# What in optics prohibits telescopes on jets travelling at night?

What in optics prohibits telescopes on jets travelling at night? It would be a nice addition to our sky sweep to have telescopes on at least long distant night flights. Anything in optics preventing it?

• What would be the advantage of a telescope on a plane compared to one on the ground? – The Photon Jan 23 at 21:15
• Reduced atmospheric distortion and absorption – JQK Jan 23 at 21:18
• Vibration would be a problem. – garyp Jan 23 at 21:59

## Why airborne telescopes are handy

There have, in fact, been several airborne observatories built. SOFIA is the only one currently in existence; like the other airplane-borne telescopes, it involves a heavily-modified jet aircraft with an opening for the telescope.

Airborne observatories are great platforms for some infrared telescopes. Infrared astronomy is inhibited for ground-based telescopes because of absorption in the atmosphere, by gases including water vapor. One way around this is to place telescopes in space (like Spitzer). Another is to build them at high altitudes, reducing absorption a little bit. A final way to is place the telescope on an airplane, which can fly higher than any ground-based telescope can be built. SOFIA could fly at about 40,000 feet (compare to, say, the Paranal Observatory, at 8600 feet above sea level).

## The problems

Now, of course, in return for an airborne platform, you sacrifice collecting size, as the telescope dish must be small, even mounted on a wide-body aircraft. That said, SOFIA isn't insignificant in size (2.5 meters isn't bad). There are other problems, though.

First, there's the fact that while the telescope is in operation, there's a large hole in the side of the plane. That needs to be accounted for in order to keep the aircraft stable in flight. All airborne observatories in existence have been made from converted commercially-available airplanes that were definitely not originally meant for this, but the aerodynamics problem can be solved by making some modifications to the fuselage near the opening, adding in a gentle ramp that guides the air away.

Next, consider the fact that the stratosphere is much colder than the ground, and if you suddenly open a hatch at ~40,000 feet that was previously at the ground temperature, you're going to have problems (the air temperature while cruising is usually about -50$$^{\circ}$$ Celsius). There are two ways this is dealt with. First, the telescope is constructed out of specialized glass that doesn't expand much when faced with changes in temperature. Additionally, the telescope cabin is cooled before takeoff and then flooded with dry nitrogen before landing, which deals with the temperature changes and prevents moisture from ruining the equipment.

SOFIA also has to be lightweight. Just like with a space-based telescope, mass matters, and so weight reduction is needed wherever possible. Plus, you need to add counterweights to the front of the plane to keep its center of gravity where it is. More weight means more fuel needed and so less observing time per flight.

Finally, of course, the equipment needs to be stabilized against vibrations and turbulence during the flight. Gyroscopes and specialized "air bladders" are able to compensate for most movement and keep the telescope properly aligned. It's a delicate operation, but is performed to extraordinary precision. This turbulence could also affect image quality, but a shear layer control device mounted aft of the opening apparently counteracted this by reducing the problematic air layer, and meant that the aircraft can be handled normally by the pilots.

You also have the issue that the telescope can only be pointed in one direction at any time; if you want to point it the opposite way, you need to turn the plane around.

• Can you comment as to the turbulence of air going over the opening? How smooth is the airflow? – ZeroTheHero Jan 23 at 22:53
• @ZeroTheHero I've made an edit. The fuselage modifications surrounding the opening reduce turbulence in the problematic thin layer of air, which improves the seeing and reduces noise. It also means that the plane handles normally, as expected. – HDE 226868 Jan 23 at 23:06

Nothing prevents it except for economics. Some planes use telescopes for star tracking to obtain position information.

For commercial aircraft there would be a substantial cost associated with modifying the aircraft and maintenance without a return on investment.

• Research-grade telescopes represent substantial cost whatever the design. As documented in the other answer, there are clear niches where spending that cost on a flying telescope is well worth the effort. – Emilio Pisanty Jan 23 at 22:24