The spectrophotometric standard star observations are used to
determine the efficiency of the instrument+telescope system. We plot
only data obtained on <a href="http://www.eso.org/observing/dfo/quality/FORS2/qc/zeropoints/zeropoints.html">photometric nights</a>.

More information can be found on
the <a href="http://www.eso.org/observing/dfo/quality/FORS2/qc/efficiency_qc1.html">plot tutorial page</a>