Scanning the Plates

    After the plates are received at STScI, they are placed on a machine called the PDS, or Photo Densitometric Scanner. This machine consists of a movable stage on which the plate rests, a laser, and a PMT (PhotoMultiplier Tube). The light from the laser passes through the plate, and the amount transmitted is measured by the PMT. The plate is moved so that each point passes over the laser. In our case, a measured 'pixel' is 1 second of arc on a side.
    On the plates, a star appears black, and background is transparent. Thus, less light gets through the plate where there is a star. The amount of light that passes through the plate is called the transmission. The transmission is lower where there is a bright object. We actually record the photographic density, which is the inverse of the transmission. Therefore, the density is higher for brighter objects.

   With the 1" pixels, a full plate scan is 23040x23040 pixels; this creates 1.2Gb of image data. The entire survey will therefore produce about 3 Terabytes of image data.


Here's some useful numbers:
Pixel size: 1" (one arcsecond)
Pixels/plate: 23,040x23,040, or 530,841,600
Objects/plate: 500,000+
Total pixel data set: 3 Tb, or 1,423,717,171,200 pixels

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