Astronomers observe objects in the infrared portion of the electromagnetic spectrum using optical components, including mirrors, lenses and solid state digital detectors. For this reason it is classified as part of optical astronomy. To form an image, the components of an infrared telescope need to be carefully shielded from heat sources, and the detectors are chilled using liquid helium.

The Spitzer Space Telescope is a dedicated infrared space observatory currently in orbit around the Sun. (Note the black side to the telescope, to maximize infrared radiation.) NASA image.

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 temperature increase induced by sunlight was higher outside the visible spectrum, beyond the red color (note that the expected temperature peak should be at the solar energy peak, not in the infrared : Herschel observation is due to the glass spectral index depending on the wavelength, the prism diluting the lower wavelengths over higher surfaces, hence diminushing the energy received by Herschel thermometers). These "calorific rays", as Herschel called them, 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 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 Herschel Space Observatory and the Spitzer Space Telescope and Wide-field Infrared Survey Explorer (WISE) are 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 the Herschel Space Observatory, Spitzer, ISO, and IRAS 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:

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 help fill in the gaps between the atmospheric windows listed above.