SOFIA (for Stratospheric Observatory for Infrared Astronomy) is an aircraft-based Infrared (IR) observatory operated by NASA and the German Aerospace Center (DLR) that began operation in 2010. The aircraft is a Boeing 747SP, which cruises at 41,000 feet, high enough that 85% of the full range of infrared is visible. The opening for the telescope opens at altitude, and has no glass. The telescope is 2.5 meters in diameter and is built to be very light and has a very short focal length.
Kuiper Airborne Observatory (KAO)