Optical interferometry with HST/FGS at V>15

Abstract

The Hubble Space Telescope's Fine Guidance Sensor FGS1r has been used to observe cool white dwarf stars with apparent magnitudes that are near the FGS's faint limit. We had expected to discover that about 10% of these stars are actually binary white dwarf systems. Furthermore, we expected the binaries to have angular separations much larger than the size of the FGS white light fringes, making them easy to resolve. Although we did find 10% of the stars to be binaries, most have angular separations less than 25 milli-arcseconds, well below the HST diffraction limit. Instead of two widely separated fringes, we observed fringes that displayed subtle differences, in amplitude and morphology, from those of point sources. A major complication for our program was the need to address and remove the effects of the detector's dark current, which for the faintest targets contributed up to 40% of the counts. This paper outlines the process we employed to extract the science from the data. Our scientific motivation is briefly discussed