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.

Kuiper Airborne Observatory

[Anonymous Coward]

Been there, done that, in 1974 even

http://en.wikipedia.org/wiki/Kuiper_Airborne_Observatory [wikipedia.org]

Re:Earth-observing?

[tomatensaft]

Author probably meant "Earth-based observing", although calling a Boeing 747 flying at 10-12 km above sea level "Earth-based" is kind of a stretch, even by Hubble telescope standards... :)

Impressive engineering

[hcdejong]

The telescope will be exposed to the elements during flight: this photo [usra.edu] of the telescope installation shows that the aircraft will be flying around with a 3x3 m hole in its fuselage.

The buffetting and general vibration levels must be huge.
here [usra.edu] is how they plan to compensate.

Re:Earth-observing?

[michaelwv]

It has a mirror that's more than two-and-a-half times larger, with correspondingly better resolution and sensitivity, and instrumentation that's several generations more advanced than Kuiper. Also, notably it will be flying and observing in the next decade and Kuiper hasn't flown since 1995. Science continues and new questions arise every day that need new observations to answer them.

Re:Vibration?

[tweak13]

It will probably be mounted on some kind of stabilized gimbal mount, much like the kind used to mount cameras to helicopters. A helicopter mount has to deal with probably 100x the vibration that a 747 in smooth air would have. Keep in mind they are going to not only be able to pick which days they fly, but pick the location as well. It won't be very hard for them to find good conditions. Example of helicopter mount here [axsys.com]

Re:Vibration?

[Quantos]

This is taken from the SOFIA site.

At visible wavelengths, it is neither atmospheric turbulence, the refractive action of mobile air cells which push light rays around, overhead (actually there is not much air left overhead) that causes the blurring problem, nor the aircraft and telescope shaking that causes the problem, but rather the "shear layer" stream of air shooting past the open airplane cavity where the telescope sits, at 500 mph. This air motion worsens the resolution (the opposite of blurring) to 3 arc secs at visible wavelengths.

But the problem at the long wavelengths is different - it's diffraction. Basically, the far-infrared light observed by SOFIA passes through the shear stream of air unperturbed. But this light has such a long wavelength, 100x to 1000 times the wavelength of visible light, that the SOFIA telescope is of insufficient size to focus it sharply, and blurriness results. At wavelengths in the far-infrared, like 60 micrometers, there is significant blurring due to this effect. The telescope is actually held extremely steady while observing occurs, even in turbulence. It's held about as stable as a mountaintop telescope sitting on a 10 meter cement foundation, but diffraction still blurs the image.

So how do you do this? First, you isolate the telescope from the airplane by mounting it on a spherical pressurized oil bearing. The plane shakes and quakes, but the telescope doesn't feel it. Second, you direct the wind away from the telescope by shaping the side of the airplane so as to deflect it, and install a little deflector fence on the edge of the telescope cavity as well. Third, you stabilize the telescope against sudden motion (wind does get through) by spinning three orthogonal gyroscopes which are rigidly attached to the structure, and fourth, you steer the telescope so as to keep it steady, by tracking a distant star and giving the telescope magnetical nudges to point it toward a fixed direction.

Re:Impressive engineering

[michaelwv]

I think by "too expensive" he means that there are instruments that work 80% of the time but need to be (kicked | disassembled | mucked around with) occasionally. Making these instruments 10x lighter, 98% reliable, and with no need for outside intervention might be one to two orders of magnitude more expensive to develop for a spacecraft that you never get to touch again (or that you have to pay ~$500 million for each repair). So an instrument that was $10 million is now several hundred million dollars. Even for a spacecraft that's real money. When people put stuff up in space you want to be conservative. So spacecraft are not good platforms for the latest and greatest instruments and new ideas. Part of the idea of an observatory that returns to the ground a lot is that you can try out new instruments much more easily and much more cheaply. In the case of the infrared it's important enough to get above the water vapor that it's worth making a flying observatory.

Re:Vibration?

[Quantos]

They have one, that's where I pulled that from. I just forgot to add the link.
Here [usra.edu] it is.