Largest airborne telescope

The largest airborne telescope is the Stratospheric Observatory for Infrared Astronomy (SOFIA), which is a joint project of NASA (USA) and the DLR (DEU; known in English as the German Aerospace Center). This observatory is a Boeing 747SP that has been heavily modified to accommodate a 2.5-m (8-ft 2-in) aperture reflecting telescope in its rear fuselage. The telescope made its first airborne observations on 26 May 2010 and is still operational as of Feb 2019.

The water vapour in the air interferes with infrared telescopes, making it difficult to observe faint objects from the ground. SOFIA can avoid this distortion because its cruising altitude of around 12,000 m (41,000 ft) puts it above almost all the water vapour in the atmosphere.

Making an aircraft into a working observatory is an astonishing feat of engineering. The telescope has to be light enough to fly, but sturdy enough to provide a stable platform. It also has to be isolated from the vibration of the aircraft’s engines and the buffeting of the wind. The aircraft, meanwhile, has to be able to not only accommodate the huge telescope (which has an oversized, 2.7 m diameter primary mirror), but also to keep flying even after the observatory door – measuring 5.5 m (18 ft) tall by 4.1 m (13.5 ft) wide – has opened. The opening is just a gap in the hull because glass would interfere with the telescope.

In addition to providing a clearer image, using an aircraft as an observatory also gives it the advantage of mobility. SOFIA can, for example, fly along the path of an eclipse or occultation (wherever in the world the shadow falls) to observe objects that are otherwise hidden.