On the optimal calibration of VVV photometry
  Prompted by some inconsistencies in the photometry of the VISTA Variables in
the V\'ia L\'actea (VVV) survey, we conduct a revision of the standard
calibration procedure of VISTA data in the $J$, $H$, and $K_S$ passbands. Two
independent sources of bias in the photometric zero-points are identified:
First, high sky backgrounds severely affect the $H$-band measurements, but this
can mostly be minimized by strict data vetting. Secondly, during the zero-point
calibration, stars serving as standards are taken from the 2MASS catalog, which
can suffer from high degrees of blending in regions of high stellar density,
affecting both the absolute photometric calibration, as well as the scatter of
repeated observations. The former affects studies that rely on an absolute
magnitude scale, while the latter can also affect the shapes and amount of
scatter in the VVV light curves, thus potentially hampering their proper
classification. We show that these errors can be effectively eliminated by
relatively simple modifications of the standard calibration procedure, and
demonstrate the effect of the recalibration on the VVV survey's data quality.
We give recommendations for future improvements of the pipeline calibration of
VISTA photometry, while also providing preliminary corrections to the VVV
$JHK_\mathrm{S}$ observations as a temporary measure.
