In the early days of photography, before celluloid and cellulose acetate
film were used as backing
for photographic emulsions, glass plates coated with light-sensitive silver
salts were used
to make photographic images. Coated plates would be loaded into cameras,
exposed, and processed to generate glass negatives from which prints could
be made. They are distinct from the Daguerrotype, which was a sheet of
metal upon which positive -- rather than negative -- images were made.
Although the majority of practical photography has been done using film
for almost a century, glass plates were used in astronomy up until the
last decade. The reason was
that it was difficult to use CCD cameras to record wide-field images of the
sky at high resolution without having a huge CCD, and an accompanying huge
amount of computer memory and storage (think of huge in, say, 1985 terms).
Glass plates were also used rather than acetate film because plates held up
better over time for archival purposes. They would not become distorted or
stretched during or
after the observation, which is important when doing astrometry. The
obvious downside is that they're quite fragile.
They were most
commonly used in Schmidt telescopes whose purpose is primarily for imaging
wide-fields. One of the more famous examples of this kind of survey
was the Palomar Observatory Sky Survey, which recorded the entire sky
visible from California on thousands of glass plates. Negative (paper)
prints were then made and distributed to observatories around the world.
There were several problems with the glass plate, one of which was
fragility. They're easy to break, especially if one goes fumbling around in
the dark with them. Another problem is that all photographic
emulsions are nonlinear. This means that the depth of the exposure does not
necessarily increase linearly with time, which makes photometry
difficult. Another problem is that their quantum efficiency is very low,
in the low percent range. That means that only one or two out of every
one hundred photons is recorded. This makes for very long observations,
and ties up an inordinate amount of telescope time.
The CCD or charged coupled device replaced photographic plates at most
astronomical observatories by the early 1990s, as the technology improved
and the costs for cameras and computer equipment came down (driven in part
by technology spin-offs from the Hubble Space Telescope). A few telescopes
used plates up until fairly recently, but new, extremely large format CCD
cameras (8192x8192 pixels) have been developed for the largest observatories,
and small CCDs are relatively inexpensive even for
amateur astronomers.
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