  Photographic plate archives contain a wealth of information about positions
and brightness celestial objects had decades ago. Plate digitization is
necessary to make this information accessible, but extracting it is a technical
challenge. We develop algorithms used to extract photometry with the accuracy
of better than ~0.1m in the magnitude range 13<B<17 from photographic images
obtained in 1948-1996 with the 40cm Sternberg institute's astrograph (30x30cm
plate size, 10x10deg field of view) and digitized using a flatbed scanner. The
extracted photographic lightcurves are used to identify thousands of new
high-amplitude (>0.2m) variable stars. The algorithms are implemented in the
free software VaST available at http://scan.sai.msu.ru/vast/
