These plots show the difference at a given ref_level between the flux obtained from 2nd order 
polynomial fit of flux vs. exposure time and the flux predicted by the linear term only, normalized by the polynomial flux. 

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For more information see the
<a href="http://www.eso.org/observing/dfo/quality/XSHOOTER/qc/detmon_qc1.html">plot tutorial page</a>.