Gravitational Lensing of Quasars as Seen by the Hubble Space Telescope Snapshot Survey

As part of the ongoing HST Snapshot Survey, 152 additional high- luminosity z > 1 quasars were observed, bringing the total to 184 quasars. Each exposure was searched for evidence of gravitational lensing. Only one quasar among those observed, 1208+1011, is a candidate lens system with subarcsecond image separation. Six other quasars have point sources within 6". Ground-based observations of five of these cases show that the companion point sources are foreground Galactic stars. The observed lensing frequency, after accounting for known lenses excluded from the sample, and including 32 quasars observed from a pilot sample described in a previous paper, is 3/184. The predicted lensing frequency of the sample is calculated for a variety of cosmological models. The effect of uncertainties in some of the observational parameters upon the predictions is discussed. The observed lensing frequency is consistent with predictions of models with a zero cosmological constant λ. Assuming the best estimates for the parameters involved in the calculation, flat cosmologies with λ > 0.85 can be ruled out at greater than 95% confidence. From the engineering perspective, we find that the software corrections for stellar aberration when the telescope is guided on gyroscopes results in drift rates of 1-2 milliarcseconds s^-1^, about 3 times smaller than before the correction was implemented. The mean and rms pointing error under gyro control is 20" +/- 13". We find no correlation of the drift rate with time, right ascension, declination, or pointing error.