When an Astronomer Involved in a Research Project Says He

Multiple Choice

When an astronomer involved in a research project says he is going to look at the observatory's plate collection, what will he be looking at?

A) a collection of souvenir plates, with painting of other observatories from around the world (astronomers collect these as a hobby)
B) a series of metal disks used for changing the aperture of reflecting telescopes
C) a series of glass plates, with light-sensitive chemicals on them, on which the appearance of the sky was recorded
D) a series of insulated plastic shields, which are used to prevent stray infra-red radiation from the observatory from interfering with observations
E) what the special-of-the-day is in the observatory cafeteria