Airborne environment: a challenge for telescope design

Telescopes are traditionally built and operated on earth. Others are built for operation in space. For both environments much of experience and data are available, which allow for a relatively straight-forward design. Not so an airborne telescope, which will be used not only for science but also for public outreach--a great idea, realized with SOFIA, the Stratospheric Observatory for Infrared Astronomy--with the consequence, that school classes will fly with the observatory. Therefore the telescope must be aircraft certified by the FAA, the Federal Aviation Administration, because it is part of the cabin pressure vessel. Also the environment for the telescope in the open cavity of the observatory aircraft is unfriendly compared with other telescope environments. It is surrounded by aircraft vibrations, high frequent excitations by air turbulence and temperature differences of 70 degree(s)C between the mirrors and the location of the science instruments. The paper explains the major design features of the SOFIA telescope, which are dominated by the harsh requirements.