History of Infrared Astronomy
Early Infrared Astronomy
After Sir William Herschel's discovery of infrared ,which showed that the Sun emits infrared radiation, astronomers tried to see if other objects in the universe gave out infrared waves. In 1856, astronomers used thermocouples (devices which convert heat into electric current) to detect infrared radiation from the Moon. Much later, in 1948 (decades before the first Moon landing), more sophisticated infrared studies of the Moon would show that its surface was covered with a fine powder. In the early 1900's, infrared radiation was successfully detected from the planets Jupiter and Saturn and from some bright stars such as Vega and Arcturus. However, the insensitivity of the early infrared instruments prevented the detection of other near-infrared sources. Work in infrared astronomy remained at a low level until breakthroughs in the development of new, sensitive infrared detectors were achieved in the 1960's.
During the past few decades, infrared astronomy has become a major field of science due to the rapid advances in infrared detector technology. Many of these advances arose from U.S Department of Defense research into infrared array technology in the 1980's. Infrared radiation, having longer wavelengths and lower energy than visible light, does not have enough energy to interact with the photographic plates which are used in visible light astronomy. Instead infrared astronomers rely on electronic devices to detect radiation. Early infrared astronomers used thermocouples and thermopiles (a group of thermocouples combined in one cell).
In the 1950's astronomers started to use Lead-sulphide (PbS) detectors to study infrared radiation in the 1 to 4 micron range. When infrared radiation in this range falls on a PbS cell it changes the resistance of the cell. This change in resistance can be measured and is related to the amount of infrared radiation which falls upon the cell. To increase the sensitivity of the PbS cell it was cooled to a temperature of 77 degrees Kelvin by placing it in a flask filled with liquid nitrogen.
A major breakthrough came in 1961, with the development of the germanium bolometer. This instrument was hundreds of times more sensitive than previous detectors and was capable of detecting all infrared wavelengths. Basically, a cool thin strip of germanium is placed in a container which has a small opening in it. When infrared radiation comes through the opening and hits the germanium, it warms the metal and changes its conductivity (a measure of how much electrical current flows through an object). The change in conductivity can be measured and is directly proportional to the amount of infrared radiation entering the container. The germanium bolometer works best at an extremely low temperature (much lower than liquid nitrogen). The best way to cool the bolometer to such a low temperature is to surround it with liquid helium which cooled it to 4 degrees Kelvin. This is only a few degrees above absolute zero. To do this a metal Dewar (similar to a well insulated thermos flask) was developed which was able to hold the liquid helium in which the germanium bolometer was immersed. This type of infrared detector is sensitive to the entire range of infrared wavelengths. To study a particular wavelength of infrared emission from astronomical objects, astronomers place filters in front of the detectors, which filter out all but the desired wavelengths.
Infrared detector technology continues to advance at a rapid rate. Astronomers now use InSb and HgCdTe detectors for the 1 to 5 micron range. These operate in a way similar to the PbS detectors but use materials which are much more sensitive to the infrared. The development of infrared array detectors in the 1980's caused another giant leap in the sensitivity of infrared observations. Basically a detector array is a combination of several single detectors. These arrays allow astronomers to produce images containing tens of thousands of pixels at the same time. Infrared arrays have been used on several infrared satellite missions. In 1983 the IRAS mission used an array of 62 detectors. Astronomers now commonly use 256x256 arrays (thats 65,536 detectors!).
Due to these breakthroughs in infrared technology, infrared astronomy has developed more rapidly than any other field of astronomy and continues to bring us exciting new views of the universe.
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.
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.
Infrared Astronomy Takes Off
In addition to absorbing most of the infrared radiation from cosmic sources, the Earth's atmosphere itself radiates in the infrared which interferes with infrared observations. This is why it is best to get above as much of the atmosphere as possible to observe in the infrared. To do this, infrared detectors have been placed on balloons, rockets and airplanes, allowing astronomers to study longer infrared wavelengths. Even though these methods can only observe a small part of the sky for short periods of time, they have contributed much to infrared astronomy.
The first cooled telescopes were those placed on rockets which could observe the sky for several minutes before reentry. The first infrared all sky map resulted from a series of rocket flights by the Air Force Cambridge Research Laboratory. This project, called Hi Star, surveyed the cosmos at wavelengths of 4, 10 and 20 microns. Although the total observation time accumulated by these flights was only about 30 minutes, they successfully detected 2363 reliable infrared sources which were published in the AFCRL Infrared Sky Survey. About 70% of these sources matched sources found by the Mount Wilson 2.2 micron survey. Rockets also found bright infrared emission from HII regions (regions of ionized hydrogen) and the center of our galaxy.
Plans are being made by NASA for a new airborne observatory. SOFIA - The Stratospheric Observatory For Infrared Astronomy will be an optical/infrared/sub-millimeter telescope mounted in a Boeing 747 and is expected to be fully operational by the year 2004.
Infrared Astronomy From Earth Orbit
In the 1970s, astronomers around the world began to consider the possibility of placing an infrared telescope on a satellite in orbit around the Earth. This telescope would be above the Earth's atmosphere and could view the sky at the mid- and far-infrared wavelengths which could not be detected on Earth. It could view a large area of the sky and observe regions for a longer period of time.
By 1977, an international collaboration was formed by the Netherlands, United States and Great Britain to develop IRAS - The Infrared Astronomical Satellite. The American team built the telescope, detectors and cooling system. The British built the satellite ground station and control center and the Dutch team built the spacecraft which included the on-board computers and pointing system.
In November 1989, NASA launched the COBE satellite to study both infrared and microwave characteristics of the cosmic background radiation (the remains of the extreme heat that was created by the Big Bang). Over a ten month period, COBE mapped the brightness of the entire sky at several infrared wavelengths and discovered that the cosmic background radiation is not entirely smooth, showing extremely small variations in temperature. These variations may have led to the formation of galaxies.
The Infrared Telescope in Space (IRTS), launched in March 1995, was Japan's first infrared satellite mission. During its 28 day mission, IRTS surveyed about 7% of the sky with four instruments: A Near and Mid Infrared Spectrometer which covered wavelengths of 1.4 to 4 microns and 4.5 to 11 microns respectively, a Far Infrared Line Mapper which studied Oxygen and Carbon spectral lines at 63 and 158 microns, and a Far infrared Photometer which studied the sky at four bands centered at 150, 250, 400, and 700 microns. This data should add to our knowledge of cosmology, interstellar matter, late type stars and interplanetary dust.
The European Space Agency launched the Infrared Space Observatory (ISO) in November 1995. ISO, which observed at wavelengths between 2.5 and 240 microns, not only covered a much wider wavelength range than IRAS but was also thousands of times more sensitive than IRAS and viewed infrared sources with much better resolution. ISO took data for about 2.5 years (3 times times longer than IRAS). It ceased operations in April 1998 when its supply of liquid helium ran out. ISO contained instruments which measured details of both the shorter and longer wavelength regions of the infrared spectrum, an infrared camera which had two infrared arrays, and a photometer. Unlike IRAS, which was an infrared survey mission, ISO is operated like a ground based observatory, having astronomers submit observing proposals to study specific astronomical objects in detail. As hundreds of astronomers from several countries study the data from ISO, important new discoveries about our universe are expected to emerge. ISO has already detected dry ice in interstellar dust and hydrocarbons in some nebulae.
The Midcourse Space Experiment (MSX) was launched in April 1996 and lasted until its liquid helium coolant ran out in Feb 1997. During its 10 months of operation, MSX gathered a vast amount of data at 4.2 - 26 microns. MSX studied the infrared emission from the gas and dust which permeates the universe. MSX had 30 times the spatial resolution as IRAS and surveyed areas of the sky which were missed by IRAS.