- Infrared astronomy
Infrared astronomy is the branch of
astronomyand astrophysicswhich 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 microwavesare called infrared (or sometimes "submillimeter" waves).
Scientists classify infrared astronomy as part of
optical astronomybecause optical components (mirrors, lenses and solid state digital detectors) are usually used.
After the use of prisms by
Isaac Newtonto split white light into a spectrum, it was found in 1800 by William Herschelthat 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 Smythin 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 telescopescover the near-infrared as well as the visible). The far infraredextends to submillimeter wavelengths, which are observed by telescopes such as the James Clerk Maxwell Telescopeat 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 nitrogenin 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 Earthinclude Mauna Kea Observatoryat 4205 meters above sea level, the ALMA site at 5000 m in Chileand regions of high altitude ice-desert such as Dome Cin 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 Telescopeis 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.
One of the most common infrared detector arrays used at research telescopes is
HgCdTearrays. These operate well between 0.6 and 5 micrometrewavelengths. For longer wavelength observations or higher sensitivity other detectors may be used, including other narrow gap semiconductordetectors, low temperature bolometerarrays or photon-counting SuperconductingTunnel 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
Astronomers' infrared spectrum
Infrared space telescopes such as Spitzer,
IRAS, ISO and the forthcoming Herschel Space Observatorycan 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, Hawaiiand 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, IRASand ISO help fill in the gaps between the atmospheric windows listed above.
Far infrared astronomy
* [http://coolcosmos.ipac.caltech.edu/cosmic_classroom/ir_tutorial/ Caltech IR tutorial]
* [http://cfa-www.harvard.edu/cfa/oir/ Harvard Optical and Infrared Astronomy Group]
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