FORS2: Detector monitoring

HC PLOTS 
conversion factor 

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.
QC1_parameters
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_linear  HC   [docuSys coming] 
*Class: KPI  instrument performance; HC  instrument health; CAL  calibration quality; ENG  engineering parameter
**There might be more than one. 
Trending
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 200910 rarely reach 60000 ADU we changed this ref_level from 60000 ADU to 40000 ADU, starting 200910.
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.
History
Date 
event 
20110117  checking the flat field data showed that the FlatBlue+3 lamp was unstable. This might explain some of the variations in nonlinearity observed since 200907. 
20101001  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 nonlinearity is determined, was reduced to 40000 ADU. 
April  September 2009  the flats reached only rather low flux levels, so the nonlinearity is not well determined. 
20210609  DET.OUT1.GAIN/CONAD for 100Kbps/2p/high_gain 1x1 adjusted in headers; (PPRS080731) 
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 biascorrected 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 biascorrected 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 nonlinearity is the difference between the polynomial flux and the one predicted by the linear fit at userdefined flux level (column flux_non_linear), normalized with the polynomial flux.
