Digitized Sky Survey
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astronomical survey 
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The Digitized Sky Survey (DSS) is a digital version of several photographic atlases of the night sky, and an ongoing project to produce more digital versions of photographic astronomical datasets.
The term Digitized Sky Survey originally referred to the publication in 1994 of a digital version of an all-sky photographic atlas. For the northern sky, the National Geographic Society - Palomar Observatory Sky Survey provided almost all of the source data. For the southern sky, the Southern Sky Atlas and its Equatorial Extension (together known as the SERC-J) and the southern Galactic Plane survey (SERC-V), from the UK Schmidt Telescope at the Australian Siding Spring Observatory, were used. The publication of a digital version of these photographic collections has subsequently become known as the First Generation DSS.
After the original 1994 publication, more digitizations were made and released as the Second Generation DSS. They include the Palomar Observatory Sky Survey II, made with the Oschin Schmidt Telescope at Palomar Observatory for the northern sky. Sources for the southern sky included the 'Galactic Red' survey, the Equatorial Red Survey, and the Second Epoch Survey, all made with the UK Schmidt Telescope at Anglo-Australian Observatory.
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.
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