Astronomical Instruments

Contents

Optical Telescopes

• Optical Telescopes -- The purpose of a telescope is to collect light and then to have the image magnified. The larger the telescope's main light-collecting element, whether lens or mirror, the more light is collected. It is the total amount of light collected that ultimately determines the level of detail. All optical telescopes fall into one of three classes (see Figure 01). In the refracting telescope, light is collected by a 2-element objective lens and brought to a focal plane. By contrast the reflecting telescope uses a concave mirror for this purpose. The mirror-lens, or catadioptric, telescope employes a combination of both mirrors and lenses, resulting in a shorter, more portable optical tube assembly. All telescopes use an eyepiece (located behind the focal plane) to magnify the image formed by the primary optical system. Other instruments can be placed in the focal plane for various purposes, e.g., a photo-electric cell to measure the luminosity, the slit of a spectrograph to analyse the light, or a thermo-couple to measure temperature. The advantage of reflecting telescope is that it has no chromatic aberration. Moreover, mirrors can be manufactured to

		much larger dimensions than lenses. Large lenses sage in the middle and distort the received image. Reflectors can also be made from a great variety of materials, because all that matters is the reflecting surface, whereas lenses have to be made from special types of glass. Figure 02 is the aerial view of Mauna Kea in Hawaii. It shows the domes that house many of the world's largest telescopes.
Figure 01 Telescope, Types [view large image]	Figure 02 Mauna Kea [view large image]

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
Subaru	Mauna Kea, Hawaii	8.3	Observational performance optimized
VLT (Very Large Telescope)	Cerro Paranal, Chile	8.2	4 units combined as an interferometer
Gemini North	Mauna Kea, Hawaii	8.1	Twin of Gemini South
Gemini South	Cerro Pachon, Chile	8.1	All sky coverage with Gemini North
Next Generation Space Telescope	Halo orbit	7 - 9	Scheduled for launch in 2007
Hale	Mt. Palomar, Ca.	5.0	Previous generation (1950-1990) limit
New Technology	Cerro La Silla, Chile	3.5	Adaptive opticsa
Hooker	Mt. Wilson, Ca.	2.5	Discovery of cosmic expansion (1917)
Hubble Space Telescope	Low Earth orbit	2.4	Observations outside the atmosphere
Solar Tower	Kitt Peak, Arizona	1.8	Study of the Sun
Yerkes	Williams Bay, Wisconsin	1.0	World's largest refractor (1897)

Table 01 Optical Telescopes

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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.

Figure 03 Interferometer

[view large image]

The table below lists some of the radio telescopes in the world:



Observatory	Location	Resolution (arcsec)	Characteristics
VLBI	Intercontinental	> 0.001	Very Long Baseline Interferometer
VLA	Socorro, NM	> 0.04	Largest (dish) synthesis array
Arecibo	Puerto Rico	> 0.2	Largest fixed dish
Effelsberg	Effelsberg, Germany	> 0.6	Largest single dish
Parkes	NSW, Australia	> 0.9	Largest in southern hemisphere

Table 02 Radio Telescopes

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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
Figure 04 Atomspheric Absorption [view large image]	Figure 05 Infrared Telescope[view large image]	celestial body, and the instruments must be cooled continuously by immersion in liquid nitrogen or helium (Figure 05).

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
FIRST	Orbiting	3.0	To be launched in 2007
NASA IRTF	Mauna Kea, Hawaii	3.0	Since 1979
SOFIA	Airborne	2.5	Scheduled for 2004
SIRTF	Heliocentric orbit	0.85	Launched in August, 2003
ISO	Geocentric orbit	0.6	Launched in November, 1995
IRAS	Geocentric orbit	0.6	Operated for ten 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.


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High Frequency Observations

		Observations of High Frequency Radiation -- As shown in Figure 04, radiation with wavelength shorter than 310 nm are absorbed in the Earth's 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 spacecraft.
Figure 06 Grazing Telescope [view large image]	Figure 07 Scintillator [view large image]

Although attempts to study the sun's 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 possible the first UV observations of objects beyond the Milky Way. The Extreme Ultraviolet Explorer (EUVE, 1990-1999) was the first orbiting observatory to focus on that part of the spectrum. Ultraviolet spectroscopy has been particularly valuable because many of the most abundant atoms and ions in the universe have their strongest lines in that region of the spectrum. Right now there are no dedicated ultraviolet observatories in orbit. The Hubble Space Telescope can perform a great 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 10^{8 o}K. 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 Sun's 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 of electromagnetic radiation. Because their wavelengths are far smaller than the sizes of the atoms in a mirror, gamma rays cannot be focued by reflection, and the early gamma-ray satellites were unable to form images of sources or even determine their positions with confidence. Modern gamma-ray imaging systems and spectrometers, such as those carried on board the Compton Gamma Ray Observatory (CGRO, 1991-2000), make use of scintillators (see Figure 07), which are devices that convert gamma rays into visible photons that are more easily detected and analyzed. Among known gamma-ray sources are the Milky Way, some pulsars, 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.



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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.


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References:

1. A complete listing of the World's Largest Optical Telescopes -- http://www.seds.org/billa/bigeyes.html