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GIRAFFE Quality Control:
system efficiency

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   system efficiency
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system efficiency
Efficiency (vs. time) all plotted under CURRENT
Efficiency (vs. wavelength) all plotted under CURRENT
QC1 database (advanced users): browse | plot
   Click on CURRENT to see the current trending (Health Check).
   Click on HISTORY to see the historical evolution of the trending.

Since August 2006, a set of standard star measurements is taken regularly with the Argus IFU of GIRAFFE, under CL (clear) or PH (photometric) conditions, to measure the overall system efficiency. A set of three settings is taken with the low-resolution grating (L427.2, L543.1, L881.7), three more with the high-resolution grating (H447.1A, H651.5B, H875.7). These data are taken approximately 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 extraction). Contrary to the usual reduction, the extracted flat-field is not divided into the standard star spectrum, in order to achieve the efficiency curve. The signal from all 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.

top Efficiency

QC1 parameters

parameter QC1 database: table, name procedure
mean efficiency at central grating wavelength giraffe_std..mean_effic find central wavelength, average efficiency values in window of 2 nm size around that wavelength
qc_numsat giraffe_std..qc_numsat number of saturaed pixels in raw frame, to discover saturation
mean signal in recombined image giraffe_std..mean_signal replace pixel (4,4) by average from neighbours (to protect against bad column); take average, to find cases with no STD star signal


The mean efficiency is plotted in boxes 1-6 of the SYSTEM_EFFICIENCY trending plot for 3 HR settings and 3 LR settings.

Efficiency plots. Boxes 1-6 show the mean efficiency displayed over time. The green bars mark the current ETC predictions. While there is good agreement of measured and predicted efficiencies for the blue and middle settings, the ETC under-estimates the red efficiency by a factor larger than 2.

These data are also plotted over wavelength, in the SYSTEM_EFFICIENCY_WLN trending plot .

Efficiency plot vs. wavelength. Box 1 shows the mean efficiency for the HR settings displayed over wavelength, box 2 the same for LR. The green curve marks the ETC predictions.



The trending plot covers the full history of this parameter.

date event
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