This tutorial provides information for the the Twilight Flats Health Check trending.
The Twilight Flats trending is based on the measurement of QC1 parameters from the twilight flat field calibration data. Following the calibration plan, the twilight flat fields are taken during evening (usually) and morning (rarely) twilights with clear skies. This is done whenever they are needed to calibrate SCIENCE data.
HAWK-I instrument does not have an internal lamp. In the infrared, twilight flats are taken by sampling a large flux range by integrating on the twilight sky as the Sun sets or rises. Twilight flats are therefore one of the most time critical calibrations because of the narrow time window during which they can be acquired.

The standard deviation of the pixel values in a stack of the input frames is monitored for each detector in J, H, Ks & Y filters.