This is the instrument performance plot for the system efficiency of 
GIRAFFE. It is based on measurements of a flux standard star with the Argus
IFU under photometric conditions. There are three settings taken with the
low-resolution grating (L427.2, L543.1, L881.7) and three more with the
high-resolution grating (H447.1A, H651.5B, H875.7). These data are taken
approximately every month. </font><p><font size="2">

They are calibrated with the <i>robotic</i> lamp flats. The flats are
used only for the localization solution (required for the extraction). 
Contrary to the usual reduction, the extracted flat-field is not divided into
the standard star spectrum, in order not to corrupt the efficiency
calculation. </font><p><font size="2">

We display the QC1 parameter mean_effic which is the ratio of detected
photons and the incident photons. The signal from all fibres (excluding SKY and
SIMCAL) is combined and averaged. </font><p><font size="2">

For comparison we also plot the current ETC values at the central wavelength of 
the setting. Check out the <a href=http://www.eso.org/observing/etc/bin/gen/form?INS.NAME=GIRAFFE+INS.MODE=spectro>ETC</a> for more.</font><p><font size="2">

The reference values are defined as best-performance values. The efficiency is expected
to degrade over time, mainly because of the degrading coating of the gratings.
The HR grating has been recoated in 2011-06, the effect is visible in plots #1
and #2. Efficiency drops by about -0.0033 per year in
H447.1A and in H651.5B. We felt unable to determine a reference value for the
two red settings in plots #3 and #6, where the measurement method is
particularly bad presumably due to insufficient sky subtraction in the data.