7. INFRARED TELESCOPES:

The infrared band is defined as radiation with wavelengths ranging from about 0.7 micrometers (mm) to about 1000 mm, between optical and submillimeter radio waves. Infrared telescopes are especially good for looking at objects that are too cool (< 2000 K) to emit optical radiation. The most important such objects are newly-forming stars and planets. Infrared waves can also pass through the dark interstellar clouds of dust that block optical and ultraviolet radiation.

Although ground-based telescopes can see through the atmosphere for part of the infrared wavelength range, much of the infrared range is impossible to see through the Earth's atmosphere, as illustrated below. But at altitudes of 10 km or more, we can see most of the infrared band. Therefore, we place infrared telescopes on aircraft, high altitude balloons, and spacecraft to observe sources at these blocked wavelengths.

Currently, NASA (in partnership with Germany) is building the Stratospheric Observatory For Infrared Astronomy (SOFIA), a 2.5 meter telescope that is mounted in a 747 aircraft with a big hole cut out of the top. SOFIA will begin flying in 2005.

To observe far-infrared wavelengths (>20 mm), we have another technical problem: at room temperatures, telescopes, including their mirrors, emit such radiation. The telescope itself is too bright at infrared wavelengths. The problem is the same as trying to observe stars with an optical telescope in daylight. This glow will limit the performance of SOFIA.

The Hubble Space Telescope has an instrument called NICMOS (Near Infrared Camera and Multi-Object Spectrometer) that can observe the sky in the spectral range 0.8 - 2.5 micrometers. But the HST is not cold enough to observe infrared sources at longer wavelengths.

The only way to remove the infrared glow of the telescope is to cool its mirrors and detectors to very low temperatures -- as low as 3 K (3 K = - 270 C = - 454 F). One way to do this is to wrap an infrared telescope in space in a shroud filled with liquid helium, which boils at about 3 K.

Much of what we know about the infrared sky came from such a telescope: the Infrared Astronomy Satellite (IRAS), which was built by NASA, the Netherlands, and the UK. (The satellite was built by Ball Aerospace in Boulder.) IRAS was launched in early 1983 and mapped the entire infrared sky over a period of 10 months, after which its liquid helium coolant all boiled away and the telescope became too warm to be useful. The infrared map in the Multiwavelength Milky Way came from the IRAS satellite.

In November 1995, the European Space Agency launched the Infrared Space Observatory (ISO). Although its mirror was the same size as the mirror on IRAS, ISO was more powerful than IRAS because it had better spectrometers and detectors. ISO ran out of liquid helium and ceased to operate in April 1998.

The next giant step in space infrared astronomy will be the Space Infrared Telescope Facility (SIRTF), which NASA plans to launch in April 2003. SIRTF is a 0.85-meter infrared telescope with much better cameras and spectrometers than ISO. Like IRAS and ISO, the SIRTF is cooled by liquid helium. But it will orbit the Sun, unlike IRAS and ISO, which orbited the Earth.

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Last modified January 19, 2003
Copyright by Richard McCray