Infrared Telescopes

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Where can I purchase an infrared telescope for backyard use?

You can't. Most infrared light from celestial sources is absorbed by the Earth's atmosphere. Only a narrow window of near-infrared radiation (at wavelengths less than about 4 microns) reaches the Earth. Observations at these wavelengths requires that the infrared camera be cooled to hundreds of degrees below zero using a cryogen (such as liquid helium) and requires special solid-state infrared detectors (costing tens of thousands of dollars). Hence, it is impractical to consider a true infrared telescope for personal use.
Why can't I use commercially available infrared film to photograph astronomical objects with an amateur telescope?
You can! But be aware that this film is responsive to near-infrared light that is just beyond the red end of the visible-light spectrum (around 1 micron). At this wavelength, you would essentially be photographing very long-wavelength visible light, and astronomical phenomena would appear to differ only slightly from the more accustomed optical photos. To really measure the thermal emission that characterizes most infrared radiation, you need to observe at wavelengths of longer than about 5 microns -- and that is not easily practical from the ground.
Which astronomical observatories are capable of observing in the narrow sliver of near-infrared light that is accessible from the ground?
In theory, any ground-based telescope located on a high mountain and in a very dry climate can observe the cosmos at near-infrared wavelengths. Many of the large optical observatories located around the world are also able to conduct some near-infrared measurements using special refrigerated science instruments.
Is it true that infrared astronomy can be done from airplanes?
Yes. By getting above most of the Earth's obscuring atmosphere, it is possible to conduct infrared astronomical observations from airborne telescopes. However, these airplanes must fly at altitudes above 12-13 kilometers (40,000 feet), or somewhat higher than most commercial flights. Early infrared observations were conducted with a small telescope aboard a Learjet in the early 1970s. The Kuiper Airborne Observatory (KAO) was a dedicated Lockheed C-141 airplane with a 1-meter diameter telescope, and was used for more than 20 years until it was retired in 1995. The next-generation airborne infrared observatory, the Stratospheric Observatory For Infrared Astronomy (SOFIA), will feature a 2.5-meter telescope mounted on a dedicated Boeing 747 airplane. SOFIA is currently under development and will make its inaugural flight in 2004.
If infrared astronomy can be done from airplanes, why do we build space-borne infrared satellites?
High-flying airborne observatories can get above 85 percent of the Earth's atmosphere (and above 99 percent of its water vapor, which absorbs infrared light). But the best "seeing" is still in the near-vacuum of space. Moreover, airborne observatories are suitable for conducting observations of a particular target for limited periods of time. But to survey the entire sky at infrared wavelengths, or to carry out any large observing program, requires a space-borne satellite that can point most anywhere in the sky and observe 24 hours a day.
Which infrared observatories are currently operating in space?
The latest major infrared observatory launched into space is the Spitzer Space Telescope, a NASA mission which was launched in August 2003. SIRTF was launched into an Earth-trailing solar orbit, and is currently still in operation.

The Hubble Space Telescope (HST) carries an infrared instrument called NICMOS that can observe in certain near-infrared wavelengths, but not at mid-infrared or far-infrared wavelengths. The first space-borne infrared telescope was the Infrared Astronomical Satellite (IRAS), a collaborative effort of the US, UK, and the Netherlands. IRAS orbited the Earth for 10 months in 1983, and the mission ended when its onboard cryogen (refrigerant) was exhausted.
 
A second-generation astronomical observatory, with much greater capabilities, was the Infrared Space Observatory (ISO). This European Space Agency satellite orbited the Earth from late 1995 through early 1998.

The European Space Agency's Herschel Space Observatory is an infrared observatory planned for launch in 2009.
 

Have any other infrared telescopes been used in space?
Yes, two other small IR telescopes have recently flown in space (neither is operating anymore). The InfraRed Telescope in Space (IRTS) was a small 15-centimeter infrared telescope developed by the Institute for Space and Astronautical Science in Japan. It was launched into space in early 1995 and operated for 3 weeks. IRTS was eventually retrieved by a subsequent Space Shuttle flight and returned to earth.
 
The Mid-course Space Experiment (MSX) was a military satellite that included an infrared science instrument called SPIRIT-III. It was launched into space in 1996 and operated for about 10 months. While this telescope was routinely used for military surveillance purposes, a substantial amount of observing time was devoted to astronomical observations, primarily of the Galactic plane.
 
A small NASA mission, the Wide-field InfraRed Explorer (WIRE), was launched in early 1999. It carried a 30-cm diameter cooled infrared telescope and was scheduled to observe distant galaxies during its four-month mission. Unfortunately, the telescope cover prematurely ejected, and bright sunlight entered the telescope. This caused the onboard refrigerant to vaporize at an accelerated rate as it attempted to keep the telescope cold, and was exhausted before any useful infrared observations could be obtained. One clever astronomer has been able to use the star trackers on WIRE to conduct an ingenious experiment involving starquakes. For details, see the CNN story at http://www.cnn.com/TECH/space/9908/04/space.salvage/.
Are electronic infrared detectors similar to the devices used in military applications?
Yes. In fact, much of the technological advances in mid-infrared detectors is the result of military research, and modifications by university-based and NASA scientists of use in astronomical applications. Like the night-vision goggles used in the military, astronomical IR detectors also measure heat but from very distant sources! At short infrared wavelengths, it is possible to build arrays of infrared detectors that can produce images much as a CCD yields optical light pictures.
 
At longer infrared wavelengths, much of the detector advancements have had to be built by researchers from scratch. Astronomers are only beginning to construct small arrays of detectors at these long wavelengths, and the fabrication of these devices still requires the skills and patience of a craftsman.
Where are the largest infrared telecopes
The largest infrared telescopes are on Mauna Kea in Hawaii at an elevation of about 14,000 feet. It is well above much of the water vapor that absorbs infrared making it a great ground based infrared site.

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