This plot displays two kinds of photometric zeropoints: the frame zeropoints (calculated by the pipeline recipe fors_zeropoint) and the instrumental zeropoints (calculated by fors_photometry). The frame zeropoints are based on individual standard star exposures, while the instrumental zeropoints take into account all available standard star exposures of a certain number of nights and refer to that time range. The standard star frames are observed for both CHIP1 and CHIP2. All zeropoints are based on flux in e-/sec.

The frame zeropoints are calculated assuming a standard extinction coefficient, which may not be correct for an individual night. The instrumental zeropoints are based on all extracted sources in a given time interval around the reference night (plus/minus 14 days). The linear equations are solved assuming a constant instrumental zeropoint and a constant extinction coefficient per night. The solutions are quite sensitive to this assumption of a constant extinction value throughout each night. Only photometrically stable input nights are accepted for these values.

These zeropoints are based on the FORS2 calibration plan as part of the FORS2 absolute photometry project. It foresees the acquisition of an initial pair of standard stars for "promising" nights, and the follow-up standard star fields for actually photometric nights.

The initial pair is a set of standard star fields taken at the begin of the night, at two airmasses separated by at least 0.4. If the corresponding extinction values differ by no more than 0.05 in all filters (for b_HIGH: 0.06), the night is called at least 'Sb' ("stable at begin"). If there are follow-up photometric data in the latter course of the same night, again with stable extinction, the night is called 'S'. If there are such data but the extinction variation was larger than these thresholds, the flag becomes 'N' (not stable). In all other cases the photometric quality of the night is 'U' (unknown). For the instrumental zeropoints, only 'S' and 'Sb' nights are used.

Due to the selection window and the data processing schedule, the nightly zeropoints become available typically after a month.

For the nightly extinction values derived from the nightly zeropoints click on 'extinction'. The pipeline recipe fors_photometry is operational since 2012-06, hence the nightly zeropoints are available from the same date on. All HISTORY plots earlier than 2012-06 do not have night zeropoints.

Major mirror interventions (recoating) are marked by a blue vertical bar.

More information can be found on the plot tutorial page.