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VIRCAM Quality Control:
detector linearity

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non-linearity parameters
 
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detector non-linearity
fraction of bad pixels
flux variation
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The linearity template acquires a series of dome flats with increasing exposure time covering the full dynamic range of the detectors. A set of dark frames with the same exposure time is included.

top non-linearity

QC1 parameters

parameter QC1 database: table, name procedure
qc_linearity vircam_line, qc_linearity, qc_linerror, qc_nbadpix, qc_badfrac - The non-linearity is derived from a series of dome flat exposures with different flux levels. The reference flux is at 10000 ADU. qc_linearity is the percentage of non-linearity at 10000 ADU.

halogen lamp performance

vircam_line, qc_screen step, qc_screen_total - The calibration unit consists of several halogen lamps illuminating a screen in the VISTA enclosure. Both parameters monitor intrinsic illumination instabilities and failures of individual lamps. It describes the maximum deviation among the exposure time scaled flats and the maximum deviation in any consecutive pair of flats.

 

top ranges and saturation

QC1 parameters

parameter QC1 database: table, name procedure

qc_line_medmin , qc_line_medmax

vircam_line, qc_line_medmin, qc_line_medmax - The post-pipeline QC script measures the mode flux in all flat frames. The highest flux and the lowest mode flux of all raw flat frames is taken. (mode means: the maximum of the flux histogram is taken.). Since the detector is operated in double correlated read out mode, which is composed by three sequential events: reset, read read. The detector requires DET.MINDIT seconds to reset and read all pixel of the detector. DET.MINDIT=1.0011 sec (2009-12-01). The detector acquisition system subtracts the flux taken between the reset and the first read from the flux measured after the second read and writes it into the frame. Hence the measured flux in a flat does correspond to DIT seconds exposure time. For reasons to check the flux versus the saturation level, the flux acquired in DIT + MINDIT seconds must be compared. The qc_line_medmin and qc_line_medmax are corrected for the DIT+MINDIT exposure times:
qc_line_medmax = flux * (DIT+MINDIT)/DIT.
The correction is only relevant for small DIT, large lamp fluxes, and large MINDIT.


Saturation

Each detector has a different saturation level. The saturation levels in ADU are (2009-09):

chip13 chip 14 chip 15 chip 16
33000 35000 34000 34000
chip 9 chip 10 chip 11 chip 12
35000 35000 37000 34000
chip 5 chip 6 chip 7 chip 8
24000 36000 35000 33000
chip 1 chip 2 chip 3 chip 4

33000

32000 33000 32000

Detector #5 has the lowest saturation level

From 2009-11-20 on, ( after the video channel card replacement for chip #6 ) the saturation levels are:

chip13 chip 14 chip 15 chip 16
33000 35000 34000 34000
chip 9 chip 10 chip 11 chip 12
35000 35000 37000 34000
chip 5 chip 6 chip 7 chip 8
24000 27000 35000 33000
chip 1 chip 2 chip 3 chip 4

33000

32000 33000 32000

History

  • Before 2018-01 the qc_line_medmin and qc_line_medmax contain the counts in the lowest exposure and the counts in the longest exposure flats generated by the template. Furthermore qc_dit_min and qc_dit_max contain the exposure times of the shortest and longest exposure.
    After 2018-01 qc_line_medmin and qc_line_medmax contain the counts in the lowest and highest exposure flat not rejected by the recipe and used to construct the polynomial fit of the detector non-linearity. Furthermore qc_dit_min and qc_dit_max contain the lowest and highest exposure times of these non-rejected flats. These parameters reflect after 2018-01 the dynamic range on which the non-linearity is based.
  • The flat images (also those of the dome flats and the gain calibrations) show a long-term degrading of the received counts, probably caused by aging of the lamp, the optical coating or dust. As a side effect, previously excluded flats of a certain longer exposure time - they were too bright for the non-linearity analysis -, show now less counts and are included for the analysis, resulting in a different non-linearity correction, as the dynamic range fo the detector used is now larger. In 2018-03 chips #5 and #6 are affected. The dynamic range increased for chip #5 from 19700 ADU (DIT = 25sec) to 23000 ADU (DIT = 30sec) and for chip #6 from 22300 ADU (DIT = 25sec) to 25200 ADU (DIT = 30sec). The impact on the non-linearity correction at 10000 ADU is negligible for chip #5 and increased from 3% to 4% for chip #6 (averaged over 16 channels).

 
 
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