The term Digitized Sky Survey originally referred to the publication in 1994 of a digital version of an all-sky photographic atlas used to produce the first version of the Guide Star Catalog. For the northern sky, the National Geographic Society – Palomar Observatory Sky Survey E-band, provided almost all of the source data. For the southern sky, the J-band of the ESO/SERC Southern Sky Atlas and the "quick" V-band SERC-J Equatorial Extension, from the UK Schmidt Telescope at the Australian Siding Spring Observatory, were used. Three supplemental plates in the V-band from the SERC and Palomar surveys are included, with shorter exposure times for the fields containing the Andromeda Galaxy, the Large and the Small Magellanic Cloud. The publication of a digital version of these photographic collections has subsequently become known as the First Generation DSS or DSS1. After the original 1994 publication, more digitizations were made using recently completed photographic surveys, and released as the Second Generation DSS or DSS2. Second Generation DSS consists of three spectra bands, blue, red, and near infrared. The red part was first to complete, and includes the F-band plates from the Palomar Observatory Sky Survey II, made with the Oschin Schmidt Telescope at Palomar Observatory for the northern sky. Red band sources for the southern sky include the short red plates of the SERC I/SR Survey and Atlas of the Milky Way and Magellanic Clouds, the Equatorial Red, and the F-band Second Epoch Survey, all made with the UKSchmidt Telescope at Anglo-Australian Observatory.
Production
The Digitized Sky Survey was produced by the Catalogs and Survey Branch of the Space Telescope Science Institute. 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 of First Generation DSS 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 multi-channel operation, in order to keep the scan time under 12 hours per plate. 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.
Publication
The compressed version of the First Generation DSS was published by the STScI and the Astronomical Society of the Pacific on 102 CD-ROMs in 1994, under the name "Digitized Sky Survey." It has also been made available online by the STScI and several other facilities in databases that can be queried over the web. The moniker "First Generation" was added later. In 1996, a more highly compressed version of the DSS was published by the STScI and ASP under the name RealSky. The Second Generation DSS has appeared steadily over the course of several years. In 2006, the Second Generation DSS was finished, and distributed on CD-ROM to partner institutions. Generally, the data are available through WWW services at partner institutions.
