Little is known about the statistics of gravitationally lensed quasars at
large (7''-30'') image separations, which probe masses on the scale of galaxy
clusters. We have carried out a survey for gravitationally-lensed objects,
among sources in the FIRST 20cm radio survey that have unresolved optical
counterparts in the digitizations of the Palomar Observatory Sky Survey. From
the statistics of ongoing surveys that search for quasars among FIRST sources,
we estimate that there are about 9100 quasars in this source sample, making
this one of the largest lensing surveys to date. Using broad-band imaging, we
have isolated all objects with double radio components separated by 5''-30'',
that have unresolved optical counterparts with similar BVI colours. Our
criteria for similar colours conservatively allow for observational error and
for colour variations due to time delays between lensed images. Spectroscopy of
these candidates shows that none of the pairs are lensed quasars. This sets an
upper limit (95% confidence) on the lensing fraction in this survey of
3.3x10^-4, assuming 9100 quasars. Although the source redshift distribution is
poorly known, a rough calculation of the expected lensing frequency and the
detection efficiencies and biases suggests that simple theoretical expectations
are of the same order of magnitude as our observational upper limit. Our
procedure is novel in that our exhaustive search for lensed objects does not
require prior identification of the quasars in the sample as such.
Characterization of the FIRST-selected quasar population will enable using our
result to constrain quantitatively the mass properties of clusters.Comment: 10 pages, accepted for publication in MNRA