On the photometric error calibration for the `differential light curves'
  of point-like Active Galactic Nuclei
  It is important to quantify the underestimation of rms photometric errors
returned by the commonly used $\emph APPHOT$ algorithm in the $\emph IRAF$
software, in the context of differential photometry of point-like AGN, because
of the crucial role it plays in evaluating their variability properties.
Published values of the underestimation factor, $\eta$, using several different
telescopes, lie in the range 1.3 - 1.75. The present study aims to revisit this
question by employing an exceptionally large data set of 262 differential light
curves (DLCs) derived from 262 pairs of non-varying stars monitored under our
ARIES AGN monitoring program for characterizing the intra-night optical
variability (INOV) of prominent AGN classes. The bulk of these data were taken
with the 1-m Sampurnanad Telescope (ST). We find $\eta$ = 1.54$\pm$0.05 which
is close to our recently reported value of $\eta$ = 1.5. Moreover, this
consistency holds at least up to a brightness mismatch of 1.5 mag between the
paired stars. From this we infer that a magnitude difference of at least up to
1.5 mag between a point-like AGN and comparison star(s) monitored
simultaneously is within the same CCD chip acceptable, as it should not lead to
spurious claims of INOV.
