Title: The VISTA ZYJHKs Photometric System: Calibration from 2MASS,
Abstract: In this paper we describe the routine photometric calibration of data taken
with the VIRCAM instrument on the ESO VISTA telescope. The broadband ZYJHKs
data are directly calibrated from 2MASS point sources visible in every VISTA
image. We present the empirical transformations between the 2MASS and VISTA,
and WFCAM and VISTA, photometric systems for regions of low reddening. We
investigate the long-term performance of VISTA+VIRCAM. An investigation of the
dependence of the photometric calibration on interstellar reddening leads to
these conclusions: (1) For all broadband filters, a linear colour-dependent
correction compensates the gross effects of reddening where $E(B-V)<5.0$. (2)
For $Z$ and $Y$, there is a significantly larger scatter above E(B-V)=5.0, and
insufficient measurements to adequately constrain the relation beyond this
value. (3) The $JHK\!s$ filters can be corrected to a few percent up to
E(B-V)=10.0. We analyse spatial systematics over month-long timescales, both
inter- and intra-detector and show that these are present only at very low
levels in VISTA. We monitor and remove residual detector-to-detector offsets.
We compare the calibration of the main pipeline products: pawprints and tiles.
We show how variable seeing and transparency affect the final calibration
accuracy of VISTA tiles, and discuss a technique, {\it grouting}, for
mitigating these effects. Comparison between repeated reference fields is used
to demonstrate that the VISTA photometry is precise to better than $\simeq2\%$
for the $Y$$J$$H$$Ks$ bands and $3\%$ for the $Z$ bands. Finally we present
empirically determined offsets to transform VISTA magnitudes into a true Vega
system.