BV Photometry of the Galactic Globular Cluster
                                   NGC 1851
 
                                (Walker, A. R.)
 
 
 
 
 
                Documentation for the Computer-Readable Version
                                Nancy G. Roman
 
 
 
 
                        Doc. No. NSSDC/WDC-A-R&S 93-11
 
 
 
 
                                  March 1993
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
                  National Space Science Data Center (NSSDC)/
          World Data Center A for Rockets and Satellites (WDC-A-R&S)
                 National Aeronautics and Space Administration
                          Goddard Space Flight Center
                              Greenbelt, MD 20771

 
                                  Abstract
 
 
         BV photometry is provided for 1,234 stars in the magnitude
         range 13 - 18 in an annulus between 5 and 20 arc seconds from
         the center of NGC 1851 and for 1,247 stars in the magnitude
         range 18 - 23 at a distance of more than 240 arc seconds from
         the center. For stars in the magnitude range 16 - 21, the
         errors in B-V increase with magnitude from 0.005 mag. to
         0.022 mag.
 
 
         1 Introduction and Source Reference
 
         1.1 Introduction
 
         The authors were primarily interested in the shape of the
         color magnitude diagram of the galactic globular cluster, NGC
         1851. To prepare this diagram, they obtained a number of CCD
         frames of the cluster in B and in V with exposures ranging
         from 20s to 1500s. The longer exposures were taken at a
         distance of more than 240 arc seconds from the center of the
         cluster.
 
         1.2 Source Reference
 
         A BV Color-Magnitude Diagram for the Galactic Globular
         Cluster NGC 1851 Walker, A. R. 1992, PASP, 104, 1063.
 
 
         2 Structure
 
         2.1 File Structure
 
         The BV Photometry of the Galactic Globular Cluster NGC 1851
         as distributed here is in a single file. The stars in the
         magnitude range 13.0 to 19.0 in an annulus between 5 and 20
         arc seconds from the center of the cluster have a blank or 1
         in byte 1. The stars in the magnitude range 18.0 to 24.0 at a
         distance of at least 240 arc seconds from the center have a 2
         or 3 in byte 1. The positions are given in pixels from the
         center of the cluster.
 
         To aid in star identification, accurate (GSC) coordinates are
         given for two bright stars in Table 1.
 
         Ident  X      Y        V       B-V        RA            DEC
                (pixels)                                (2000)
          7   608.8   26.8    13.425   1.628   05 13 44.76   -40 00 28.8
         59   575.4   228.3   14.025   1.306   05 13 53.02   -40 00 44.1
 
                      Table 1: Positions of Bright Stars
 
         The pixel scale is 0.478 arcsec/pixel, with the orientation X
         increasing to the north, and Y increasing to the east.
 
                                       1

 
         The median error of the B-V colors as a function of magnitude
         is given in Table 2.
 
                           Mag. Range     Med. Error
                             16-17          0.005
                             17-18          0.009
                             18-19          0.012
                             19-20          0.016
                             20-21          0.022
 
 
            Table 2: Median Errors as a Function of Magnitude
 
 
         2.2 File Format
 
         Table 3 gives a byte-by-byte description of the contents of
         the data file.
 
                    Bytes   Unit    Format    Item
                    1 - 4             I4      Number
                    6 - 11  pixel    F6.1     X coordinate
                   13 - 18  pixel    F6.1     Y coordinate
                   20 - 25  mag.     F6.3     V magnitude
                   27 - 32  mag.     F6.3     B-V color
                   34 - 36            A3      Remark
 
                              Table 3: Format
 
 
 
         3 Acknowledgments
 
         We thank Dr. Walker for forwarding to us a machine-readable
         version of the catalog, including the data for the faint
         stars that are not in the published reference.
 
         4 References
 
 
         A BV Color-Magnitude Diagram for the Galactic Globular
         Cluster NGC 1851 Walker, A. R. 1992, PASP, 104, 1063.
 
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