Inexpensive balloon-borne observatories using modified COTS telescopes

By virtue of operating at altitudes of 35 km, above 99.5% of the Earth\´s atmosphere, balloon-borne observatories have several advantages over ground-based and airborne observatories, including space-like image quality, access to the IR spectrum from 1 to 30 microns, very stable photometry and the potential for 100-day missions. We present plans for modifying inexpensive COTS telescopes for use in the stratosphere. The stratospheric environment is a problem for thermal control of a telescope and optical tube assembly (OTA): at 4 torr (the pressure near 35 km), convection is ineffective as a cooling mechanism, but the ambient pressure is easily high enough to thermally short-circuit layers of MLI that would otherwise form an adequate blanket in vacuum. A consumer telescope on a balloon would fail to form reasonable images due to distortion of the OTA and thermal gradients across the primary mirror. We discuss simple modeling of thermal mitigation strategies and the potential to fly modified COTS telescopes (costing less than a few $10Ks) to achieve 0.25" imaging in UV and visible wavelengths, scintillation-free photometry and IR spectroscopy from 1 to 30 microns.