The gain is derived from a set of paired flat fields with identical exposure
time, using the Photon Transfer Curve method. Since the NIR arm can take only
echelle flats we specify a small window on one order. 
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2013-08-01: The thresholds have been modified (flats of more counts
are included for the analysis) resulting in a higher sensitivity of the
non-linearity QC parameter. As a side effect, the gain values become 
more affected by the non-linearity. The more high counts flats are included,
the smaller the retrieved gain value. 


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