GIRAFFE: System efficiency
| Efficiency vs. time:
| Efficiency vs. wavelength:
QC1 database (advanced users):
A set of standard star measurements is taken regularly
with the Argus IFU of GIRAFFE, under CL (clear) or PH (photometric)
to measure the overall system efficiency. A set of three
settings is taken with the low-resolution grating (L427.2, L543.1,
more with the high-resolution grating (H447.1A, H651.5B, H875.7).
These data are taken
once every 1 or 2 months.
They are calibrated with the robotic lamp
flats. The flats
are used only for the localization solution (required for the
to the usual reduction, the extracted flat-field is not divided into
star spectrum, in order to achieve the efficiency curve. The signal from
fibres (excluding SKY and SIMCAL) is summed up and divided by the
flux curve of the
standard star, to measure the overall (telescope plus optical
components plus detector)
efficiency of the instrument.
Argus STD frame. This is the pipeline-recombined image of a GIRAFFE standard star measurement taken with the Argus IFU. Each pixel corresponds to one fibre on the sky. The bright spot at lower left is due to a bad column; the dark hole at upper right is caused by the first fibres which are somewhat truncated by the CCD.
||QC1 database: table, name
| [calculated by QC procedure] || giraffe_std..mean_effic ||mean efficiency at central wavelength||KPI|| || |
|*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter|
**There might be more than 1, always check the links 'HC_plots'
The mean efficiency is plotted in boxes 1-6 of the SYSTEM_EFFICIENCY trending
plot for the 6 reference settings (3 HR and 3 LR settings).
Efficiency plots. Boxes 1-6 show the mean efficiency displayed over time. The green bars mark the current ETC predictions. The blue line marks the CCD replacement in 2008.
These data are also plotted over wavelength, in the SYSTEM_EFFICIENCY_WLN trending plot .
Efficiency plot vs. wavelength. Boxes 3 and 4 show the mean efficiency for the HR and LR settings (new CCD) displayed over wavelength. Boxes 1 and 2 show the same for the historical (old) CCD.
The trending plot covers the full history of this parameter.
| 2006-08-06 || first set of 6 efficiency monitoring frames taken |
| 2008-03-14 || Old CCD ("Bruce") decommissioned |
| 2008-05-26 || New CCD ("Carreras") operational; significantly higher efficiency in the red, lower efficiency in the blue |
| 2011-06 || The HR grating has been recoated |
| 2014-09-02 || Updated ETC reference values to reflect the current ETC including the fudge factors|
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, due to insufficient sky
subtraction in the data.
The scoring is quite loose because the definition procedure for mean_effic is not very good. Upper and lower thresholds are only to indicate a possible failure of the routine. The correct interpretation of the results requires looking at the long-term trend.
find central wavelength, average the efficiency values in window of 2 nm size around that wavelength, subtract the SKY