This plot monitors the persistence in <b>IFS background</b> calibrations 
which can be generated by previously acquired IFS optical distortion calibrations.
These optical distortion calibrations can leave a pinhole slice pattern in the IFS background pipeline product. 
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The QC parameter is simply: the median counts in the pipeline product in a window P = [770, 130 --- 850, 1440] 
minus the median counts in window B = [640, 1470 --- 720, 1570].
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Window P (pinhole) is the area where the pinhole slice pattern can show up and window B (background) is the area 
off the pinhole slice pattern areas on the detector. 

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The absolute persistence value in ADU is divided by the DIT and multiplied by 60 to represent the ADU difference for 1 min exposure time. 
Because of the linear dependence to the IFS optical bench temperature relation, this value is also divided by the T_bench value, so it corresponds to 1 deg C. 
This finally normalized persistence value is monitored.

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This kind of monitoring was introduced 2010-01-28.</font><p><font size="2">
IFS_DARK master frames are not used to calibrate science observations, only
the ancillary IFS_DARK product IFS_STATIC_BADPIXEL is used. IFS_BACK
calibrations are used to calibrate science observations.  
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Since the
background level of the IFS_BACK calibration depends on the exposure
time (DET.SEQ1.DIT) and the ambient temperature, template subtraction
(subtraction of a reference product with the same DIT and ambient
temperature) might give persistence values with samller error bars.
However the two analysis windows P and B are close to each other,
and measure the values at the identicat DIT and ambient temperature. 
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