Abbreviation for barycentric dynamical time. The order of the initials is that of the term in French. See also: dynamical time.

Abbreviation for Tracking and Data Relay Satellite System.

Abbreviation for terrestrial dynamical time. See also: dynamical time.

A familiar name sometimes applied to an asterism formed by a group of the brighter stars in the constellation Sagittarius because of its supposed resemblance to the shape of a teapot.

Comet Tebbutt.

technetium star A star whose spectrum shows the presence of the unstable element technetium. The longest-lived isotope of technetium has a half-life of 2.1 × 10 to the power of 9 years, which is short in relation to the typical lifetimes of stars. The age of the Sun, for example is 5 × 10 to the power of 9 years. The implication is that the technetium is being created within the star itself and subsequently brought to the surface; it could not have been in the material from which the star formed. Technetium has been detected only in a small group of carbon stars.

An observatory site on the island of Tenerife in the Canary Islands, shared by the Instituto de Astrofisica de Canarias with European partners. The instruments located there include several solar telescopes, a spectroheliograph, a radio telescope for studying cosmic background radiation and a 155-cm 61-inch infrared telescope.

A characteristic piece of natural glass. Tektites are found distributed on the Earth’s surface in four main areas, called strewn fields, which are in Australasia, the Ivory Coast, Moravia and Bohemia in the Czech Republic and Texas and Georgia in the USA. Individual tektites range up to 15 kilograms in mass though most are much smaller and their shape and structure suggest that the molten material from which they formed underwent a high-velocity flight through the atmosphere. The most popular theory for their origin is that they were created from terrestrial material when the impacts of large meteorites melted and ejected rock at the impact sites. Their ages and links to known impact structures support this theory.

A converging lens inserted in the light path of a telescope to reduce the effective focal length.

An optical device in an extension tube used in conjunction with an eyepiece to increase the effective focal length of a telescope.

The technique of remote control of a spacecraft, or instruments on it, and the receipt of results, by means of radio signals.

An instrument to collect electromagnetic radiation from a distant object, bring the radiation to a focus and produce a magnified image or signal. As technological advances have made it possible for astronomers to study the complete electromagnetic spectrum, telescopes of specialized design, and complementary detectors, have been devised to operate over different wavebands. The term telescope, originally coined for an optical instrument, has come to have this broader meaning in astronomy. However, telescopes for use in the radio, visible and X-ray regions, for example, employ widely differing designs and techniques. Optical telescopes fall into two main categories, refractors and reflectors, according to whether the main light-gathering element is a lens or a mirror. A refracting telescope has an objective lens at the front of the telescope tube and either an eyepiece or equipment such as a camera at the back where the image is formed. In a reflecting telescope, the objective is a concave mirror at the back of the tube. The objective of a refracting telescope is usually a compound lens, with two or more elements, of relatively long focal length. The use of a multi-element lens reduces the degree of chromatic aberration inherent in lenses. Such a lens is known as an achromatic doublet or triplet. A long focal length also helps minimize both chromatic and spherical aberration, but it also means that refractors tend to be long and bulky. In the past, exceptionally long refractors were constructed in an effort to reduce the aberrations. The abbreviation OG for object glass is sometimes used to indicate a refracting telescope. There are inherent difficulties in constructing and mounting glass lenses of large diameter, and large thick lenses absorb too much light for astronomical purposes. The world’s largest refractor has an objective lens 101 centimetres 40 inches in diameter and is at the Yerkes Observatory. All large astronomical telescopes are reflectors, and reflectors are also popular with amateurs, being less expensive than refractors and easier to make. In a reflector, the light converges to a focal point in front of the main mirror, called the prime focus. It is usually redirected, by means of a secondary mirror, to a place where detection is more convenient. Several systems are in common use. The Newtonian telescope, Cassegrain telescope, coudé focus and Nasmyth focus all have different applications. In a very large telescope, the observer may be able to work directly at the prime focus in a cage suspended within the main tube. The obstruction caused by a secondary mirror or prime focus cage has little effect on the performance of the telescope in practice. Large multi-purpose professional telescopes are usually constructed to offer observers a choice of foci. The Newtonian focus is used only on amateur visual telescopes. The primary mirrors in reflecting telescopes are made most usually from glass or a ceramic material that does not expand or contract with temperature changes. The surface must be carefully figured to the required shape, either part of a sphere or part of a paraboloid, to an accuracy of a fraction of the wavelength of light. A thin layer of aluminium is then deposited on to the glass to provide the reflecting surface. In early reflecting telescopes, such as those made by William Herschel 1738-1822, the primary mirror was a casting of speculum metal 68 per cent copper, 32 per cent tin. For this reason, the abbreviation spec. is still sometimes used to indicate a reflecting telescope. The earliest glass mirrors were coated with silver, but this has the disadvantage of tarnishing quickly when exposed to the air. In the design of the most modern large telescopes, techniques known as active optics allow thinner, more light-weight mirrors to be kept accurately in shape by means of an array of computer-controlled supports behind. This makes it possible to construct telescopes with mirrors of larger diameter than was previously possible, and mirrors composed of a number of separate segments. Both the light-gathering power and the resolving power of a telescope depend on the size of its objective. Astronomers continually aspire to larger instruments to reach fainter limiting magnitudes and achieve resolution of greater detail, though some of these objectives are also served by developing more sensitive detectors and the application of interferometers. Magnifying power is not of great significance, except with small amateur telescopes for visual use. The magnification for visual observing is changed by employing different eyepieces. The maximum magnification is usually governed by seeing conditions rather than the limit of performance of the telescope. The images formed by astronomical telescopes are inverted. Since the introduction of a lens to rectify the image would serve no useful purpose and would absorb valuable light, astronomers prefer to work directly with inverted images. The mounting of an astronomical telescope is an important part of its structure, since the observer needs to be able to point the instrument easily at selected objects and to follow them as the Earth’s rotation causes their apparent movement across the sky. Very small amateur telescopes and modern computer-controlled telescopes employ altazimuth mountings. Before the advent of computer control, the most practical method was the equatorial mounting. Many existing telescopes are on equatorial mounts, and the system remains popular for amateur instruments. See also: adaptive optics, New Technology Telescope, radio telescope, Schmidt camera, X-ray astronomy.

English name for the constellation Telescopium.

A 3.5-metre 11.5-foot reflecting telescope at the Observatorio del Roque de los Muchachos, in the Canary Islands. It was commissioned by Padua University, Italy, as a national facility for Italian astronomers and was completed in 1997. It is modelled on the European Southern Observatory’s New Technology Telescope.

An insignificant southern constellation introduced by Nicolas L. de Lacaille in the mid-eighteenth century. It contains only one star as bright as third magnitude.