THEORETICAL COLOR CALIBRATION FOR HST FOC ULTRAVIOLET FILTERS

A theoretical color calibration is carried out for three (F140W, F220W , and F342W) of the most heavily used ultraviolet filters and one visu al band filter (F480LP) in the Faint Object Camera on the HST, using t he new Kurucz model spectra which became available recently. It is kno wn that the UV filters suffer from the visible leak problem, and it is apparent in this study that the visible leak problem can cause seriou s trouble to analyses of observed data. We show here that some of the colors defined from these filters do not have a monotonic relation wit h effective temperature; and thus, the colors fail to indicate a uniqu e effective temperature in certain temperature ranges. This study show s that the problem can be solved by using one of the visual band filte rs, preferably F480LP, together with the UV filters. The expected mode l color-magnitude diagrams for a typical globular cluster with the HST ultraviolet colors are also provided.