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GIRAFFE Quality Control:
Grating stability

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Trending & QC1
   grating stability
Data Management
QC links:
temperature | grating motion | thermal correlation | history
Grating stability
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.

A set of frames is measured daily to monitor the stability of the gratings and of the fiber system. These frames carry the DPR TYPE SIMLAMP,TAL (TAL = thorium-argon lamp) and are created by the arclamp of the simultaneous calibration unit. The arc lamp is used to illuminate the simultaneous calibration fibres. The signal is correlated both in X (cross-disperion) and Y (dispersion) against a reference frame. There is a set of six such health check measurements, one (using the LR grating) for each slit system (Medusa1 and 2, IFU1 and 2, Argus), plus one for the HR grating and the Medusa1 slit system.

The data are also used to monitor the simlamp efficiency.

SIMLAMP,TAL frame (closeup). There are five simultaneous calibration fibres, illuminated by the simultaneous arclamp.

top Temperature

The gratings are observed to have both thermal and non-thermal motions. To control the thermal motions, the temperature sensor TEMP53 is used, located on top of the grating table. It delivers a value into the header of each raw frame.

QC1 parameters

parameter QC1 database: table, name procedure
temperature from sensor TEMP53, in degree Celsius giraffe_simlamp..ins_temp53_val written into raw file fits header; sensor is close to grating


As a reference evalue, the temperature TEMP53 is plotted in box 1 and repeated in box 4 of the trending plot.

top Grating motions in X and Y

Each of the Health Check data is correlated in X and Y to a reference frame. The set of reference frames is arbitrarily taken from date 2003-07-24. The difference (X_new - X_ref) is plotted vs. time. The same is done for the Y (dispersion) direction.

QC1 parameters

parameter QC1 database: table, name procedure
mean_xdiff (gratings HR and LR) (left panel, plots 2 and 3) giraffe_simlamps..mean_xdiff difference in X between new frame and reference, HR and LR gratings
mean_ydiff (gratings HR and LR) (middle panel, plot 5 and 6) giraffe_simlamps..mean_ydiff difference in Y between new frame and reference, HR and LR gratings


Boxes 2 and 3 of the trending plot display the X motions of the HR and LR grating (selected is the Medusa1 frame), boxes 5 and 6 the Y motions. Box 4 repeats the temperature record from box 1 for comparison.

Stability trending plot, panel 1 . On top there is the trending of the temperature (in degree Celsius), and below are the plots for the shift in X direction, both for the HR and the LR grating. The temperature sensor selected is TEMP53 ('surface table temperature top').

A similar column is plotted for the Y shifts (panel 2).

top Correlation of shifts with temperature

The third panel shows the shifts in X and Y versus temperature TEMP53. For both directions, there is essentially the same anti-correlation with temperature.

Correlation plot. Boxes 7 and 8 show the shift of the grating in X direction, in pixels, versus the temperature drifts (TEMP53 as above). X-axis: grating shifts in X; Y-axis: TEMP53 in degrees Celsius.

Boxes 9 and 10 display the same for the Y shifts.


Note that the Giraffe gratings occasionally show sudden non-thermal shifts. These are detected in the trending plot as corresponding shift of the correlation pattern in X direction.


date event
2007-11-14 earthquake; non-thermal jump of HR grating in X (+1) and Y (+2) pixels
2011-03-14 earthquake, non-thermal jump in Y direction; adjusted backto the old values on 2011-04-01; no science files affected
2011-06-10 earthquake, non-thermal jump in X and Y direction; adjusted back to the old values on 2011-06-22
2011-06-27 earthquake, non-thermal jump in X; adjusted back on 2011-06-30
2015-11-27 earthquake, non-thermal jump of HR in X by +8 px, LR in Y by more than 1 px; adjusted back begin of 2015 Dec; required new reference dispersion solutions for Medusa2