Hamburger Sternwarte plate archives: Historic long-term variability
  study of active galaxies based on digitized photographic plates
  At the Hamburger Sternwarte an effort was started in 2010 with the aim of
digitizing its more than 45000 photographic plates and films stored in its
plate archives. At the time of writing, more than 31000 plates have already
been made available on the Internet for researchers, historians, as well as for
the interested public. The digitization process and the Internet presentation
of the plates and accompanying hand written material (plate envelopes,
logbooks, observer notes) are presented here. To fully exploit the unique
photometric and astrometric data, stored on the plates, further processing
steps are required including registering the plate to celestial coordinates,
masking of the plates, and a calibration of the photo-emulsion darkening curve.
To demonstrate the correct functioning of these procedures, historical light
curves of two bright BL Lac type active galactic nuclei are extracted. The
resulting light curve of the blazar 1ES 1215+303 exhibits a large decrease in
the magnitude from $14.25^{+0.07}_{-0.12}$ to $15.94^{+0.09}_{-0.13}$ in about
300 days, which proves the variability in the optical region. Furthermore, we
compare the measured magnitudes for the quasar 3C~273 with contemporaneous
measurements.
