Measurements for the pixel-to-visibility-matrix (P2VM) are recorded during
daytime with the calibration unit, with one of the grisms inserted (low,
medium, or high resolution), and with the Wollaston prism either inserted
(split) or not inserted (combined polarimetry). A measurement sequence for one
set up consists of six exposures where each exposure has two inputs (i.e. one
P2VM frame. Medium resolution, combined polarimetry, baseline 1-2.
The normalized coherence should be close to 1 for a good P2VM calibration.
In April 2019, scoring thresholds have been revised. As upper threshold,
the upper envelope of historical data has been chosen, obvious outliers excluded.
Lower thresholds have been set to median - 3sigma, again after excluding strongly
deviating data points. Before April 2019, thresholds have been 1.0 and 0.85, respectively.
The thresholds for the phase RMS have been set to median +/- 0.002 in May 2019
which is about +/- 3sigma.