the Gran Telescopio Canarias at La Palma, Canary Islands, Spain, has an altitude-azimuth mounting. The significance of the latter design is that the telescope must be moved both in altitude and in 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. These guide telescopes have low magnification and a wide field of view, the latter being a desirable attribute for finding stars or other remote cosmic objects.
The parabolic shape of a primary mirror has a basic failing in that it produces a narrow field of view. This can be a problem when one wishes to observe extended celestial objects. To overcome this difficulty, most large reflectors now have a modified Cassegrain design. The central area of the primary mirror has its shape deepened from that of a paraboloid, and the secondary mirror is configured to compensate for the altered primary. The result is the Ritchey-Chrétien design, which has a curved rather than a flat focus. Obviously, the photographic medium must be curved to collect high-quality images across the curved focal plane. The 1-metre telescope of the U.S. Naval Observatory in Flagstaff, Arizona, was one of the early examples of this design.
