Astronomers In Arizona Aim For Shortest Submillimeter Wavelengths

Astronomers plan soon to make observations at the shortest, highest-frequency submillimeter wavelengths ever detected from Earth -- with a telescope that satellite holography recently proved to be the most accurate of its kind in the world.

If they succeed, they pave the way for ground-based astronomy at the shortest possible submillimeter, or microwave, wavelengths. Until astronomers complete this gap between radio and infrared/optical astronomy, they cannot piece together a coherent, comprehensive picture of the universe.

The telescope is the 10-meter Heinrich Hertz Submillimeter Telescope (HHT), a joint project of The University of Arizona Steward Observatory in Tucson and the Max Planck Institute for Radioastronomy (MPIfR) in Bonn, Germany. Astronomers from the UA and the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge this month will install a unique receiver at the $10 million telescope on Mount Graham, Ariz.

The surface shape of the telescope's primary reflector must be incredibly accurate to observe very high-frequency submillimeter wavelengths, said Thomas L. Wilson, director of the Submillimeter Telescope Observatory. The observatory, which consists of the HHT and its co-rotating enclosure, is staffed by 15 Arizona and German researchers. Wilson became observatory director in September 1997.

This fall, observatory staff measured and adjusted the accuracy of the 60 panels in the 10-meter (33-foot) primary reflector by a technique called satellite holography. It involved scheduling time on a communications satellite operated by the MIT Lincoln Laboratory for the U.S. Defense Department. Observatory staff used signals from the satellite's radio beacon and a special receiver from the National Radio Astronomy Observatory in Tucson to map the shape of the reflector twice each night. They then made fine adjustments by moving panels in or out, or tilting panels left or right.

After about three weeks of measuring and mapping, the HHT reflector was brought to an accuracy within 12 microns – about half the thickness of a human hair, or one part in a million of the diameter of the telescope. "If the HHT dish were a mile across, no surface irregularity would be greater than a few widths of a fingernail. That makes this telescope THE most accurate telescope in the radio range," Wilson said. The feat is all the more remarkable because it was done on a reflector in an open telescope on a 10,425-foot (3,200-meter) mountaintop, not in a laboratory, he added.

The Harvard-Smithsonian Center for Astrophysics developed the new technology submillimeter receiver to be installed at the HHT later this month. The new-type "HEB" receiver, or Hot Electron Bolometer, incorporates a new material called nobium nitride. The receiver was first tested on a telescope – the HHT – during three weeks last March. Astronomers measured spectra at the high frequency bands they aimed for -- the 690 GigaHertz (500 micron wavelengths) and 810 GigaHertz (350 micron wavelengths) bands. (A hertz is a unit of frequency, one cycle per second. A GigaHertz is a billion Hertz.) "We believe these are the first astronomical spectra ever reported with an HEB system on a telescope," Wilson said. "This has very great promise for studies at really high frequency wavelengths – 200 microns or shorter." Wavelengths this short approach one TeraHertz (trillion Hertz) frequencies – about 10,000 times higher frequency than an FM radio receives. Such high-frequency wavelengths – which border far infrared wavelengths -- cannot be detected with conventional, superconducting receivers currently used, Wilson said.

Submillimeter, or microwave, astronomy covers the wavelength range between 300 microns and 1,000 microns (three-tenths of a millimeter and one millimeter). Longer wavelengths fall in the radio region of the electromagnetic spectrum; shorter wavelengths are in the far infrared region. The submillimeter region is regarded by many to be astronomy's last wholly unexplored wavelength frontier. It is essential in understanding the process of star and planet formation, both in our own Milky Way Galaxy and in other galaxies throughout the universe.

One major target for submillimeter astronomy is to study the cool, molecular clouds of dust and gas that exist 100 light years or more from our sun, Wilson said. (Astronomers theorize that a supernova explosion swept away the molecular material closer to the sun, which at the time already had formed its solar system.) Astronomers need a better understanding of the physical conditions within molecular clouds to better understand why stars form, and why some stars give rise to planets, while other stars do not.

Submillimeter astronomy has remained terra incognita because of the sheer complexity of its astronomical instrumentation and a dearth of extremely good observing sites since water vapor in Earth's atmosphere absorbs submillimeter radiation. Only the driest atmosphere is sufficiently transparent to submillimeter radiation.

"Weather plays a big role in submillimeter astronomy," Wilson said. "I think a fair statement is that if you want to observe at submillimeter wavelengths, you have to reckon that half the time is lost to other-than-optimal weather. Then, during the other half of the time, there can be problems with the receiver or the telescope. The receivers which we use are really very complex. They really are cutting-edge physics in themselves."

The Steward Observatory Radio Astronomy Lab headed by Christopher Walker, the MPIfR Heterodyne Receiver Group and the MPIfR Bolometer Group have provided much of the HHT instrumentation available to the astronomical community. Wilson said the Submillimeter Telescope Observatory has started developing a suite of receivers in collaboration with the National Radio Astronomy Observatory and the Harvard-Smithsonian CfA.

The HHT is the only large submillimeter telescope that astronomers can use both day and night. It owes this unique capability to the fact that it is made of a carbon-fiber reinforced plastic that is 20 times less sensitive to temperature change than most metals, and to that fact that it is located at a high-altitude site that is generally very dry. The world's other two large submillimeter telescopes – the 10-meter Caltech Submillimeter Telescope and the 15-meter James Clerk Maxwell Telescope – are both located on Mauna Kea, Hawaii, a mountain surrounded by ocean so that it trails off clouds of water vapor when heated by daily sunshine. The HHT is shut down during the Arizona summer monsoons, but is free of the local diurnal effect that plagues the Hawaiian-based telescopes.

A small, millimeter-wavelength radio receiver mounted on the HHT enclosure measures atmospheric conditions. UA and CfA astronomers will observe at the shortest, highest-frequency wavelengths this winter when the weather is extremely good -- usually right after a big snowstorm, after the water has fallen from the sky and temperatures turn very cold, Wilson said.

"I think the chance of finding something new is much better the shorter wavelengths, where little astronomy has been done," Wilson said. "We're hoping for a big surprise, something really spectacular."

Prior to becoming director of the Submillimeter Telescope Observatory, Wilson was a staff scientist for the MPIfR in Bonn. His work there involved helping to start a 100-meter telescope in Effelsberg, Germany, and a 30-meter telescope in Spain, as well as the 10-meter HHT in Arizona. His more than 220 publications and three books include the book, "Tools of Radio Astronomy," about to published in 3rd edition. Wilson completed his doctorate in physics in 1969 at the Massachusetts Institute of Technology, where his thesis advisor was Bernard F. Burke. Burke was in Tucson last Friday to receive the 33rd Karl G. Jansky award for his pioneering work in radio astronomy.