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Trending & QC1
   gain & linearity
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This page refers to the determination of linearity and gain by the DETMON recipe from the dedicated (TPL.ID=FORS2_img_tec_detLin) calibrations. Conversion factor derived traditionally from imaging SCREEN FLATS by the fors pipeline can be found here.

Detector parameters

To measure the detector parameters a dedicated DETMON calibrations are taken. They consist of sequences of imaging screen flat field pairs in the I_BESS filter taken with the standard readout modes (100Kps/high_gain/2x2 and 200Kps/low_gain/2x2). An appropriate set of bias frames is also included in the template.


FITS key QC1 database: table, name definition class* HC_plot** more docu
QC.GAIN fors2_detmon..gain measured gain (e-/ADU; inverse of CONAD, but the same quantity as DET.OUT1.CONAD) HC [docuSys coming]
QC.LIN.EFF fors2_detmon..non_linear relative deviation from linearity at the flux level given in flux_non_linearHC [docuSys coming]
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter
**There might be more than one.


The gain values (units e-/ADU) for the MIT detectors only are trended here. The linearity is trended as a difference at a given ref_level between the flux obtained from 2nd order polynomial fit of flux vs. exposure time and the flux predicted by the linear term only, normalized by the polynomial flux. Since data taken after 2009-10 rarely reach 60000 ADU we changed this ref_level from 60000 ADU to 40000 ADU, starting 2009-10.

The EEV detectors are rarely used and detector monitoring data are rarely taken.

Scoring&thresholds Detector parameters

The gain of both MIT detectors is quite stable. It is loosely scored with constant thresholds. The deviation from linearity is also loosely scored. The only requirement is stability on the calibration timescale.


Date event
2011-01-17 checking the flat field data showed that the FlatBlue+3 lamp was unstable. This might explain some of the variations in non-linearity observed since 2009-07.
2010-10-01 changed --filter parameter (to exclude saturated data) from 62000 to 60000 ADU
October 2009 since then, the flats rarely reached 60000 ADU, so the ref_level, at which the non-linearity is determined, was reduced to 40000 ADU.
April - September 2009 the flats reached only rather low flux levels, so the non-linearity is not well determined.
2021-06-09 DET.OUT1.GAIN/CONAD for 100Kbps/2p/high_gain 1x1 adjusted in headers; (PPRS-080731)

Algorithm Detector parameters

The gain values (units e-/ADU) are calculated by the detmon_opt_lg recipe (using the Photon Transfer Curve - PTC, method). The data come from a dedicated detmon calibrations consisting of sets of paired flat fields and bias frames with identical exposure time. For every pair of flat fields with identical exposure times the difference of the average of the bias-corrected frames is compared to the difference in rms between the bias and the flat field difference. This comparison is limited to the illuminated parts of the CCDs. The factor relating the two quantities is the gain in e-/ADU. For method=PTC (Photon Tranfer Curve) a linear relation is fit to these quantities for the various exposure times.

For each exposure time the mean and the median of the bias-corrected flat fields (restricted to the illuminated area of the CCDs) is calculated. Then a second order polynomial is fit to the mean/median vs. the exposure time and the coefficients of that polynomial are stored in the columns mentioned above. In general the constant and the second order terme are 0 within their errors.

The effective non-linearity is the difference between the polynomial flux and the one predicted by the linear fit at user-defined flux level (column flux_non_linear), normalized with the polynomial flux.

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