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Boeing 747 Modified To Act As Infrared Telescope
xyz writes "A joint project of NASA and the German Aerospace Center has developed a highly modified Boeing-747SP aircraft to carry a 2.5-meter (98.4 inch) infrared telescope. The project SOFIA (Stratospheric Observatory For Infrared Astronomy) will observe radiation in the wavelengths from 0.3 microns to 1.0 millimeters, spanning the visible, infrared, and sub-millimeter portions of the electromagnetic spectrum. The observations will be taken at an altitude of 40,000 to 45,000 feet (12 to 14 km) which is above 99.8 percent of the water vapor in Earth's atmosphere, thus giving it a greater range of observations." Update: 10/31 13:27 GMT by T : Mea culpa -- headline changed to reflect that this telescope is intended for looking out at space rather than down at the Earth.
There is a set of seven nitrogen-filled airbags with active controls to isolate as much as possible. The telescope itself is gyroscopically stabilized. And for the worrisome "dumbbell" flexure mode, there is an active image compensation calculation.
It's MUCH more sophisticated than KAO was. As well as substantially larger.
Vibration transferred from the plane is relatively straightforward to solve (although it can expensive to do it to tolerance). The biggest challenge that caused the most delay (years) for SOFIA was the layer of air that was happy to be going around the side of the plane and then suddenly sees a 3m gap in the side of the plane. This led to a significant amount of turbulence and both shaking of the telescope and degradation of the "seeing" (the sharpness of images through the atmosphere and optics; in this case the very local atmosphere). Significant redesign and careful consideration of the exact shape, baffling, etc. of the hold and telescope mount was necessary to overcome this problem.
Transparent in IR is relatively easy (although not necessarily all the way out to 500 microns). Not emitting in the IR is hard. You have to be very cold (3 degrees Kelvin would be good). Much colder then you'd like the skin of the plane to be. Obviously the air is not that cold, but it's much thinner and so doesn't emit nearly as much as a solid sheet of whatever transparent material you could think of.