Infrared astronomy

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

The cheapest infrared detectors commonly used at research telescopes are HgCdTe arrays. These operate well between 1 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.