By Dan Brocius

High atop Mount Graham, Arizona, astronomers use one of the most sensitive radio receivers in the world to tune in to the radio broadcasts of the cosmos.

"This receiver won't fit in the dash of your car or tune in classic rock," says Arizona Radio Observatory (ARO) director Lucy M. Ziurys, "but it tells us volumes about the chemistry of the universe."

The "radio receiver" is the Heinrich Hertz Submillimeter Telescope (HHSMT). The precise shape of its 10-meter-diameter main reflector and its extremely sensitive detectors allow scientists to collect, focus, and measure the shortest and least explored of radio waves, the submillimeter band.

Most people think of astronomers as only looking at the light of stars. Why do astronomers care about radio waves?

"Because radio can tell us things we could never learn from visible light," says Ziurys," and we're only now able to begin mining the submillimeter portion of the radio spectrum. It's the last unexplored window into the universe from Earth."

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.

Submillimeter astronomy has remained terra incognita until recently 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.

"The 3,200-meter elevation of Mount Graham gets us above 33 percent of the atmosphere and most of the water vapor," says HHSMT operations manager Tom Folkers.

"We operate on an around-the-clock observing schedule 10 months of the year and use the summer rainy season as our maintenance period.

"The instrumentation we use to detect submillimeter waves faces incredibly tough technical challenges to work well at all," says Folkers.

"While the general concepts we draw on are standard techniques used in almost any radio," he says, "only radio astronomers push it hard enough to hear these frequencies."

Chemists in Space
During an interview in an ARO lab with the rhythmic "clunk-chunk" of a refrigator pump maintaining a cold telescope detector providing background, director Ziurys outlines the aim of her research group.

"We're interested in studying molecules in interstellar space," says Ziurys.

"We want to know how complex chemistry can evolve in the giant gas clouds of our galaxy. We're astrochemists using the tool of submillimeter astronomy because it's best suited to tell us what we want to know."

Not surprisingly, the Ziurys research group is located in the departments of chemistry and astronomy at the University of Arizona.

"While most people intuitively understand how optical astronomers can measure light from a star because our eyes can detect that energy," says Ziurys, "we have to build radio telescopes with specialized receivers to detect this part of the invisible universe."

As elements or molecules in the universe gain or lose energy, they emit weak radio signals, each acting as a tiny radio transmitter. This feeble energy makes its way across space, passing through dust and gas that stops visible light, until it encounters the Earth. Certain of these radio waves can make it through several partially-opened windows in the drier upper reaches of the atmosphere and be captured by the precision dish of the HHSMT.

The submillimeter waves ­ which are very high frequency for radio, approaching the wavelengths of visible light ­ are concentrated and reflected back to the secondary reflector, and then through a hole in the primary dish to where they are ultimately led to a receiver.

This signal from cold realms enters the vacuum of a super-cooled detector where its energy impacts an ultra-fine niobium super conducting junction. There the signal is joined with a locally generated radio signal and converted to a more manageable lower frequency. This signal is then extensively processed to produce data on the chemistry of the universe.

How does the HHSMT contribute?
The HHSMT is operated as a joint facility for the University of Arizona's Steward Observatory and the Max-Planck-Institut für Radioastronomie, Bonn, Germany. Radio astronomers from Arizona and Germany share most of the time, but outside observers may apply as well.

"The HHSMT is the most accurate radio telescope ever built," says Folkers. "By that I mean both the shape of the big reflector and how well we can point to and track astronomical objects."

To work well, any telescope's mirror surface, be it a radio or optical telescope, must be smoother than a small fraction of the wavelength of light sought. In the case of submillimeter-wave telescopes, this means the primary reflector must have no surface irregularities larger than about 20 microns (1/50,000th of a meter, about one sixth the thickness of a human hair).

The HHSMT main reflector is smooth to 15 microns. As past director Thomas L. Wilson said, "If the HHSMT 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."

The 10-meter wide dish consists of 60 lightweight carbon-fiber, aluminum-honeycomb panels joined together. The carbon-fiber material is 20 times less sensitive to temperature change than most metals. Each panel is faced with a 25 micron (.025 mm) thick aluminum foil surface to reflect radio waves. The panels sit atop a carefully designed composite supporting structure that maintains the curve of the dish.

(The Steward Observatory Mirror Lab, known for spincasting large glass mirrors for optical telescopes, and the University's Optical Sciences Center, made an important contribution early in the project by precisely generating the shapes of the glass molds the HHSMT's carbon-fiber panels were formed over.)

"The combination of light, strong, thermally stable materials and a well-designed support structure enable the reflector to maintain its shape to, literally, within the diameter of a human hair," says manager Folkers. "Remember, this has to be done on a mountaintop regardless of wind, sunlight and temperature changes," he adds.

The HHSMT is the only large submillimeter telescope that astronomers can use both day and night. 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. The daytime sun heating of the surrounding ocean causes clouds of water vapor to rise over the site effectively closing the sky to submillimeter radio waves during daylight.

Intersteller Space
The Ziurys astrochemistry group uses the HHSMT to look for molecules in the spaces between stars. Interstellar space was once thought to be too harsh an environment for anything but elements and perhaps the simplest compounds. But astronomers, radio astronomers in particular, continue to discover more and more complex chemical structures out in space.

"By now, there must be 130 chemical compounds detected in the interstellar medium," says Ziurys.

"Of particular interest to us are small molecules containing a few atoms, one of which is a metal atom with a simple attachment. We are also investigating small organic molecules related to sugars and nucleic acids."

People are interested in the origin of life. So where does life begin? Soup on a planet? Where do the soup and the planet come from?

"From the synthesis of elements by stars for starters," answers Ziurys. "But beyond that, we're looking to see how complicated chemistry can get in places like large molecular clouds. What kind of compounds are there and how do they form?"

Ziurys' group has a project underway to search for the simple sugar ribose ­ the backbone of DNA.

"We don't know what it looks like in radio," says Ziurys, "so we have to detect it in the lab first.

"Put simply, the team will beam a radio wave through a gas of ribose molecules, detect that signal with the spectrometer they built for the purpose, and see what the spectrum is. Among the challenges is the need to cool the lab molecules down to 10-20 K in a supersonic molecular beam.

"With those fingerprints in hand, we'll know what to look for," says Ziurys.

"Finding a part of the code carrier for life would be spectacular."