Space-quality data from balloon-borne telescopes: the High Altitude
  Lensing Observatory (HALO)
  We present a method for attaining sub-arcsecond pointing stability during
sub- orbital balloon flights, as designed for in the High Altitude Lensing
Observatory (HALO) concept. The pointing method presented here has the
potential to perform near-space quality optical astronomical imaging at 1-2% of
the cost of space-based missions. We also discuss an architecture that can
achieve sufficient thermomechanical stability to match the pointing stability.
This concept is motivated by advances in the development and testing of Ultra
Long Duration Balloon (ULDB) flights which promise to allow observation
campaigns lasting more than three months. The design incorporates a multi-stage
pointing architecture comprising: a gondola coarse azimuth control system, a
multi-axis nested gimbal frame structure with arcsecond stability, a telescope
de-rotator to eliminate field rotation, and a fine guidance stage consisting of
both a telescope mounted angular rate sensor and guide CCDs in the focal plane
to drive a fast-steering mirror. We discuss the results of pointing tests
together with a preliminary thermo-mechanical analysis required for
sub-arcsecond pointing at high altitude. Possible future applications in the
areas of wide-field surveys and exoplanet searches are also discussed.
