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Vibration-proofing for an airborne telescope

01 March 2008

A ContiTech air spring system is providing vibration-free working conditions on board the flying observatory SOFIA (Stratospheric Observatory for Infrared Astronomy). SOFIA is a Boeing 747 SP aircraft, converted into a flying observatory that conducts research on the genesis of stars and planetary systems from an altitude fourteen thousand metres.

At the heart of SOFIA is a 17-tonne infra red telescope assembly, including a primary mirror of 2.7m diameter. The assembly is isolated from aircraft borne vibration and the effects of wind flow (once the hatch is opened) by the air spring system and an arrangement of silicon-oil-filled dampers. With the aid of an electronic control system, the air spring system holds the telescope exactly in position relative to the aircraft's fuselage, ensuring that it remains on target.

Conditions within the telescope chamber, which is sectioned off from the rest of the flying observatory, include temperatures as low as -60ºC and variable air pressures (dependent upon altitude) down to just 20% that at sea level.

A complex design, comprising twelve air springs arranged in an axial direction, equalise the variance in pressure between the aircraft cabin and the telescope chamber, as well as isolating X-axis vibrations.

Another twelve radially directed air springs are incorporated into the telescope raising mechanism, as well as isolating it from Y- and Z-axis vibrations. Three dampers, located at angles of 120 degrees between the radially arranged air springs, operate in all six degrees of freedom.

Pressure is fed into the system via height control valves that regulate specific air spring groups independently one from another. The axially arranged air springs are supplied via a redundant system of conduits. The degree of isolation provided by the air spring system is sufficient for frequencies above the eigenfrequency of the vibration isolation system (approx. 2.5 Hz). A special fire-resistant and self-extinguising elastomer was developed for this application. The material also had to have a conductive surface (to eliminate electrostatic charges) and be resistant to very high levels of UV and cosmic radiation.

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