High-resolution imaging follow-up of doubly imaged quasars
  We report upon three years of follow-up and confirmation of doubly imaged
quasar lenses through imaging campaigns from 2016-2018 with the Near-Infrared
Camera2 (NIRC2) on the W. M. Keck Observatory. A sample of 57 quasar lens
candidates are imaged in adaptive-optics-assisted or seeing-limited
$K^\prime$-band observations. Out of these 57 candidates, 15 are confirmed as
lenses. We form a sample of 20 lenses adding in a number of previously-known
lenses that were imaged with NIRC2 in 2013-14 as part of a pilot study. By
modelling these 20 lenses, we obtain $K^\prime$-band relative photometry and
astrometry of the quasar images and the lens galaxy. We also provide the lens
properties and predicted time delays to aid planning of follow-up observations
necessary for various astrophysical applications, e.g., spectroscopic follow-up
to obtain the deflector redshifts for the newly confirmed systems. We compare
the departure of the observed flux ratios from the smooth-model predictions
between doubly and quadruply imaged quasar systems. We find that the departure
is consistent between these two types of lenses if the modelling uncertainty is
comparable.
