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Recognizing the importance of these products for telescope operations and planning tools, a number of international partners are collaborating with us and supporting our efforts with the DSS-II and/or GSC-II projects. These include OATo, ESO, GEMINI, ESA/ST-ECF, ESA/SA, CADC, CDS, AAO, NAOJ, BAO. A preliminary release of the GSC-II will be provided to the GEMINI and VLT telescopes at the end of this year, while an improved version with better calibrations will be made available to the community at the end of 2000.
The originally published Digitized Sky Survey consisted of the Palomar POSS-I E survey and the SERC-J survey scanned at 25 micron resolution with modified PDS microdensitometers (Laidler et al. 1994). These have been substantially redesigned and modified to be multi-channel laser scanning machines and we are now scanning the additional plate material at 15 microns. STScI has negotiated access to the original plates of the POSS-II survey, and in partnership with the Anglo-Australian Observatory (AAO) undertook the Second Epoch Southern (SES) survey with the UK Schmidt (Lasker & Cannon 1990). In addition, we have begun scanning the SERC-I plates so that we will have the entire sky in at least 3 passbands (J, F, IV-N) and a sufficient baseline to determine proper motions. These data are being compressed using the same HCOMPRESS algorithm (White & Postman 1992) used for the published DSS-I and made available on a number of web servers around the world. The J and F bands are nearing completion and the I band will be completed by 2001. A decision on the format of a final distribution will depend on community interest.
Interim data deliveries (GSC 2.1.x) will be supplied for telescope operations over the next year whilst the final calibrations are derived and applied. The first general community release (GSC 2.2) will occur around the end of 2000, with a final version available (GSC 2.3) a year later. The published version of the catalog will be exported from our object database (described in section 4) as binary FITS tables for each HTM region on the sky (approximately 1 square degree). Access software to read entries from this distribution format has already been developed and integrated into a number of tools including ESO SKYCAT and STScI SEA.
Our other goal in the design of COMPASS is to promote database interoperability between GSC-II and other astronomical archives. Following the reasoning of having manageable amounts of data of roughly the same quantity as in the GSC-I custom database, the sky is partitioned into approximately equal areas using the Hierarchical Triangulated Mesh (HTM), a quad tree based on a spatial subdivision of the celestial sphere into approximately equal area spherical triangles (Barrett 1995). Each of the triangular subdivisions corresponds to an Objectivity database within the all-sky Federated database. This strategy has also been adopted by the SDSS Science Archive and 2MASS and is an ideal example for supporting interoperations between large-scale astronomical archives. The HTM can serve as a common identification basis for any user or agent selected spatial region of the sky. Within each triangular region database we are creating several containers: one for each plate to store the measured and calibrated parameters for each source; a container for each astronomical catalog with reference sources; and an Index container which has derived multi-plate parameters and links references to the same source in the plate and catalog containers. There will typically be 5-8 observations of the same source measured on different plates, and each plate will be split up among 50-60 region databases. Depending on the nature of the query, we can easily retrieve sources grouped by plate or region after determining the appropriate list of region databases containing the sources. Access to the source parameters is achieved by iterating over the index in each region database, retrieving the derived data or using the references to directly access the raw data from the individual plates.
Barrett, P. 1995 in ASP Conf. Ser., Vol. 77, Astronomical Data Analysis Software and Systems IV, ed. R. A. Shaw, H. E. Payne, & J. J. E. Hayes (San Francisco: ASP)
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