Production
The "First Generation" Digitized Sky Survey was produced by the Catalogs and Survey Branch (CASB) of the Space Telescope Science Institute (STScI). They scanned plates using one of two Perkin-Elmer PDS 2020G microdensitometers. The pixel size was 25 or 15 micrometres, corresponding to 1.7 or 1.0 arcseconds in the source material. The scanning resulted in images 14,000 x 14,000 or 23,040 x 23,040 pixels in size, or approximately 0.4 and 1.1 gigabytes each. The scanning takes a little under seven hours per plate to complete. Due to the large size of the images, they were compressed using an H-Transform algorithm. This algorithm is lossy, but adaptive, and preserves most of the information in the original. Most of the First Generation DSS files were shrunk by a factor of seven.
Similar methods were used in the production of the "Second Generation" DSS, but the microdensitometers have since been modified for dual-channel operation.
The CASB has also published several companion scientific products. The most notable is a photometric calibration of part of the "First Generation" DSS. It allows photometric measurements to be made using the digital northern POSS-E, southern SERC-J, and southern Galactic Plane SERC-V data.
Read more about this topic: Digitized Sky Survey
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