Microvariability survey with the Hubble Space Telescope Fine Guidance Sensors - Exploring the instrumental properties

The suitability of the Hubble Space Telescope (HST) Fine Guidance Sensors (FGS) for a magnitude limited survey of microvariability among guide stars is investigated. The linearity, relative sensitivity, and dead-time constants of the three FGSs are determined, and the photon noise limitation of the FGS photometry is confirmed. The noise spectrum is found to be "white" and a procedure is described which provides an estimate of the mean noise amplitude for any guide star observation. We derive a criterion, given a specific probability, to predict an upper limit for peaks in the amplitude spectrum of time series. We give examples for constant, known variable, and new variable guide stars, and illustrate that our FGS noise model will permit the automatic detection of microvariability within our survey of HST data. Presently, the lowest noise level found in FGS data obtained between November 1992 and December 1993 is 200 ppm, but better data by an order of magnitude are expected to be obtainable during the life-time of HST.