The Hubble Space Telescope snapshot survey. III. Further observations in search of gravitationally lensed quasars

We present new HST Snapshot Survey observations of 163 high-luminosity z > 1 quasars, bringing the total number observed to date in the survey to 347 quasars. Each exposure was searched for evidence of gravitational lensing. Among the newly observed quasars, six have additional point sources at separations of 2″-6″, but in all six cases ground-based data indicate that gravitational lensing is not involved. No point sources were found at smaller separations. Counting the one new lens candidate the survey has uncovered (1208 + 1011, described in previous papers) and the known lenses excluded from the sample, the observed frequency of lensing of quasars by galaxies is 5/351. This frequency is in the range predicted by models with a vanishing cosmological constant, but an order of magnitude lower than expected in models strongly dominated by a cosmological constant. The HST pointing accuracy under gyroscope control has improved and is now 14″ ± 9″ (mean and rms).