Background and Technology

Ground Based Infrared Observations

Infrared detectors attached to ground based telescopes can detect the near-infrared wavelengths which make it through our atmosphere. The best location for ground based infrared observatories is on a high, dry mountain, above much of the water vapor which absorbs infrared. At these high altitudes, astronomers can study infrared wavelengths centered at 1.25, 1.65, 2.2, 3.5, 4.75, 10.5, 19.5 and 35 microns. Telescopes as well as our atmosphere emit infrared radiation which can complicate the observation of cosmic sources. Infrared telescopes are designed to limit the amount of this thermal emission from reaching the detectors. All ground based infrared detectors are cooled to extremely low temperatures to reduce their emission. In addition, astronomers making ground based observations measure both the emission from our atmosphere and from the object that they are observing. They then subtract the atmospheric emission from the infrared emission of a celestial object to get an accurate measurement.

In the mid-1960's, the first large-area near-infrared survey of the sky, the Two Micron Sky Survey, was made at the Mount Wilson Observatory using liquid nitrogen cooled PbS detectors which were most sensitive at 2.2 microns. The survey covered approximately 75 percent of the sky and detected about 20,000 infrared sources. Many of these sources were stars which had never been seen before in visible light. These stars were much cooler than our Sun and had surface temperatures of 1,000 degrees to 2,000 degrees Kelvin. Our Sun has a surface temperature of about 6,000 degrees Kelvin. The brightest 5,500 of these sources made up the first catalog of infrared stars. The 24 inch telescope used in this first infrared survey of the sky is shown to the left. A partial infrared survey of the southern sky was also made in 1968 at the Mount John Observatory in New Zealand.
New observatories, specializing in infrared astronomy, became possible in the 1960's due to advances in infrared detectors. The largest group of infrared telescopes can be found on top of Mauna Kea (a dormant volcano) on the island of Hawaii. At an elevation of 13,796 ft., the Mauna Kea Observatories , which were founded in 1967, are well above much of the infrared absorbing water vapor.

Richard Wainscoat, Institute for Astronomy,
University of Hawaii

By the early 1970s, it was found that the centers of most galaxies emit strongly in the infrared, including our own galaxy, the Milky Way. Quasars and other active galaxies were also found to be strong infrared emitters. All of this new information came from near-infrared observations which could be made from the ground. Today, most of the larger ground based telescopes have been modified to accommodate infrared detectors. Many ground based infrared telescopes are now using adaptive optics to create very sharp images. Adaptive optics removes the blurring of an astronomical image due to turbulence in earth's atmosphere.

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