On the optimal calibration of VVV photometry

Prompted by some inconsistencies in the photometry of the VISTA Variables in the Via Lactea (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: intra-array variations in the detector's response, and the blending of local secondary standard stars. Their combined effect on the data results in both a space-varying bias in the absolute photometric calibration, and a time-varying error in the photometric zero-points on various time-scales. 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 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 $JHK_S$ observations as a temporary measure.