Astronomical Instruments and Devices

equipment for conducting and analyzing astronomical observations. Astronomical instruments and devices can be subdivided into observing instruments (telescopes), light-gathering and analyzing devices, auxiliary observation devices, time devices, laboratory devices, auxiliary computing equipment, and demonstration devices.

Optical telescopes collect the light from the celestial bodies being studied and construct their images. According to optical scheme, they are grouped into (1) mirror systems—reflectors (or catoptric systems); (2) lens systems—refractors (or dioptric systems); and (3) hybrid mirror-lens systems (catodioptric), which include the Schmidt telescope and Maksutov telescope. Functionally, telescopes are categorized as (1) instruments for conducting a wide range of astrophysical studies of stars, nebulas, galaxies, planets, and the moon, which consist primarily of large reflectors equipped with filmholders, spectrographs, and electrophotometers; (2) instruments for the simultaneous photographing of large segments of the sky (dimensions of up to 30° x 30°), which include the wide-angle Maksutov and Schmidt telescopes and also the wide-angle astrographs of the photographic refractor type; (3) astronomical instruments for highly accurate measurements of the coordinates of celestial objects and the time periods of their transit across the meridian; (4) solar telescopes for studying the physical processes occurring in the sun; and (5) meteor cameras, cameras for photographing artificial earth satellites, cameras for recording the aurora borealis, and other special telescopes. Astronomical research in the radio-frequency bands is conducted with the use of radio telescopes. The largest optical telescope in the world in the mid-20th century is the 5-meter reflector at the Mount Palomar Observatory (USA). In the USSR in 1968 mounting began in the Northern Caucasus of a reflector with a 6-meter mirror.

Meridian circles, transit instruments, vertical circles, zenith telescopes, prismatic astrolabes, and other devices are used to determine the coordinates of celestial bodies and to calculate time. In astrogeodetic expeditions, transit-type portable instruments, zenith telescopes, and theodolites are used. Large solar telescopes, which are usually stationary, are grouped into tower telescopes and horizontal telescopes; light is directed into them by one (siderostat, heliostat) or two (coelostat) moving flat mirrors. Coronagraphs, chromosphere telescopes, and photosphere telescopes are used to observe the sun’s corona, chromosphere, and photosphere.

Artificial earth satellites, which move quickly across the sky, take photographs with satellite cameras which are capable of accurately registering the opening and closing of the shutter.

Auxiliary equipment is used during observations. This includes eyepiece micrometers for measuring angular distances; filmholders for photography; and light-gathering and analyzing equipment: astronomical spectrographs (slit, slit-less, prismatic, diffraction, and interference) for photographing the spectra of the sun, stars, galaxies, and nebulas and also objective prisms, which are mounted in front of the telescope’s lens and make it possible to obtain the spectra of many stars on one photographic plate. Small and medium-size astronomical spectrographs are mounted on the telescope so that the spectrograph’s slit is in the telescope’s focus (in the first focal point and in the Newtonian, Casse-grain, and Nasmyth focuses). Large spectrographs are mounted stationary inside the coudé focus.

In most cases, visual observations have been supplanted by observations with objective light-sensitive devices, such as special highly sensitive photographic plates and devices for electrophotometric recording of the radiation of celestial bodies using photomultipliers and light intensifiers with the aid of electronic-optic devices. Observations with objective light detectors also include television observation, electronic photography, and the use of infrared light detectors.

In antiquity the main time-measuring devices were solar clocks and gnomons and later, mural quadrants, with whose help the time at which the sun or the stars crossed the meridian was determined. Modern astronomy uses transit instruments with photoelectric recording for this purpose. The most accurate pendulum devices for keeping time are the Short and Fedchenko clocks. However, today these have been replaced by quartz and molecular (or atomic) clocks.

Various laboratory devices are used to process photographs obtained from observations. These devices include coordinate measuring machines for measuring the positions of images of celestial bodies on the photographs, blink comparators for comparing two photographs of the same section of the sky taken at different times, comparators for measuring the wave lengths of spectral lines on spectrograms, micro-photometers for measuring the intensity distribution in a spectrum on a spectrogram, and stellar microphotometers for determining the magnitude of stars from photographs.

Computers are used for calculations related to the processing of results of observations. Demonstration devices include tellurians and planetariums, which make it possible to view astronomical phenomena on the interior surface of a spherical dome.

The history of astronomical observation includes four main periods, which are characterized by different methods of observation. In the first period, in early antiquity, observers, using special devices, learned how to determine time and to measure the angles between celestial bodies in the celestial sphere. Improvement in reading accuracy was achieved for the most part by increasing the dimensions of the instruments. The second period dates from the beginning of the 17th century and is linked with the invention of the telescope and the resulting increase in the range of visual observations. The introduction into astronomical observations of spectral analysis and photography led to the third period in the mid-19th century. Astrographs and spectrographs afforded the possibility of obtaining information on the chemical and physical properties of celestial bodies. The development of radio engineering, electronics, and astronautics in the mid-20th century led to the rise of radio astronomy and extra-atmospheric astronomy, which mark the fourth period.

The first astronomical instruments are considered to be gnomons, vertical columns attached to a horizontal plane, which determined the elevation of the sun and the direction of the meridian and established the onset of the equinox and solstice. The Babylonians are considered to have invented time measurement and division; but in Egypt, and especially later in ancient Greece, significant changes were introduced into these methods. The development of astronomical instrument designs in ancient China proceeded, apparently, independent of analogous work in the Near and Middle East and in the West. Reliable information about ancient Greek astronomical instruments became the property of subsequent generations through the Almagest. Along with the methodology and results of astronomical observations, Ptolemy described astronomical instruments—gnomons, armillary spheres, astrolabes, quadrants, and parallactic rules—some of which were used by his predecessors (especially Hipparchus) and some of which were invented by him. Many of these instruments were subsequently improved and used for many centuries.

In the early Middle Ages, the achievements of ancient Greek astronomers were taken over by scholars in the Near and Middle East and in Middle Asia; these scholars improved the existing instruments and worked out a number of original designs. Important works on the use and construction of astrolabes, solar clocks, and gnomons were written by al-Khwarizmi, Alfraganus, al-Khujandi, al-Biruni, and others. Significant contributions in the development of astronomical instruments were made by the astronomers of the Maragheh observatory (Nasir al-Din al-Tusi, 13th century) and the Samarkand observatory (Ulug Beg, 15th century), where a gigantic sextant with a radius of about 40 meters was installed.

The achievements of these astronomers became known in Northern Italy, Germany, England, and France through Spain and Southern Italy. In the 15th and 16th centuries, European astronomers used, along with instruments of their own designs, instruments described by Eastern scholars. G. Purbach, Regiomontanus (J. Müller), and especially Tycho Brahe and J. Hevelius built many original high-precision instruments which became widely known.

The beginning of telescopic astronomy is usually associated with the name of Galileo, who with the use of a telescope he built in 1609 (the telescope had been invented shortly before in Holland) made outstanding discoveries and gave them proper scientific explanations. In 1611, J. Kepler published a description of a new telescopic system which had, in addition to a large field of view, an additional important advantage: it gave a true image of a celestial object in the focal plane which could be measured by placing an accurate scale in the focal plane (cross hairs). The invention of eyepiece cross hairs and micrometers in the 1740’s through the 1770’s, which is associated with the names of W. Gas-coigne, C. Huygens, J. Picard, and A. Auzout, significantly increased the possibilities of the telescope by making it not only an observing but also a measuring instrument. The single-lens objectives of the first refractors produced images of poor quality—colored and blurred. A somewhat improved image was achieved by increasing the focal length of the objective, which led to the construction of long, unwieldy telescopes.

In the 17th and 18th centuries, in various countries, several designs for reflectors were developed. N. Zucchi in 1616 proposed a reflector design with a single concave mirror tilted at a small angle to the axis of the tube, which permitted elimination of a second mirror, necessary in most later designs. But Zucchi himself did not build a telescope according to his proposed design. A single-mirror reflector was first built by M. V. Lomonosov (described in 1762). Later, a large single-mirror reflector was built by W. Herschel. M. Mer-senne, J. Gregory, and N. Cassegrain worked out new designs for reflectors—reflectors with two mirrors—in 1638, 1663, and 1672, respectively. In 1668–71, I. Newton proposed a design and built telescopes in which the second mirror was flat and was tilted 45° to the axis of the tube to reflect rays into the eyepiece located on the side. The relative simplicity of their construction had the result that the number of such reflectors and the dimensions of the instruments being built began to grow rapidly. They were preferred for a long time.

At the same time, refractors continued to be perfected. In 1742, the possibility of preparing an achromatic lens was theoretically proved by L. Euler, and in 1758, J. Dollond created such a lens. Later, in the first quarter of the 19th century, owing to the improvements in optical glass made by P. Guinand and J. Fraunhofer, prerequisites for creating more improved refractors with achromatic lenses appeared.