astrometry

astrometry The measurement of positions, parallaxes, and proper motions on the sky. It can be divided broadly into two categories: global and small-field astrometry. Global astrometry is concerned with mapping and cataloguing positions and motions over large areas of sky. It was traditionally based on optical observations made with meridian instruments and astrolabes. Currently, optical interferometers are being developed to achieve greater accuracy. A stellar reference frame of bright stars is derived from such observations, and interpolation to fainter stars is achieved by photographic surveys. The accuracy of all ground-based optical astrometry is limited by thermal and mechanical instabilities in the telescopes, but also mainly by uncertainties in the amount of atmospheric refraction. The Hipparcos satellite was launched to overcome these limitations. Global astrometry at radio wavelengths is carried out by interferometers, with both short and very long baselines.

In small-field astrometry, relative positions are measured within the area observable with long-focus telescopes, by means of photographic plates or, more recently, CCDs. Its main aims are to measure relative proper motions and trigonometric parallaxes of stars, to discover astrometric binaries, and to identify faint optical counterparts of objects detected at other wavelengths.