Fourier analysis of Hubble Space Telescope Fine Guidance Sensor data of binary stars and application to the multiple system HD 157948

A Fourier-based method is presented for the analysis of binary and multiple star data taken with the Hubble Space Telescope Fine Guidance Sensors (FGSs). Relative astrometry and magnitude differences are obtained as with standard FGS analysis techniques, and although the FGS system is essentially unfiltered, this method also permits the characterization of color differences between components of binary or multiple star systems based on the wavelength dependence of the interference fringes produced by the instrument. Using the multiple system HD 157948, we show that the method produces astrometric and photometric measurements that are consistent with previous FGS analysis for the three components that lie within the field of view of FGS and gives color differences relative to the primary for two of the three known companions. Speckle observations of the system with the WIYN 3.5 m telescope at Kitt Peak National Observatory are also presented, which, in combination with the FGS results and other data available in the literature, permit B - V colors and absolute V magnitudes to be obtained of all four components in the system. Their colors and magnitudes are plotted on the H-R diagram, and a comparison with theoretical isochrones indicates that the results are consistent with theory for a relatively young, somewhat metal-poor system.