Taking the Photographic Plates

A more detailed description of the photographic survey, POSS-II, is also available.

   Photographic plates for the sky survey are taken at the 48" Oschin Schmidt Telescope at Palomar Observatory. A dedicated staff of 3 observers works every clear night to complete the survey. Each plate is actually a 14" square piece of glass, with a photographic emulsion. Each covers 36 square degrees of sky. The Sky Survey uses plates taken in three colors:
ColorEmulsion/Filter
J - blueIIIaJ + GG395
F - redIIIaF + RG610
N - nearIRIVN + RG9
An example plate (J779) is shown at left. This is a greatly reduced image so it is nearly impossible to see any objects, but it really contains about 500,000 stars and galaxies! The funny looking boxes at the lower right corner are known as densitometry spots; they allow us to calibrate the brightness of objects on the plate.

   The plates used for the Sky Survey were donated by the Eastman Kodak Company. They are hypersensitized by the survey observers immediately before being used. The highly sensitive plate is placed in a metal holder which bends them slightly to match the curved focal plane of the telescope. They are then placed into the telescope, and exposed to the night sky for about one hour. After this, the plates are removed from the telescope, and developed in a darkroom in the telescope building. This is done in absolute darkness; no red lights here! Once developed, the plates are rinsed and ready to ship to various observatories to be copied, and finally to STScI to be scanned.

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