Experimental trigonometric parallaxes in the Hyades region using a Schmidt telescope of moderate size
As an experiment, parallaxes and proper motions were measured for 51 stars on 64 plates with a single field centre in the Hyades region, using the Cambridge 43161 cm Schmidt telescope and the GALAXY automatic measuring machine at the Royal Observatory, Edinburgh. Twelve of the stars were background objects selected to control systematic effects. The internal standard errors for the parallaxes were + arc sec at brightness 11m increasing to 1 at i6 , and for the proper motions 1 and 1 arc sec yr-' respectively. The limiting accuracy was set by photographic noise. The mean parallax for I 2 of Van Altena's probable main sequence members with apparent magnitude between 10m.3 and 14m.5 was arc sec. Five of his probable or possible subdwarf members were measured and shown to form a group spatially and dynamically separate from the main sequence members. It is concluded that a Schmidt telescope of twice the size of the Cambridge instrument would give useful statistical parallaxes en masse for low luminosity stars.