This database is based on the electronic version of the SAO catalog from the Astronomical Data Center, which is itself based on an original binary version of the Smithsonian Astrophysical Observatory Star Catalog (SAO, SAO Staff 1966). Subsequent improvements by T. A. Nagy (1979) included the addition of cross-identifications from the Table of Correspondences SAO/HD/DM/GC (Morin 1973). As a prelude to creation of the 1984 version of the SAO, a new version of the SAO-HD-GC-DM Cross Index was prepared (Roman, Warren, and Schofield 1983). The 1984 version of the SAO contained the corrected and extended cross identifications, all errata published up to January 1984 and known to the ADC, numerous errors forwarded to the ADC by colleagues, and errors discovered at the ADC during the course of this work. Clayton A. Smith of the U. S. Naval Observatory provided J2000.0 positions and proper motions for the SAO stars. Published and unpublished errors discovered in the previous version (1984) have been corrected (up to May 1991). For this HEASARC representation, some parameters such as the RA and Dec in radians have been omitted. This online version of the SAO Catalog was created by the HEASARC in March 2001 based on ADC/<a href="https://cdsarc.cds.unistra.fr/ftp/cats/I/131A">CDS Catalog I/131A</a>, which itself is originally derived from a character-coded machine-readable version of the Smithsonian Astrophysical Observatory Star Catalog (SAO, SAO Staff 1966) prepared by T.A. Nagy in 1979, and subsequently modified over the next decade or so. The first machine-readable version contained format modifications, cross identifications and other changes, and was the starting point of the version in this database. Additional changes were made to the SAO catalog over time (namely more cross identifications and corrections) which resulted in a new version in 1984. Finally, the most recent version of the catalog was published in 1989. It included J2000 positions for all the objects, and corrections to errors known as of May 1989. This is a service provided by NASA HEASARC .