This is the FULL trending plot for the monitoring of the GIRAFFE
CCD linearity and gain. These parameters are derived from image flats. 
These data are taken about monthly. Since 2008-07, they are acquired with the
common detector monitoring template and reduced with the corresponding common
recipe. </font><p><font size="2">

The CCD was upgraded between 2008-03 and 2008-05. Data from the new CCD are
marked.</font><p><font size="2">

A sequence of image flats is exposed
between 0.5 and 220 secs. 
A third-order polynomial is fitted to the mean exposure level as a function of
exposure time. An effective non-linearity correction is derived from this and
plotted in box 1. This is available only since 2008. </font><p><font size="2">

The gain (in electrons per ADU) is 
also monitored here, as box 2. It is measured from
the difference between two identical raw frames, by comparing the square root
of the signal to its measured rms. Plotted here is the asymptotic value for
exptime &rarr; 0 which is the true gain. Find the complete correlation as plot no. 4 
<a href=/qc/GIRAFFE/reports/HEALTH/trend_report_DETRELS.html target=_blank>here</a>.
The value stored in
the file headers is very close to the one plotted here.