A balloon-borne telescope is a type of airborne telescope, a sub-orbital astronomical telescope that is suspended below one or more stratospheric balloons, allowing it to be lifted above the lower, dense part of the Earth's atmosphere. This has the advantage of improving the resolution limit of the telescope at a much lower cost than for a space telescope. It also allows observation of frequency bands that are blocked by the atmosphere.
Balloon-borne telescopes have the disadvantage of relatively low altitude and a flight time of only a few days. However, their maximum altitude of about 50 km is much higher than the limiting altitude of aircraft-borne telescopes such as the Kuiper Airborne Observatory and Stratospheric Observatory for Infrared Astronomy, which have a limiting altitude of 15 km. A few balloon-borne telescopes have crash landed, resulting in damage to, or destruction of the telescope.
The balloon obscures the zenith from the telescope, but a very long suspension can reduce this to a range of 2°. The telescope must be isolated from the induced motion of the stratospheric winds as well as the slow rotation and pendulum motion of the balloon. The azimuth stability can be maintained by a magnetometer, plus a gyroscope or star tracker for shorter term corrections. A three axis mount gives the best control over the tube motion, consisting of an azimuth, elevation and cross-elevation axis.
|Name||Active||Description and purpose|
|Stratoscope I||1957–59||12-inch telescope attached to a polyethylene balloon. This was the first balloon-borne astronomical telescope. It took photographic images of the sun, showing granulation features. In 1959 it was flown again, this time with a television transmitter.|
|Stratoscope II||1963–71||36-inch telescope with a tandem balloon system.|
|THISBE||1973–76||Infrared telescope used for observations of extended sources, including OH airglow, the zodiacal light, and the central galaxy region.|
|HIREGS||1991–98||High-resolution spectrometer for examining gamma ray and hard X-ray emissions from solar flares and galactic sources. It used an array of liquid nitrogen-cooled germanium detectors.|
|BOOMERanG experiment||1997–2003||Microwave telescope with cryogenic detectors that was carried on long-duration flights over the antarctic. It was used to map the cosmic microwave background radiation (CMBR).|
|MAXIMA||1998–99||Microwave telescope with a cryogenic receiver that was used to measure the CMBR.|
|HERO||2001–10||Hard X-ray telescope that flew successfully beginning in 2001 but crashed in 2010, destroying the telescope.|
|BLAST||2003–||Submillimetre telescope with a 2 m aperture. It was destroyed during the third flight, but was rebuilt and completed a fourth flight in 2010.|
|InFOCμS||2004–||Hard X-ray telescope with a 49 cm2 collecting area.|
|HEFT||2005||Hard X-ray telescope that uses grazing-incidence optics.|
|Sunrise||2009–||1 m ultraviolet telescope with image stabilization and adaptive optics for observing the Sun.|
|PoGOLite||2011–||Telescope for polarised hard X-rays and soft gamma-rays.|
|Spider||2015–||Submillimeter telescope searching for primordial gravitational waves.|
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