infrared astronomy

infrared astronomy, study of celestial objects by means of the infrared radiation they emit, in the wavelength range from about 1 micrometer to about 1 millimeter. All objects, from trees and buildings on the earth to distant galaxies, emit infrared (IR) radiation. The study of such radiation from celestial objects is of particular importance for several reasons. Cosmic dust particles effectively obscure parts of the visible universe, such as the center of our galaxy, the Milky Way, but this dust is transparent in the IR wavelengths. Most of the energy radiated by objects ranging from interstellar matter to planets lies in the IR wavelengths; IR observations are therefore significant in studying asteroids, comets, planetary satellites, and interstellar dust clouds where stars are forming. Finally, because the expansion of the universe shifts energy to longer wavelengths, most of the visible radiation emitted by stars and galaxies during the early stages of the formation of the universe is now shifted to the IR range; studies of the most distant objects in the IR spectrum are necessary if astronomers are to understand how the universe was formed.

The beginnings of IR astronomy can be traced to the discovery of IR radiation in the spectrum of the sun by English astronomer Sir William Herschel about 1800. It is reported that Irish astronomer Lord William Rosse detected IR radiation from the moon about 1845. As early as 1878 the American inventor Thomas Alva Edison observed a solar eclipse from a site in Wyoming using a sensitive IR detector, and during the 1920s the first systematic IR observations of celestial objects were made by Seth B. Nicholson, Edison Pettit, and other American astronomers. However, modern IR astronomy did not begin until the 1950s because of the lack of appropriate instrumentation. Since then, special interference filters and cryogenic systems (to minimize IR interference from the radiation emitted by the equipment itself) have been introduced for ground-based observations, and aircraft, balloons, rockets, and orbiting satellites have been successively employed to carry the equipment above the water vapor in the earth's atmosphere.

The Kuiper Airborne Observatory (KAO), operated by the National Aeronautics and Space Administration (NASA), had its first flight in 1975. Named for the American astronomer Gerard P. Kuiper, the KAO was a C-141 jet transport that carried its 36-inch (91-cm) telescope to altitudes of up to 45,000 ft (13,720 m). Before it flew its last mission in 1995, the KAO was instrumental in the discovery of the rings of Uranus, the atmosphere around Pluto, and the definitive detection of water during the crash of comet Shoemaker-Levy 9 into Jupiter. Also sponsored by NASA is the Infrared Telescope Facility, a 10-ft (3-m) IR telescope located at an altitude of 14,000 ft (4,270 m) on the summit of Mauna Kea in Hawaii; established in 1979, it effectively is the U.S. national IR observatory. Also near the summit of Mauna Kea is the 12.5-ft (3.8-m) United Kingdom Infrared Telescope (UKIRT), the largest telescope in the world used solely for IR observations.

The first IR satellite to be launched (1983) was the Infrared Astronomical Satellite (IRAS), a joint venture of the United States, Great Britain, and the Netherlands. Orbiting the earth for 10 months, IRAS performed an all-sky survey that yielded catalogs of hundreds of thousands of IR sources, more than half of these previously unknown, including asteroids and comets; detected a new class of long-lived "cool" galaxies that are dim in the visible region of the spectrum; located a protoplanetary disk around a nearby star; and showed clearly for the first time the bulge near the center of the Milky Way. In 1989 the second IR satellite, the Cosmic Background Explorer (COBE), was launched by NASA. Operating through 1993, COBE detected small temperature variations in the cosmic microwave background radiation that provided vital clues to the nature of the early universe and its evolution since the "big bang." The European Space Agency (ESA) launched the Infrared Space Observatory (ISO) in 1995. Operating until May, 1998, ISO monitored nearby planets, asteroids, and comets. It found water vapor in the atmospheres of Saturn, Neptune, Uranus, and Titan, Saturn's largest moon; detected water vapor and fluorides in the interstellar medium; and studied the "cool" galaxies first seen by IRAS. The near-infrared camera multiobject spectrometer (NICMOS) was placed aboard the Hubble Space Telescope in 1997. Consisting of three cameras and three spectrometers, it has been used to study interstellar clouds where stars are being formed, young stars, and the atmospheres of Jupiter and Uranus.

The Spitzer Space Telescope, a cryogenically cooled satellite observatory with a 2.8-ft (0.85-m) telescope, was launched in Aug., 2003, and placed in a solar orbit in which it trails the earth by 5.4 million mi (8.7 million km); it is expected to have a two-to-five-year operating lifetime. In May, 2009, ESA launched the Herschel Space Telescope, with a 138-in. (3.5-m) mirror; it also is cryogenically cooled. Positioned some 930,000 mi (1.5 million km) from earth on a mission expected to last three years, it is designed to observe wavelengths from the infraread to the submillimeter. A KAO replacement, the Stratospheric Observatory for Infrared Astronomy (SOFIA), a joint project of NASA and the German space agency, DLR, that consists of a Boeing 747-SP aircraft modified to accommodate a 8.2-ft (2.5-m) reflecting telescope (the largest airborne telescope in the world) began test flights in 2009.

Study of astronomical objects by observing the infrared radiation they emit. Its techniques enable examination of many celestial objects that give off energy at wavelengths in the infrared region of the electromagnetic spectrum but that cannot otherwise be seen from Earth because they do not emit much visible light or because that light is blocked by dust clouds, which infrared radiation can penetrate. Infrared astronomy originated in the early 19th century with the work of William Herschel (see Herschel family), who discovered infrared radiation while studying sunlight. The first systematic infrared observations of other stars were made in the 1920s; modern techniques, such as the use of interference filters for ground-based telescopes, were introduced in the early 1960s. Because atmospheric water vapour absorbs many infrared wavelengths, observations are carried out with telescopes sited on high mountaintops and from airborne and space-based observatories. Infrared astronomy allows studies of the dust-obscured core of the Milky Way Galaxy and the hearts of star-forming regions and has led to many discoveries including brown dwarf candidates and disks of matter around certain stars.

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Infrared astronomy is the branch of astronomy and astrophysics which deals with objects visible in infrared (IR) radiation. Visible radiation ranges from 400 nm (blue) to 700 nm (red). Longer wavelengths than 700 nm but still shorter than microwaves are called infrared (or sometimes submillimeter waves).

Scientists classify infrared astronomy as part of optical astronomy because optical components (mirrors, lenses and solid state digital detectors) are usually used.

Discovery

After the use of prisms by Isaac Newton to split white light into a spectrum, it was found in 1800 by William Herschel that the hottest part of the band of light from the Sun was actually past the red end of the spectrum. These "heat rays" even displayed some spectral lines. Charles Piazzi Smyth in 1856 detected infrared radiation in the light of the Moon.

Modern infrared astronomy

Nearing infrared radiation (infrared radiation with wavelengths close to that of visible light) behaves in a very similar way to visible light, and can be detected using similar electronic devices. For this reason, the near infrared region of the spectrum is commonly incorporated as part of the "optical" spectrum, along with the near ultraviolet (most scientific instruments such as optical telescopes cover the near-infrared as well as the visible). The far infrared extends to submillimeter wavelengths, which are observed by telescopes such as the James Clerk Maxwell Telescope at Mauna Kea Observatory.

Like all other forms of electromagnetic radiation, infrared is utilised by astronomers to learn more about the universe. As infrared is essentially heat radiation, infrared telescopes (which include most major optical telescopes as well as a few dedicated infrared telescopes) need to have their detectors shielded from heat and chilled with liquid nitrogen in order to actually form images. This is particularly important in the mid infrared and far infrared regions of the spectrum. The principal limitation on infrared sensitivity from ground-based telescopes is the water vapour in the Earth's atmosphere, which absorbs a significant amount of infrared radiation. For this reason most infrared telescopes are built in very dry places at high altitude (above most of the water vapour in the atmosphere). Suitable locations on Earth include Mauna Kea Observatory at 4205 meters above sea level, the ALMA site at 5000 m in Chile and regions of high altitude ice-desert such as Dome C in Antarctic.

However, as with visible-light telescopes, space is the ideal place for their use and most optical telescopes launched into space (such as the Hubble Space Telescope) can also perform infrared observations. The recently launched Spitzer Space Telescope is dedicated solely to infrared observations.

Another way of doing infrared astronomy is by the use of airborne observatories such as SOFIA (Stratospheric Observatory for Infrared Astronomy) and the Kuiper Airborne Observatory.

By flying at high altitude (Stratosphere) less water vapour will be between the telescope and space leading to a smaller IR absorption of the atmosphere.
The residual IR background (due to the absorption left) is statically removed by applying a chopping reduction technique of the observed field and a blank region.

The highest resolution infrared observations are performed by ground-based astronomical interferometers.

Infrared technology

One of the most common infrared detector arrays used at research telescopes is HgCdTe arrays. These operate well between 0.6 and 5 micrometre wavelengths. For longer wavelength observations or higher sensitivity other detectors may be used, including other narrow gap semiconductor detectors, low temperature bolometer arrays or photon-counting Superconducting Tunnel Junction arrays.

Special requirements for infrared astronomy include: very low dark currents to allow long integration times, associated low noise readout circuits and sometimes very high pixel counts.

Astronomers' infrared spectrum

Infrared space telescopes such as Spitzer, IRAS, ISO and the forthcoming Herschel Space Observatory can observe across almost all of the infrared spectrum. However, most infrared astronomy is still done at ground-based telescopes, and these are limited to observations through a small number of spectral "windows", at wavelengths where the Earth's atmosphere is transparent. The main infrared windows are listed below:

Wavelength range	Astronomical bands	Telescopes
(micrometres)
0.65 to 1.0	R and I bands	All major optical telescopes
1.25	J band	Most major optical telescopes and most dedicated infrared telescopes
1.65	H band	Most major optical telescopes and most dedicated infrared telescopes
2.2	K band	Most major optical telescopes and most dedicated infrared telescopes
3.45	L band	Most dedicated infrared telescopes and some optical telescopes
4.7	M band	Most dedicated infrared telescopes and some optical telescopes
10	N band	Most dedicated infrared telescopes and some optical telescopes
20	Q band	Some dedicated infrared telescopes and some optical telescopes
450	submillimeter	Submillimeter telescopes

Between these windows there are generally regions where infrared observations are more difficult or impossible from the ground due to the opacity of the atmosphere. Dedicated infrared and submillimeter telescopes are generally built at very high altitude sites like Mauna Kea Observatory, Hawaii and the ALMA site in Chile, or even flown on aircraft like SOFIA, providing the best sensitivity available from Earth based observatories. Data from space-based observatories like Spitzer, IRAS and ISO help fill in the gaps between the atmospheric windows listed above.

See also

• Far infrared astronomy
• Infrared spectroscopy

• Infrared
• Infrared detector
• Radio window
• Atmospheric window
• Astronomical window
• Optical window

External links

• Caltech IR tutorial
• Harvard Optical and Infrared Astronomy Group