This plot displays the extinction calculated by the pipeline recipe fors_zeropoint for each individual STD_IMA frame, and the nightly extinction as calculated by fors_photometry. The frame extinction is calculated with a constant instrument zeropoint which is a reasonable assumption. The true zeropoint may however deviate from the assumed one which comes from a static table. The frame extinction values are plotted for all nights (with 'stable' flag values 'S', 'Sb', 'N' and 'U'; open circles), as well as for stable nights ('S' or 'Sb', filled circles). For a definition of this flag see the 'IMG zeropoint' plots.

The nightly zeropoints are plotted here only for stable nights ('S' or 'Sb').

We plot here only values for chip 1. For the other chip, you may want to use the link to the QC1 database.

The nightly extinctions are based on the FORS2 calibration plan as part of the FORS2 Absolute Photometry (FAP) 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 with SCIENCE observations requiring photometric conditions.

In the lower panel, the airmasses of the individual standard star fields taken in the course of the night are plotted. One can clearly see the initial low-high-airmass pair if acquired. We also mark 'S'/'Sb' nights by a green bar, 'N' nights in red, and 'U' nights in black.

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

The pipeline recipe fors_photometry is operational since 2012-06, hence the nightly extinctions are available from the same date on. All HISTORY plots earlier than 2012-06 do not have night extinctions.

More information can be found on the plot tutorial page.