User Manual
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instrument is 6.2 t, spread between 2 t for the warm part or AO system and 4.2 t for the cold
part. The optics inside the cryo-vessel is cooled to 65 K. The detectors are stabilized at 35
K within ~5 mK
.
3.1.5 The CRIRES
In order to stable exposures with high repeatability, the concept of a metrology system was
developed to allow for a 0.1pixel
reproducibility. The strategy is to centre a defined set of
emission lines of the Kr and Ne pen ray lamps on the science detector by finetuning the
positions of the cross-disperser grating and Echelle grating and refining further via the use
of a piezo driven tip-tilt mirror that has actuators aligned with the main- and cross-dispersion
axes. This is an iterative process which may take a few minutes, the exact duration depends
upon the unpredictable behaviour of the cross-disperser grating and Echelle grating
functions. The metrology then ensures that these emission lines are indeed located at their
fiducial positions on the science FPA before any science exposure (or any calibration
exposure when used during daytime) follows. Those science/calibration exposures
obtained after a successful application of metrology will have the following metrology
keywords written to their headers (values below are examples):
HIERARCH ESO OCS MTRLGY DX = 0.002 / [pixels] Final mean x residual relative to
fiducial
HIERARCH ESO OCS MTRLGY DY = 0.039 / [pixels] Final mean y residual relative to
fiducial
HIERARCH ESO OCS MTRLGY ECHCORR = -54 / [Enc] Difference between the nominal echelle
encoder value and the resulting value after metrology
HIERARCH ESO OCS MTRLGY ECHNMOV = 1 / Number of echelle grating moves
HIERARCH ESO OCS MTRLGY NITER = 5 / Total number of iterations performed
HIERARCH ESO OCS MTRLGY PIEZO1 = 4.340 / [V] Cross-dispersion piezo voltage at the
conclusion of metrology
HIERARCH ESO OCS MTRLGY PIEZO2 = 3.990 / [V] Main-dispersion piezo voltage at th e
conclusion of metrology
HIERARCH ESO OCS MTRLGY ST = T / Success or failure of metrology
HIERARCH ESO OCS MTRLGY TIME = 1612768172.0 / [s] Timestamp for the successful
conclusion of metrology
HIERARCH ESO OCS MTRLGY TOTDX = -1.430 / [pixels] Average total applied correction
in the main dispersion direction
HIERARCH ESO OCS MTRLGY TOTDY = 0.194 / [pixels] Average total applied correction
in the cross-dispersion direction
HIERARCH ESO OCS MTRLGY XDGWCORR = 0 / [Enc] Difference between the nominal XDGW
encoder value and the resulting value after metrology
HIERARCH ESO OCS MTRLGY XDGWNMOV = 0 / Number of XDGW moves
The metrology can be activated or deactivated in the acquisition and observing templates.
When it is enabled during the acquisition the metrology runs in parallel to the telescope
preset, therefore no overheads are associated to the metrology (see Table 9).