The ESO grade values are given at the OB level and the quality control classification rules are based on:
- sky transparency as given by the weather officer (a person that goes out regularly to check if there are any clouds on the sky) + zero point variations during the night
- moon (FLI and moon distance) - which is known at any given time and pointing
- seeing - image quality measured on the pipeline reduced data - one value is given per stacked pawprint
- ellipticity - as measured from pipeline reduced data - one value is given per stacked pawprint
- airmass - comparison of the airmass at the time of observation vs. constraint in the OB
For all these constraints the telescope operator verifies whether they are
- fully satisfied
- almost satisfied
- not satisfied
this then constitutes the quality control grade:
A - all within constraints; OB completed
B - mostly within constraints - some constraint is ~10% violated; OB completed
C - out of constraint - OB set to must repeat
D - out of constraint, BUT OB set to completed and will not be repeated - this may happen if some constraint is more than ~10% violated, but for some (scientifically valid) reason it is decided not to repeat the observation - this decision is typically taken by the support astronomer not by telescope operator; it happens very rarely
The qc grade "D" could happen if for example twilight constraint was violated (observation started before the end of the astronomical twilight, but the OB requested to have twilight=0), where for K-band observation during the evening/morning twilight does not influence the background.
Some more information on the VIRCAM specific classification rules regarding seeing and ellipticity are available here:
QC grades R and Z do not exist, but these have been used for: :-)
R - aborted OB - this will be repeated later
Z - unclear status/grade - automatic retrieval of the status/grade returned null value
X - Not applicable. It is always associated with the status M (must repeat)
NONE and empty string are not delivered.
Be careful with deprecating automatically the observations that have qc grade "C". This can be done automatically only if you are 100% certain that the qc grade in the headers correspond to the genuine QC grade/status of the OB.