PRECISION ASTROMETRY WITH SUPERCOSMOS

We describe the new, fast, high-precision microdensitometer SuperCOSMO S, Some aspects of hardware and software design that enable high-preci sion astrometry from photographic plates are explained. We show that t he positioning repeatability of the measuring machine is less than 0.1 mu m standard error in either coordinate, and the absolute positional accuracy is about 0.15 mu m standard error. Furthermore, measurements of the same plate in different orientations show that the sampling er rors are small (e.g. similar to 0.2 mu m, rising to similar to 1.0 mu m at the plate limit, for stellar images in a IIIaJ emulsion), thus al lowing the extraction of relative positional information from Schmidt plates at accuracies less than 1 mu m. We demonstrate that SuperCOSMOS is capable of measuring the positions of bright stars (i.e. those mor e than similar to 4 mag above the plate limit) to a precision similar to 0.5 mu m with survey-grade photographic plates employing fine-grain ed emulsions.