We have completed a robotic wide-angle imaging survey of the southern sky (δ = +15 • to −90 • ) at 656.3 nm wavelength, the Hα emission line of hydrogen. Each image of the resulting Southern H-Alpha Sky Survey Atlas (SHASSA) covers an area of the sky 13 • square at an angular resolution of approximately 0.8 arcminute, and reaches a sensitivity level of 2 rayleigh (1.2 × 10 −17 erg cm −2 s −1 arcsec −2 ) per pixel, corresponding to an emission measure of 4 cm −6 pc, and to a brightness temperature for microwave free-free emission of 12 µK at 30 GHz. Smoothing over several pixels allows features as faint as 0.5 rayleigh to be detected.

Following previous suggestions of other researchers, this paper discusses the prospects for astrometric observation of MACHO gravitational microlensing events. We derive the expected astrometric observables for a simple microlensing event with either a dark or self-luminous lens and demonstrate that accurate astrometry can determine the lens mass, distance, and proper motion in a very general fashion. In particular, we argue that in limited circumstances ground-based, narrow-angle di †erential astrometric techniques are sufficient to measure the lens mass directly and other lens properties (distance, transverse motion) by applying an independent model for the source distance and motion. We investigate the sensitivity of di †erential astrometry in determining lens parameters by Monte Carlo methods and derive a quasi-empirical relationship between astrometric accuracy and mass uncertainty.

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We surveyed the IRAS data base at the positions of the 1808 06-B9.5 stars in The Bright Star Catalog for extended objects with excess emission at 60 jum, indicating the presence of interstellar dust at the location of the star. Within 400 pc the filling factor of the interstellar medium for dust clouds with a density > 0.5 cm -3 is 14.6±2.4%. Above a density of 1.0 cm -3 , the density distribution function appears to follow a power law of index -1.25. When the dust clouds are mapped onto the galactic plane, the sun appears to be located in a low-density region of the interstellar medium of width about 60 pc extending at least 500 pc in the direction of longitudes 80°-260°, a feature we call the "local trough." 1127