Digitized Sky Survey
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Versions and source material 
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 Anglo-Australian 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.
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.
The compressed version of the First Generation DSS was published on 102 CD-ROMs in 1994, under the name "Digitized Sky Survey." It has also been published by STScI and several other facilities in databases that can be queried over the web. The moniker "First Generation" was added later.
The Second Generation DSS has appeared steadily over the course of several years. In 2006, the Second Generation DSS (second epoch POSS-II and SES surveys) was finished, and distributed on CD-ROM to partner institutions. Generally, the data are available through WWW services at partner institutions.
- Association of Universities for Research in Astronomy
- Anglo-Australian Observatory
- Beijing Astronomical Observatory
- Canadian Astronomical Data Center
- Centre de Donnee Stellaire
- European Southern Observatory
- Gemini Observatory
- Hohenkarpfen Observatory
- National Astronomical Observatory of Japan
- Palomar Observatory
- Royal Observatory Edinburgh
- Space Telescope Science Institute