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A terrestrial stratospheric telescope is ideally suited for making infrared observations of Venus' night hemisphere during
inferior conjunctions. The near-space environment at 35 km altitude has low daytime sky backgrounds and lack of
atmospheric turbulence, both of which are necessary for observing Venus' night side at the diffraction limit when Venus
is close to the Sun. In addition, the duration of the observing campaign will be around 3 weeks, a time period that is
achievable by current long duration flights. The most important advantage, however, will be the ability of a balloonborne
telescope to clearly image Venus' night side continuously throughout a 12-hr period (more for certain launch site
latitudes), a capability that cannot be matched from the ground or from the Venus Express spacecraft currently in orbit
around Venus. Future missions, such as the Japanese Venus Climate Orbiter will also not be able to achieve this level of
synoptic coverage. This capability will provide a detailed, continuous look at evolving cloud distributions in Venus'
middle and lower cloud decks through atmospheric windows at 1.74 and 2.3 μm, which in turn will provide
observational constraints on models of Venus' circulation.
The science requirements propagate to several aspects of the telescope: a 1.4-m aperture to provide a diffraction limit of
0.3" at 1.74 μm (to improve upon non-AO ground-based resolution by a factor of 2); a plate scale of 0.1" per pixel,
which in turn requires an f/15 telescope for 13 μm pixels; pointing and stability at the 0.05" level; stray light baffling; a
field of view of 2 arc minutes; ability to acquire images at 1.26, 1.74 and 2.3 μm; and ability to operate aloft for three
weeks at a time. The specific implementations of these requirements are outlined in this paper. Briefly, a 1.4-m
Gregorian telescope is proposed, with stray light baffling at the intermediate focus. A three-stage pointing system is
described, consisting of a coarse azimuthal rotator, a moderate pointing system based on a star tracker and ALT/AZ
gimbals, and a fine pointing system based on analog photodiodes and a fine steering mirror. The science detectors are
not discussed here, except to specify the requirement for moderate resolution (R > 1000) spectroscopy.
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