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Dietmar Lilienthal (foreground, gesturing), German program manager for the SOFIA, points out a feature of the NASA 747SP to a group of German dignitaries.Image above: Dietmar Lilienthal (foreground, gesturing), German program manager for the SOFIA, points out a feature of the NASA 747SP to a group of German dignitaries. (NASA photo by Tom Tschida)

For as long as humans have stared up into the night sky, curiosity about the heavens has drawn their gaze.

It is this fundamental need to know the unknown - and a desire for the ever-larger telescopes needed to find it - that has led Americans and Germans to forge a partnership aimed at unlocking the mysteries of the heavens.

In the Stratospheric Observatory for Infrared Astronomy, or SOFIA program, the two countries have joined to assemble the world's largest and most modern airborne observatory. Through that partnership, scientists are working toward the day when groundbreaking astronomical missions will contribute to understanding of those mysteries. And they hope to see that day come in as little as two years.

Dietmar Lilienthal, German program manager for the SOFIA, and German scientist Jürgen Wolf recently offered insights into their country's development of the flying observatory's telescope and instruments as well as what astronomers worldwide hope to discover when the SOFIA reaches full operational capabilities.

The German Aerospace Center, or DLR, is the agency leading the German portion of the SOFIA program, which accounts for about 20 percent of the program's content. The DLR contracted with German industry partners to build the observatory's 17-metric-ton telescope. German science operations, including integration of science instruments, are managed through the Deutsches SOFIA Institut, or DSI, of the Universität Stuttgart.

The German-built telescope is 2.7 meters (2.5 meters of usable surface for observation, more than eight feet) in diameter, slightly larger than the Hubble Space Telescope. The principle component, the main mirror, was built by SCHOTT Lithotec of Mainz, Germany. The mirror is constructed of Zerodur, the ceramic material used in stovetops. Use of the ceramics will prevent thermal expansion; significant temperature changes can affect the optical stability of mirrors made of other materials.

SOFIA chief pilot Gordon Fullerton, center, gives a group of VIP visitors from Germany a tour of the NASA 747SP cockpit.Image above: SOFIA chief pilot Gordon Fullerton, center, gives a group of VIP visitors from Germany a tour of the NASA 747SP cockpit. (NASA Photo by Tom Tschida)

The mirror along with its stabilization system, called the primary mirror cell, weighs about two tons. The Zerodur mirror isn't the first of its type but it is one of the largest. It also is very complex. In order to reduce the telescope's weight SCHOTT staff members milled 130 holes in the rear of the main mirror, a process that did not affect its optical surface. The process created a honeycomb-like structure that reduced the mirror's weight by a factor of five from the original four-ton block of Zerodur from which it was carved.

A joint effort by German companies MT Mechatronics (formerly known as MAN Technology) and Kayser-Threde contributed optics, the bearing and the electronics for the telescope.

The mirror took two years to develop and will require further preparation before actual science missions can begin. The mirror and its stabilization system will be removed from the 747SP by crane and transported by truck to a facility located at Ames Research Center, Moffett Field, Calif. Once there, the surface of the mirror will be ground and prepared for an aluminum coating that will be added to the mirror then polished to maximize reflectivity of light from the heavens.

"It is the largest airborne infrared telescope ever built and has a mirror nine times larger than the mirror in the Kuiper Airborne Observatory's telescope and ten times higher in optical efficiency, which will allow scientists to see much fainter objects," Lilienthal said, referring to the SOFIA predecessor. The KAO was based at Ames Research Center during its 21-year career.

In addition to the telescope, German contributions will include engineers and scientists, fuel, telescope system spare parts, the SOFIA's paint scheme and the aircraft's four Pratt & Whitney JT-9D-7J engines. The Germans also are contributing two science instruments to the SOFIA's existing inventory.

German infrared astronomers in the past have collaborated with their U.S. counterparts on the Kuiper Airborne Observatory and are looking forward to future endeavors aboard the SOFIA.

"One of the highlights will include study of the galactic center, where there is a black hole," Lilienthal said. "One of the most interesting questions in astronomy is to find out what is the driving engine in spiral galaxies, and we believe that to be black holes. The better we understand black holes, the better we will understand the formation of galaxies.

"With the infrared telescope, we have the chance to use long wavelengths to peer deeply into gas dust clouds."

To support the program's science goals, the German team is relying on two additional instruments. The first, called the German Receiver for Astronomy at Tetahertz Frequencies, or GREAT, is a high-resolution heterodyne spectrometer, Wolf said, which will give readings on gas and its velocity with a greater sensitivity than ever before possible. An institutional consortium that included the Max Plank Institute for Radio Astronomy of Bonn, Germany, the Bonn University of Cologne, Germany, and the DLR developed the spectrometer.

A spectrometer is an instrument incorporated onto the telescope system to detect the energy signatures of atoms and molecules in dust clouds. Spectrometers were a hallmark of the Kuiper Airborne Observatory science and will be important on the SOFIA - five of nine instruments to be used in early missions are spectrometers.

The second is the Field-Imaging, Far-Infrared-Line Spectrometer, or FIFI LS. The FIFI LS boasts a leap in sensor technology that will enable viewing of extra-galactic, super-luminous bursts of stars. It will be used to study nearby galaxies and could provide insight into star birth and formation and the content of gas clouds like molecular hydrogen. Researchers at the Max Plank Institute for Extraterrestrial Physics in Garching, Germany, developed the instrument. The SOFIA will offer researchers a quantum leap in capability as they explore new paths to discovery. It will be through the work of the U.S./German cooperative effort, however, that the awe and excitement of new discoveries in astronomy will be brought home to people around the globe.

Jay Levine
X-Press Editor