The zeropoint trending is based on photometric standard star calibration data in J, H, Ks, & Y broad band filters. The data are obtained every photometric or clear night during which HAWK-I SCIENCE observations are executed.
Each standard star dedicated observation consists of 4 star images where the target is placed at the center of each detector sequentially. This is repeated for all filters.
The computed zeropoints, corrected for atmospheric extinction, are monitored for each detector separately.

As of 2015-10-01 new HAWKI pipeline, prepared by CASU (v.2.+), is used. The zeropoints derived by this pipeline (recipe hawki_standard_process) differ significantly (by about 2 mag) from the previous values. See further for more information.