Infrared astronomy
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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 400nm (blue) to 700nm (red). Longer wavelengths than 700nm but still shorter than microwaves are called infrared.
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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
Near 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 esentially 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 principle limitation on infrared sensitivity from ground-based telescopes is the water vapour in the Earth's atmosphere, which absorbs a lot 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 5000m 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 soley to infrared observations.
The highest resolution infrared observations are performed by ground-based astronomical interferometers.
Astronomers' Infrared Spectrum
Infrared space telescopes such as Spitzer, IRAS, ISO and the forthcoming Herschel Space Observatory can observe across the 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 |
|---|---|---|
| 0.65 - 1.0 micrometers | R and I bands | All major optical telescopes |
| 1.25 micrometers | J band | Most major optical telescopes and most dedicated infrared telescopes |
| 1.65 micrometers | H band | Most major optical telescopes and most dedicated infrared telescopes |
| 2.2 micrometers | K band | Most major optical telescopes and most dedicated infrared telescopes |
| 3.45 micrometers | L band | Most dedicated infrared telescopes and some optical telescopes |
| 4.7 micrometers | M band | Most dedicated infrared telescopes and some optical telescopes |
| 10 micrometers | N band | Most dedicated infrared telescopes and some optical telescopes |
| 20 micrometers | Q band | Some dedicated infrared telescopes and some optical telescopes |
| 450 micrometers | 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
- Most telescopes listed in Category:Telescopes can observe at some infrared wavelengthsde:Infrarotastronomie