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VISIR Quality Control:
FLAT FIELD frames

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One FLAT FIELD frame for the imaging detector is taken every day within the VISIR daytime calibrations. It consists of 6 raw frames, where each is illuminated with different flux level. The changes of the flux are achieved by illuminating detector with the "warm calibration unit" simulating radiation of black body with different temperatures:

  • 1st frame: 20 C (keyword INS.EXTS.ACT)
  • 2nd frame: 10 C
  • 3rd frame: 0 C
  • 4th frame: -10 C
  • 5th frame: -20 C
  • 6th frame: -30 C
The FLAT FIELD frames are always obtained with the same instrument setup: filter (INS.FILT1.NAME)=PAH1, pixel-field-of-view (INS.PFOV)=0.127, detector integration type (DET.DIT)=0.016s the number of detector sub-integrations (DET.NDIT)=1875 and readout mode (DET.NCORRS.NAME)=Imager_syncABAB. The FLAT FIELD frames are not used for calibrating science data. They are utilized only to monitor performance of the VISIR's detectors.

There are also additional, technical flat fields taken every day for both imaging and spectroscopic detector with a different readout mode (DET.NCORRS.NAME)=Imager_nocorr_syncABAB and Spectro_nocorr_syncABAB. However, currently they are not being used for monitoring.

FLAT FIELD master frame of the imaging detector . This is a pipeline product of the "visir_img_ff" recipe. It was build by combining the 6 input raw frames. Note bright horizontal stripes which are characteristic for the detector. FLAT FIELD 2nd raw frame . This is a 2nd raw FLAT FIELD frame which corresponds to the illumination by the black body with temperature of 10 K.

Other VISIR detector parameters that are monitored:
read-noise dark
read-noise12 dark
dark level dark
det. temperature dark

top nbadpix

The following plot shows the temporal evolution of the the number of bad pixels in the fltafield frame of the imaging detector. For each flatfield image the median intensity is determined. Then it's plotted against intenisty in each pixel. Ideally, all pixels should have an equal gain, thus the plots should be straight lines with slope of 1. However, some pixels have relative gain greater and some smaller than 1. Those are flagged as bad pixels.

QC1 parameters

  • number of bad pixels (QC1 database table visir_flat_img, column nbbadpix)
    This is the number of pixels, for which the calculated relative gain falls outside the range from 1/5 to 5.

 
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