Coordinates

The collection of coordinates for stars recorded in the database is quite an important issue, because the forms used to query the photometric database offer the possibilities of making selections based on coordinates.

Field Stars

A very large effort has been devoted to collecting available positions from the literature to add coordinates to the catalogue. Positions for the HD, HDE and many DM stars have been provided by the Strasbourg Data Center thanks to the courtesy of Dr Daniel Egret. Positions for fainter stars have been taken directly from the Durchmusterung catalogues and precessed to 1950. Data for various kinds of stars have been collected from the relevant catalogues or lists published.

However, many thousands of stars remained without coordinates after this first compilation because they were identified on charts only. Thanks to the recent availability of the Guide Star Catalogue (GSC) (Lasker et al. 1988) and the superb program developed by Luc Weber (Geneva Observatory) it has been possible to obtain coordinates for more than 5000 stars brighter than about 14.5, depending on the GSC local limit. The coordinates were found by comparing visually the published charts with that of the GSC plotted on the screen. However a number of charts were found to be completely unusable due to inappropriate scale, incompleteness, and so on. Therefore some stars which should normally have had coordinates do not.

The coordinates were then used to perform more cross-identifications with the Durchmusterung catalogues, which finally became quite easy and reliable with the program developed by Bernard Pernier (Geneva Observatory). In addition, the coordinates made it possible to find identical stars still recorded under different names. This mostly applies to anonymous stars. Nothing could be done for stars fainter than the limit of the GSC, and about 4000 additional stars do not yet have any position. The solution of the problem of coordinates will still require large efforts. It may be worth making this effort because faint stars are potentially interesting for the magnitude scale calibration of Schmidt plates.

Cluster Stars

Stars in clusters offered also several problems because few positions were available. All coordinates contained in the database for stars in open clusters were used, but this was not sufficient. Therefore, rectangular (x,y) positions in mm or arbitrary units were transformed to equatorial coordinates with a second order astrometric program, on the basis of at least 6 standard stars. The lists of standard stars were often enlarged with positions found in the GSC.

The GSC provided most of the coordinates for small clusters and standard sequences for the larger ones. In total, more than 5000 stars have been cross-identified with the GSC fields. Finally, cluster maps have been measured with a Hipad Plus digitizing tablet (Houston Instruments) and the (x,y) positions have been transformed to equatorial coordinates. For most clusters, the residuals of the transformation were less than 1 second of arc, which is quite sufficient for the precision used in the present database.


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Last update: 7 November 1995