Astronomical instruments can be divided into twomajor categories. The first category might includeall of the instruments which are used in the actualprocess of observing celestial objects. Some ofthese, like the meridian transit, are designed forspecific tasks such as the precise determination ofan observers position on the earth or a starsposition in the sky; other observationalinstruments are principally collectors of theradiation emitted by stars, planets, nebulas, andgalaxies. These latter, which are generally referredto as telescopes, enable objects invisible to thenaked eye to be seen, photographed, or otherwise detected.
In the second category may be grouped the auxiliary instruments which are used to standardize, record, or analyze the data obtained by the observational equipment. Devices to provide an accurate standard of time, to determine the brightnesses of stars, to record their spectra, or to measure the positions of stars on photographic plates, are examples of instruments belonging to this second category.
It should be mentioned at the outset that the radiation gathered from a celestial object by a conventional astronomical telescope lies in the visible and near visible region of the electromagnetic spectrum. Over the past few decades, however, an entirely different type of astronomical telescope has come into wide use. These instruments, known as radio telescopes, have been developed as the result of the discovery in 1928-1932 by Karl G. Jansky of the Bell Telephone Laboratories that the center of our own galaxy is a powerful emitter of electromagnetic radiation in the radio wavelength region. Since Janskys initial discovery, many other celestial "radio sources" have been found. The operating principles and the evolution of radio telescopes, as well as the significance and importance of the new field of radio astronomy which they have fostered, are treated under Radio Astronomy and Radar Astronomy. Also described elsewhere are certain other electronic devices, such as "image intensifiers," which already belong, or soon will belong, to the growing list of techniques employed in modern astronomy.
Optical TelescopesOptical Telescopes -- The purpose of a telescope is to collect lightand then to have the image magnified. The larger the telescopesmain light-collecting element, whether lens or mirror, the morelight is collected. It is the total amount of light collected thatultimately determines the level of detail. All optical telescopes fallinto one of three classes (see Figure 01). In the refractingtelescope, light is collected by a 2-element objective lens andbrought to a focal plane. By contrast the reflecting telescope usesa concave mirror for this purpose. The mirror-lens, orcatadioptric, telescope employes a combination of both mirrorsand lenses, resulting in a shorter, more portable optical tubeassembly. All telescopes use an eyepiece (located behind the focalplane) to magnify the image formed by the primary opticalsystem. Other instruments can be placed in the focal plane forvarious purposes, e.g., a photo-electric cell to measure theluminosity, the slit of a spectrograph to analyse the light, or athermo-couple to measure temperature. The advantage ofreflecting telescope is that it has no chromatic aberration.Moreover, mirrors can be manufactured to much largerdimensions
than lenses. Large lenses sage in the middle and distort the receivedimage. Reflectors can also be made from a great variety of materials,because all that matters is the reflecting surface, whereas lenses have tobe made from special types of glass. Figure 02a is the aerial view ofMauna Kea in Hawaii. It shows the domes that house many of the worldslargest telescopes.
Figure 01 Telescope, Types Figure 02a Mauna Kea The magnification of a telescope is given by the formula: M = OF / f, -------------------- (1) where OF is the focal length of the objective, and f that of the eyepiece. The resolution (limit) of a telescope is given by the formula: R (in sec of arc) = 2.3 x 105 x ( /A), ------------------- (2) where is the wavelength and A is the diameter of the aperture. For example, if A = 100 cm and = 4000x10-8 (yellow light) then R = 0.092".
Interferometer is used to measure the size of astronomical objects. By careful analysis of the resulting interference pattern, the position of a point source, or fine detail in an extended one, can be resolved. The same formula for the resolution above is applicable except the aperture A is replaced by the baseline between the two receivers. This technique has been used extensively with radio telescopes; it is now also applied to the optical telescopes as well. The table below lists some of the optical telescopes in the world (a complete listing can be found in reference 1):
Observatory Location Aperture (m) Characteristics Keck Mauna Kea, Hawaii 10.0 36 segment mirror Keck II Mauna Kea, Hawaii 10.0 Interferometry optical Hobby-Eberly Mt. Fowlkes, Texas 9.2 inexpensive, spectroscopy only Observational performance Subaru Mauna Kea, Hawaii 8.3 optimized 4 units combined as an VLT (Very Large Telescope) Cerro Paranal, Chile 8.2 interferometer Gemini North Mauna Kea, Hawaii 8.1 Twin of Gemini South All sky coverage with Gemini Gemini South Cerro Pachon, Chile 8.1 NorthNext Generation Space Telescope Halo orbit 7-9 Scheduled for launch in 2007 Previous generation (1950-1990) Hale Mt. Palomar, Ca., 5.0 limit New Technology Cerro La Silla, Chile 3.5 Adaptive opticsa Discovery of cosmic expansion Hooker Mt. Wilson, Ca 2.5 (1917) Observations outside the Hubble Space Telescope Low Earth orbit 2.4 atmosphere Solar Tower Kitt Peak, Arizona 1.8 Study of the Sun Yerkes Williams Bay, Wisconsin, 1.0 Worlds largest refractor (1897)
Table 01 Optical Telescopes Figure 02b Large Telescopes of the World By 2010 there are about 50 telescopes on Earth with at least 2.5 meters in aperture diameter. Figure 02b below shows the locations for these giant telescopes (from "Astronomy", Vol. 38, Issue 11, November 2010).
Radio Telescopes · Radio Telescopes -- It is an instrument for collecting radio waves from celestial objects. The radiation is reflected from a parabolic dish to an aerial (dipole), situated at the focus, from which the signals are led to a radio receiver. Because the wavelength of radio waves is very large (from 0.3 mm to 30 cm), a radio telescope with an aperture comparable to the optical telescope would have a very poor resolution according to Eq. (2). A considerable increase in resolution can be obtained by using an interferometer - an array of identical antennae spaced at regular intervals as shown in Figure 03, which shows that the elevation angle of a celestial object (from the horizon) can be calculated from the time difference and the distance between the receivers (the baseline). The angular separation (resolution) is obtained from the difference of the elevation angles corresponding to the resolution of the time difference of T1 and T2.
Interferometer The table below lists some of the radio telescopes in the world: Observatory Location Resolution (arcsec) Characteristics Very Long Baseline VLBI Intercontinental > 0.001 Interferometer Largest (dish) synthesis VLA Socorro, NM > 0.04 array Arecibo Puerto Rico > 0.2 Largest fixed dish Effelsberg Effelsberg, Germany > 0.6 Largest single dish Largest in southern Parkes NSW, > 0.9 hemisphere Table 02 Radio Telescopes
Infrared Telescopes Infrared Telescopes -- Infrared radiation (wavelength between 1 and 1000 mm) from space is mostly absorbed by the atmosphere (see Figure 04): so the largest infrared telescopes are built on the tops of high mountains, installed on special high flying aircraft or balloons, or better yet on satellites orbiting the earth. However, atmospheric absorption is not the only obstacle to analyse this type of radiation on earth: the main problem, which also occurs in space, is to distinguish the signal collected from the "background noise", i.e., from the enormous infrared emissions of the Earth or
of the instruments themselves, since object which is not at absolute zero, emits infrared radiation. So everything around the instruments (including the telescope) produces "backround noise". Therefore, special photo- graphic film is used to produce a "thermograph" of a celestial body, and the instruments must be cooled continuously by immersion in liquid nitrogen or helium (Figure 05).
Figure 04 Atomspheric AbsorptionFigure 05 Infrared Telescope The table below lists some of the infrared telescopes in the world: Observatory Location Aperture (m) Date UKIRT Mauna Kea, Hawaii 3.8 Since 1978 To be launched in FIRST Orbiting 3.0 2007 NASA IRTF Mauna Kea, Hawaii 3.0 Since 1979 Started operation in SOFIA Airborne 2.5 February, 2006 Launched in August, SIRTF Heliocentric orbit 0.85 2003 Launched in ISO Geocentric orbit 0.6 November, 1995 Operated for ten IRAS Geocentric orbit 0.6 months in 1983 Table 03 Infrared Telescopes
Acronym: UKIRT - United Kingdom Infrared Telescope. FIRST - Far Infrared Space Telescope. IRTF - Infrared Telescope Facility. SOFIA - Stratosphere Observatory for Infrared Astronomy. SIRTF - Space Infrared Telescope Facility; renamed to Spitzer Space Telescope in honor of the late astrophysicist Lyman Spitzer Jr., who first conceived of a large telescope in orbit. ISO - Infrared Space Observatory. IRAS - Infrared Astronomical Satellite.
High Frequency Observations Observations of High Frequency Radiation -- As shown in Figure 04, radiation with wavelength shorter than 310 nm are absorbed in the Earths stratosphere, high above any terrestrial observatory. These short wavelength radiations can be studied only by instruments carried on very-high--altitude balloons, rockets, satellites, and
Figure 06 Grazing TelescopeFigure 07 Scintillator Although attempts to study the suns UV spectrum from balloons were made during the 1920s, it was not until 1946 that rocket-borne instruments made this possible. Only limited additional progress was made until 1962, when the first Orbiting Solar Observatory (OSO) satellite was launched by the National Aeronautics and Space Administration (NASA). It returned thousands of UV spectra, including the first exteme-ultraviolet (wavelengths below 91 nm) observations of the solar corona. The International Ultraviolet Explorer (IUE) was in orbit between 1978 and 1996.
Its large telescope (0.45 m aperture) made possiblethe first UV observations of objects beyond theMilky Way. The Extreme Ultraviolet Explorer(EUVE, 1990-1999) was the first orbitingobservatory to focus on that part of the spectrum.Ultraviolet spectroscopy has been particularlyvaluable because many of the most abundantatoms and ions in the universe have their strongestlines in that region of the spectrum. Right nowthere are no dedicated ultraviolet observatories inorbit. The Hubble Space Telescope can perform agreat deal of observing at ultraviolet wavelengths,but it has a very fairly small field of view.
X-rays are typically emitted by gaseous bodies with temperatures ranging from 106 to 108 oK. Conventional telescopes cannot be used at x-ray (or EUV) wavelengths because mirrors abosrb x-rays rather than reflect them, unless the x-rays graze the surface at a very shallow angle. Satellites such as ROSAT (in orbit between 1990- 1999), and the Chandra X-ray observatory (since July, 1999) utilize "grazing incidence" telescopes (GRITs), which bring x-rays to a focus by reflecting them at shallow angles from the surfaces of nested sets of tapering, tubelike reflectors as shown in Figure 06. X- ray observations have discovered a variety of objects, ranging from hot patches and cool "holes" in the Suns outer atmosphere to swirling discs of hot gas surrounding collapsed stars and black holes to hot clouds of gas in intergalactic space.
Gamma rays are the most energetic form ofelectromagnetic radiation. Because their wavelengthsare far smaller than the sizes of the atoms in a mirror,gamma rays cannot be focued by reflection, and theearly gamma-ray satellites were unable to form imagesof sources or even determine their positions withconfidence. Modern gamma-ray imaging systems andspectrometers, such as those carried on board theCompton Gamma Ray Observatory (CGRO, 1991-2000),make use of scintillators (see Figure 07), which aredevices that convert gamma rays into visible photonsthat are more easily detected and analyzed. Amongknown gamma-ray sources are the Milky Way, somepulsars, and some
quasars. Most puzzling of all are the 2600 gamma-ray bursts detected by CGRO. The next generation gamma-ray observatory is GLAST (Gamma-ray Large Area Space Telescope) scheduled to be launched in 2005. It is designed for making observations of celestial gamma-ray sources in the energy band extending from 20 MeV to more than 300 GeV. Figure 08a is a gamma-ray sky animation - constructed from simulating the first 55 days of GLAST observations of cosmic gamma-ray sources. It shows the plane of our Milky Way Galaxy as a broad U-shape, with the center of the galaxy toward the right. Besides the diffuse Milky Way glow, the simulated objects include flaring active galaxies, pulsars, gamma-ray bursts, the flaring Sun, and the gamma-ray Moon. The GLAST was
Figure 08a Gamma-ray Skyfinally launched on June 11, 2008 many years behind schedule. It will studygamma-rays from extreme environments in our own Milky Way galaxy, aswell as supermassive black holes at the centers of distant active galaxies,and the sources of powerful gamma-ray bursts.
Figure 08b is another GLAST gamma-ray sky map taken in the period from August 4 to October 30, 2008. The map highlights the "top ten" list of five sources within, and beyond the Milky Way. Within our galaxy: the Sun traces a faint arc across the sky during the observation dates, LSI +61 303 is an X-ray binary star, PSR J1836+5925 is a type of pulsar that is only seen to pulse at gamma-ray energies, and 47 Tuc is a globular star cluster.Figure 08b Gamma-ray Sky
A fifth galactic source (unidentified), just above the center of the galactic plane, is a variable source and has no clear counterpart at other wavelengths. Beyond our galaxy: NGC 1275 is a large galaxy at the heart of the Perseus galaxy cluster, while 3C 454.3, PKS 1502+106, and PKS 0727-115 are active galaxies billions of light-years away. Another unidentified source, seen below the galactic plane, is likely beyond the boundaries of the Milky Way. Its nature remains a mystery.
Table 04 lists the three types of gamma-ray sources in the Milky Way (MW). It isthought that they may be associated with the unknown dark matter particle(dmp). The mp denotes the mass of proton (nucleon). Gamma-rayInstrument Process Source Distribution Energy Annihilation of ~ 0.003 mp light Around theINTEGRAL 511 kev e-e+ dmp center of MW Annihilation of ~ 60 mp Faint galactic EGRET ~ 1 Gev dmp neutralino background Annihilation of ~ 20000 mp heavy Point source at HESS ~ 100 Gev dmp dmp MW center
Table 04 Gamma-ray Sources in Milky Way Acronym: INTEGRAL - INTErnational Gamma-Ray Astrophysics Laboratory (a space instrument combining fine spectroscopy and imaging of gamma-ray emissions in the energy range of 15 keV to 10 MeV ). EGRET - Energetic Gamma Ray Experiment Telescope (space telescope for detecting 30 MeV - 30 GeV gamma-rays). HESS - High Energy Stereoscopic System (a system of ground based Cherenkov Telescopes for the investigation of cosmic gamma rays in the 100 GeV energy range).
Meanwhile the Swift satellite (Figure 09) was launchedinto a low-Earth orbit on November 20, 2004. Itsmission is to solve the mystery of Gamma-ray bursts(GRBs). They are the most powerful explosions in theUniverse since the Big Bang. They occur approximatelyonce per day and are brief, but intense, flashes ofgamma radiation. They come from all differentdirections of the sky and last from a few millisecondsto a few hundred seconds. So far scientists do notknow what causes them. Do they signal the birth of ablack hole in a massive stellar explosion? Are they theproduct of
Figure 09 Swift the collision of two neutron stars? Or is it some other exotic phenomenon that causes these bursts? Swift is designed to look for faint bursts coming from the edge of the universe. On September 2005, astronomers announce that they have detected a cosmic explosion (GRB) at the very edge of the visible universe. The explosion occurred soon after the first stars and galaxies formed, perhaps 500 million to 1 billion years after the Big Bang. It was probably caused by the death of a massive star. It is believed that this observation opens the door to the use of GRBs as unique and powerful probes of the early universe.
The key to unravel the nature of GRB comes in part from the discovery that they are narrowly focused beams. This realization allowed astronomers to estimate energies for individual bursts and hypothesize the number of total bursts occurring over a given time interval. The usual assumption of spherical emission would over estimate the released energy and under estimate the number of GRB. By 2007, astronomers can link the most common type of GRB, those lasting 20 seconds or longer, with the collapse of massive stars about 30 or more times larger than the Sun. While the short GRB (lasting just a few milliseconds) came from a neutron star crashing into a black hole or another neutron star.
Footnotes: aIn adaptive optics, light from the primary mirror is directed onto a smaller flexible mirror behind which a large number of actuators are located. The actuators distort the shape of the mirror to cancel out distortions in the incoming wavefronts of light that have been caused by the atmosphere. Wavefront distortions are sensed by monitoring a suitable bright star, or an artificial "star" generatged by shining a powerful laser beam into the upper atmosphere. All these operations are computerized. Computers have so revolutionized astronomy, in fact, that researchers rarely look through the telescope or work in the dome during observations. Instead, they operate the instrument in comfort from a control room.