  The application of photography to astronomy was a critical step in the
development of astrophysics at the end of the nineteenth century. Using
custom-built photographic telescopes and objective prisms, astronomers took
images of the sky on glass plates during a 100-year period from many observing
stations around the globe. After each plate was developed, astronomers and
their assistants studied and annotated the plates as they made astrometric,
photometric and spectroscopic measurements, counted galaxies, observed stellar
variability, tracked meteors, and calculated the ephemerides of asteroids and
comets. In this paper, the authors assess the importance of the plate
annotations for future scientific, historical, and educational programs.
Unfortunately, many of these interesting annotations are now being erased when
grime is removed from the plates before they are digitized to make the
photometric data available for time-domain astrophysics. To see what
professional astronomers and historians think about this situation, the authors
conducted a survey. This paper captures the lively discussion on the pros and
cons of the removal of plate markings, how to best to document them if they
must be cleaned off, and what to do with plates whose annotations are deemed
too valuable to be erased. Three appendices to the paper offer professional
guidance on the best practices for handling and cleaning the plates,
photographing any annotations, and rehousing them.
