An account is given of an investigation to obtain accurate relative star positions from overlapping photographic plates. The method described is quite general, but in the first instance is to be applied to plate material obtained with the new Cape astrometric camera.

The Cape 4-component f/10 20-cm astrometric camera produces a field of 5° square with a scale of 100″ per mm in the focal plane. The telescope is fitted with a yellow filter 12 mm from the focal plane, which in combination with the Ilford-R40 emulsion used, transmits more or less uniformly in the range 5300<λ<6500 and little elsewhere. Within this spectral range, the maximum longitudinal aberration is only 0·75 mm and the images sensibly round out to the edge of the plate. The instrument is currently being used to obtain a complete coverage of the Southern hemisphere down to mv~11·5 such that each plate, effectively 4° square, has overlapping plates centred at each corner and at the middle of each side. In practice, this system of overlapping is applied to a number of zones in declination each about 10° broad. Each plate is taken within 30m of the meridian and is given two 3m exposures above and below the 1950 standard positions, the geometrical plate centre being estimated to be within 20″ of the tangential point. At the time of writing, coverage of the zone −30° to −52° is almost complete, and preparations for the zone 0° to −30° well advanced.