View source for Hubble Guide Star Catalog – Astrographic Catalog/Tycho

The '''Hubble Guide Star Catalog – Astrographic Catalog/Tycho''' ('''GSC-ACT''') is an [[astronomical catalog]]. It contains the twenty million [[star]]s of the Hubble GSC, version 1.1, recalibrated using the [[U.S. Naval Observatory]]'s ACT ([[Astrographic Catalog/Tycho]]).  By 2009 it was considered obsolete, superseded by such catalogues as [[GSC-2.2]], [[B1.0]], [[A2.0]], and especially [[UCAC-3]].<ref>{{Cite web|url=https://www.projectpluto.com/gsc_act.htm|title=GSC-ACT files and information|last=Gray|first=Bill|website=www.projectpluto.com|access-date=2017-03-03}}</ref>
GSC-ACT was a re-calibration of the Guide Star Catalog version 1.1 for Hubble.<ref>{{Cite web|url=http://vizier.u-strasbg.fr/cgi-bin/VizieR?-source=GSC-ACT|title=VizieR|website=vizier.u-strasbg.fr|access-date=2017-03-03}}</ref> The original GSC dated to 1989,<ref>{{Cite web|url=https://www.projectpluto.com/gsc_act.htm|title=GSC-ACT files and information|last=Gray|first=Bill|website=www.projectpluto.com|access-date=2017-03-03}}</ref> and one of the reasons for an update was improvements to star catalogs that took place after that time.<ref>{{Cite web|url=http://tdc-www.harvard.edu/catalogs/gsc_act_all.htm|title=GSC-ACT files and information|last=Gray|first=Bill|website=tdc-www.harvard.edu|access-date=2017-03-03}}</ref> In this case the ACT catalog was used, which had for example more stars and whereas the GSC 1.1 had 3rd order fit for plate distortions, the ACT has a five-order available.<ref>{{Cite web|url=http://tdc-www.harvard.edu/catalogs/gsc_act_all.htm|title=GSC-ACT files and information|last=Gray|first=Bill|website=tdc-www.harvard.edu|access-date=2017-03-03}}</ref> Another improvement of the GSC 1.1 was the GSC 1.2.<ref>{{Cite web|url=http://tdc-www.harvard.edu/catalogs/gsc_act_all.htm|title=GSC-ACT files and information|last=Gray|first=Bill|website=tdc-www.harvard.edu|access-date=2017-03-03}}</ref>
GSC-ACT uses five classification codes:<ref>{{Cite web|url=http://ledas-www.star.le.ac.uk/blasta/gscahelp.php|title=GSC-ACT Catalog|website=ledas-www.star.le.ac.uk|access-date=2017-03-03}}</ref>
*1        Star
*2        Galaxy
*3        Non-Star
*4         Artefact
==See also==
*[[Guide Star Catalog]]
*[[Tycho catalog]]
*[[Tycho-2 Catalogue]]
==References==
{{Reflist}}
==External links==
* [http://www.projectpluto.com/gsc_act.htm Overview of the GSC-ACT]
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[[Category:Astronomical catalogues of stars]]
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