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Published online by Cambridge University Press: 26 July 2016
Schmidt plate-based surveys of the sky are currently the largest sources of positions and colours of stellar and non-stellar objects (see e.g. Lasker 1993). This situation is likely to remain such through the next decade when new and improved CCDs will probably replace the photographic medium. But even then deep photographic surveys will constitute important first epoch material for the derivation of better proper motions. Accordingly, we should do our best to improve astrometry and photometry of Schmidt plates.