Precision astrometry with SuperCOSMOS

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