The tropopause, typically at 16 to 18 km altitude at the lower latitudes, dips down to only 8 km in the polar regions, allowing access to the cold, dry and nonturbulent lower stratosphere by tethered aerostats. These can float as high as 12 km, have long operating lifetimes, and are extremely reliable. In contrast to free-flying balloons, they can stay on station for weeks at a time, and payloads can be safely recovered for maintenance and adjustment and relaunched in a matter of hours. We propose to use such a platform, located first near Fairbanks, Alaska, and later in the Antarctic, to operate a new-technology 4 m telescope with diffraction-limited performance in the near infrared. Thanks to the low ambient temperature (about 200 K), thermal emission from the optics is of the same order as that of the zodiacal light in the 2-3 micron band. Since this wavelength interval is the darkest part of the zodiacal light spectrum from optical wavelengths to 100 micron, the combination of high-resolution images and a very dark sky make it the spectral region of choice for observing galaxies, QSOs and clusters of galaxies at the formation epoch of galaxies.
Atmospheric Transparency and Radiance
At 12 km altitude, the pressure is about 17% of the atmospheric pressure at sea level, and the column amount of precipitable water vapour above it is only a few microns. The remaining opacity from 2 to 10 micron (outside the 2.7 and 4.7 micron H2O lines) gives a transmission better than 90%. Similarly, the sky radiance in this wavelength range is one order of magnitude lower than at excellent ground based observatories.
Winds at Altitude
The mean wind speed at 12km altitude is 17m/s for Fairbanks and 7m/s for the South Pole. Because of the lower atmospheric density at this altitude, these velocities are equivalent to 7m/s and 2.9m/s at sea level respectively.
Seeing
Models show that the Fried parameter r0 is expected to be as large as 2m (0.05 arcsec) on average in the visible at 12km altitude, and can reach 8 meter 5% of the time.
Summary extracted from the following document:
Title: POST: polar stratospheric telescope
Authors: FORD, H. C.; BELY, P.
Origin: Proposal to build a 6-m, diluted aperture telescope on a high altitude, tethered aerostat for testing future space telescope technology and for near-infrared astronomical observing.
Publication Date: 09/1994