XSHOOTER: Active Flexure Compensation
To compensate for possible flexure of XSHOOTER a pinhole arc frame is
taken before every science frame, from which the line shift relative
to reference positions is determined (and then compensated by the
piezo-controlled mirrors). This shift in X and Y is stored together with
other parameters for later analysis. These QC parameters are specific to
the acquisition of the individual science OB (pointing) and less related
to the instrument health.
The values are not monitored. They are available as shift_x and shift_y
in the flexure QC1 DB.