Optical dark frames are acquired once a month in all three read modes
to monitor the dark radiation. The exposure time is 100 sec,
the QC paramaters in the plot are scaled to ADU/h.
The plot shows the optical CCD dark current values and RMS as derived
from the zimpol dark recipe for read mode StandardImaging. The recipe
expresses dark values in units of the used EXPTIME (here per 100 sec),
the values in the plot are scaled to ADU/hour.
odd/even means the cycle
zero/pi means the polarimetric phase
The recipe arranges the phase=pi rows and the phase=zero rows of the
raw frame and writes them in different extensions of the pipeline
product. The QC parameters describe the median and the rms of these
imaging products. The QC parameter 'median' retrieved and stored is
given in ADU per 100 sec, the values in the plots are scaled to ADU/hour.
From 2015-04-01 to 2015-11-01: The ZIMPOL CCDs accumulate counts when not being read out since a longer time (several hours). The first ZIMPOL calibration in the day-time calibration batch is the imaging dark, of which the first of two frames is contaminated, while the second one is clean and corresponds to a proper measurement of the dark current. In case the calibration is acquired in cube mode, only the first layer of the data cube is affected, while all further subintegrations are not affected. The clean dark current corresponds to a value of ~3 ADU/100sec, the contaminated and useless value is about ~10 ADU/100sec. The issue is solved since 2015-12-01.
As occured on 2016-09-23, the saturated modemeff calibration acquired directly in advance of the ZIMPOL imaging mode dark calibration left a double as large dark current on the detector, a kind of persistence.