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It is important to quantify the underestimation of rms photometric errorsreturned by the commonly used $\emph APPHOT$ algorithm in the $\emph IRAF$software, in the context of differential photometry of point-like AGN, becauseof the crucial role it plays in evaluating their variability properties.Published values of the underestimation factor, $\eta$, using several differenttelescopes, lie in the range 1.3 - 1.75. The present study aims to revisit thisquestion by employing an exceptionally large data set of 262 differential lightcurves (DLCs) derived from 262 pairs of non-varying stars monitored under ourARIES AGN monitoring program for characterizing the intra-night opticalvariability (INOV) of prominent AGN classes. The bulk of these data were takenwith the 1-m Sampurnanad Telescope (ST). We find $\eta$ = 1.54$\pm$0.05 whichis close to our recently reported value of $\eta$ = 1.5. Moreover, thisconsistency holds at least up to a brightness mismatch of 1.5 mag between thepaired stars. From this we infer that a magnitude difference of at least up to1.5 mag between a point-like AGN and comparison star(s) monitoredsimultaneously is within the same CCD chip acceptable, as it should not lead tospurious claims of INOV.

On the photometric error calibration for the `differential light curves' of point-like Active Galactic Nuclei