long-focus photographic astrometry
long-focus photographic astrometryA branch of photographic astrometry in which (usually) refracting telescopes of long focal lengths of eight meters or more and focal ratios of f/15 to f/20 are used to produce sharp photographs of both the star under consideration plus three or more distant reference stars. Accuracies of about 0.01 arc seconds can be reached, distances being determined from annual parallax. The technique is now also employed in the determination of orbital motion and masses of binary stars and of secular parallax.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006