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SOFIA SUPER-TELESCOPE FITS INTO 747

When every now and then astronomers travel to the ends of the earth, for example to scan the sky with infrared telescopes on Mauna Kea in Hawaii, a 4,200 metre high extinct volcano, there is method in this apparent madness. Any astronomer who makes his way to such a remote spot is driven by the search for optimal observation conditions.

Today infrared astronomy is an important field in research on the universe. Like the visible spectrum (light), infrared rays form a part of the electromagnetic spectrum. But becuase the infrared spectral region is around ten times greater than the visible spectrum, it holds out the prospect of enormous observation potential to astronomers.

However, most infrared radiation is absorbed due to water vapour, carbon dioxide and ozone in the atmosphere so that infrared observatories are positioned on high mountains. But even at these heights the majority of the infrared wavelengths are swallowed up. To utilise the scientific potential of infrared observations, researchers must reach still higher.

One option is to position telescopes on satellites, thus avoiding any impairment of their operating range due to the atmosphere. But airborne observatories which operate at altitudes of between 12 and 15 km (39,000-49,000 ft) also offer a number of advantages. These airborne observatories make their observations from the lower part of the stratosphere, thus keeping above the troposphere, the atmospheric layer in which weather events normally occur. At these altitudes astronomers can then capture most of the infrared radiation which enters the Earth's atmosphere.

The American space agency NASA and the German Aerospace Centre (DLR) are currently working together on just such a project. The infrared SOFIA (Stratospheric Observatory for Infrared Astronomy) observatory is to be installed on board a Boeing 747 and in two years' time will have teams of German and American astronomers at its disposal.

Expectations and hopes are running high that SOFIA will do justice to its heavily symbolic name (in Greek and Latin "sophia" means "wisdom"). If the mission were to succeed in unravelling the last secrets from light in the infrared spectrum, this would be a milestone in airborne astronomy. In actual fact, SOFIA's chances of performing measurements of the highest quality and hence of eclipsing terrestrial infrared telescopes are very good. For many celestial bodies are only visible in the infrared spectrum of light. Spiral galaxies, for example, radiate up to a thousand times more strongly in the infrared spectrum than in the visible spectrum. As thermal radiation passes through dark gas clouds and interstellar dust clouds without any problems, the young stars and planet systems which are in the process of being formed there can now be observed. Distant galaxies and dust haze, the different evolutionary stages of the universe, the origin of planets and sun systems, the centre of our Milky Way (which is suspected of harbouring a black hole) also suddenly become "visible" as long as the "telescope" is at the right altitude.

In this respect even the latest infrared satellites will barely be able to hold their own against SOFIA. In the stratosphere in which SOFIA will operate the vast majority of the water vapour has been left behind. But Dr. Ruth Titz of DLR, Berlin-Adlershof sees further critical advantages. "Unlike satellites, our system can undergo continuous improvement and be deployed flexibly. With an aircraft platform, we can choose the trajectory to follow during the eight-hour flight time so that one object can be targeted for several hours. The measuring instruments on SOFIA can be upgraded to the latest technology without great expense, something that is impossible with a satellite."

Whereas with satellite technology the instrument design is frozen many years before satellite launch (up to ten years), even though everyone knows far better hardware and software will become available on the market within the year, with an aircraft it is possible to substitute any certified component instantly.

Another major advantage of this concept is the flexibility with which it can be deployed: "As a flying observatory can in principle take off and land at any major airport, it can be used in any part of the world to observe transient or highly localised astronomical events, such as, for example, stellar occultations and solar and lunar eclipses," explains Professor Hans Peter Röser, head of the DLR Institute of Space Sensor Technology and Planetary Exploration in Berlin.

The aircraft platform for the SOFIA project is a Boeing 747 SP, a reduced-length version of the jumbo jet which was in use on scheduled services from 1977 to 1995 with the US airlines Pan Am and United Airlines. This is now to pick up where its American predecessor left off.

The era of airborne astronomy in fact began in 1969 when a refurbished Learjet was sent in pursuit of infrared waves. But this converted business aircraft only had room for two astronomers, who also had to make do with a 30 cm telescope. The Kuiper Airborne Observatory (KAO), a converted Lockheed L200 Starlifter troop-carrier (military designation C141) equipped with a 91.5 cm reflector, was much larger and more versatile and completed around 60 to 80 flights a year (each lasting seven hours) from 1974 to 1994.

"For German teams it was very difficult to get any flying time on the American KAO. Only two German teams regularly flew with instruments they have developed themselves," says Ruth Titz. So it is hardly surprising that while the KAO was still in service the idea arose of designing a successor aircraft which would permit even more sensitive measurements, especially as the KAO observatory was mainly used by American scientists.

After a 10-year discussion and study phase, development of SOFIA began in 1997. The 747 SP is currently undergoing conversion at Raytheon's Waco plant, Texas. A significant part of the work here entails initially preparing the 747 for integration of the 20-tonne telescope system. At the start of the study phase the plan was still to mount the telescope in the front section of the aircraft. However, this solution would have necessitated complex and costly modifications, such as building a pressure-sealed tunnel between the cockpit and the mid section of the fuselage.

The telescope is now to be accommodated in the rear of the airliner. This requires that an approximately 10 m2 aperture is made in the outer skin of the 747, which will be covered by a louver-like sliding door. Once the aircraft has reached its planned cruise altitude on an observation mission, this door will be pushed upwards, giving the primary mirror a clear view of the sky.

During operation, the telescope will be exposed to extreme external conditions: at an altitude of 14 km (46,000 ft) it must withstand a temperature of around -60*C and an atmospheric pressure which is only about one-fifth of that at sea level. On top of vibration caused by the aircraft engines, the telescope will have to endure being buffeted by turbulent air flowing past the telescope port, which will be open to the outside. For this reason the telescope is to be mounted on air cushions.

The external shape of the port has been optimised in numerous wind tunnel tests so that the (contiguous) laminar flow over the aft fuselage is essentially maintained and does not have any negative effect on the flow of air on the control surfaces nor, therefore, impair the 747's ability to maintain controlled and stable flight. Wind loading, which could potentially affect the stable alignment of the telescope, is to be offset by a special telescope drive system.

The rear section of the aircraft containing the telescope system is separated off from the air-conditioned passenger cabin by a pressure bulkhead so that the research staff and control instruments are protected during flight from the low air pressure and the cold. The section of the fuselage between the wings will accommodate the scientists and telescope operators. Seats will be available for passengers in the front section of the aircraft and the upper deck.

The telescope system itself lies at the heart of the project. Under the collaborative agreement, the DLR is responsible for design and construction of the entire telescope. The contract for this work was let to German industry at the start of 1997. The MAN Technologie and Kayser-Threde companies formed a consortium for this purpose.

The telescope system consists of a 2.7m wide glass ceramic primary mirror which can withstand even extreme temperature fluctuations unscathed. There is also a smaller secondary mirror 34 cm wide and a tertiary mirror which separates the infrared and visible rays and passes them on to the scientific instruments.

The primary mirror was supplied as a blank by Schott Glaswerke in Germany. It is made of ZerodurTM, a special material developed by Schott. In an 18-month long process known as "lightweighting", the weight of the mirror was then reduced from 4,500 kg to around 900 kg by REOSC in France, acting as subcontractors to Kayser-Threde.

During this process, a honeycombed structure was cut into the back side of the mirror. The walls of the mirror are now only 7 mm thick in places. The front of the mirror is currently being shaped and polished. This phase should be complete by the end of 2000.

The entire design of the telescope is geared towards maximum weight savings combined with high stability. Every tonne less weight carried increases the usable observation time by approximately eight minutes. The structure of the telescope system is therefore composed largely of carbon fibre strengthened synthetic material. The telescope will be delivered to NASA in 2002.

On the American side, NASA is responsible for purchasing, modifying and operating the aircraft, and also for the ground-based infrastructure. For this purpose it has let work packages to a private team headed by the Universities Space Research Association (USRA), an amalgamation of 80 American universities and United Airlines.

As the DLR is contributing 20 percent of the project and operating costs, amounting to around $160 million (DM 350 million), German scientists will get 20 percent of the available observation time on SOFIA. The lion's share of the costs is being borne by NASA. "It is only thanks to free-market competition and the cost reductions which this enables that implementation of the project has been made possible. SOFIA was literally in danger of collapsing more than once during the 10 years or so that it took to plan the project," says Dr. Titz.

Starting at the end of 2002, SOFIA is to complete 160 flights a year and 30 German research teams will be available. SOFIA will function not just as an observatory but also as a unique educational and public relations platform, just as KAO did in its day, allowing over 60 teachers to look over the shoulders of the scientists as they worked. As part of NASA's Education and Public Outreach (EPO) programme, once again teachers and scientific journalists will be able to follow events on the telescope on board SOFIA. The educators will even be able to communicate with their school classes from on board via an audio-visual link. And at SOFIA's home base, at the NASA Ames Research Centre in Moffet Field near San Francisco, the hangar is being converted to a training and conference centre for children and adults.

From page 40 of FLUG REVUE 7/2000