WFI Nightly Photometric Zeropoint Monitoring
During photometric nights a selected standard star, the Standard Star of the Month, is measured using the MIDAS Amag program and the zeropoints plotted in this page. The measurements are done on the raw WFI frames without previous bias subtraction or flat fielding. The group of stars measured are taken from Landolt and selected to fit near the bottom of chip 51. They are measured at approximately the same airmass every night. Typically we observe TPhe at airmasses of 1.05 and RU149 at airmasses of 1.15. The standard extinction law for La Silla is assumed. These measurement are only useful for quality assurance and should not be used to reduce science data.
The plot below shows the zero points in every chip. It include data from non photometric nights. After night 800 we have been carefull to measure only stars near the bottom of chip 51, and only during photometric nights. The data can be found here .