FORS1 Quality Control: QC1 from screen flat frames

FORS1:
QC1 from screen flat frames

QC1 PLOTS: screen flat / fixed-pattern noise / conversion factor

QC1 ASCII DATA

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QC1 database (expert mode)

QC1 database (expert mode) QC1 database (expert mode)

This page refers to the two CCDs installed since March 27, 2007. Data from the previous CCD can be found here.

SCREEN_FLAT_IMG: noise parameters, conversion factor

Modes

Parameters are measured for the 5 standard filters UBVRI Bessel, and for the standard CCD modes

Parameters

Efficiency: median signal devided by exposure time. The median value is computed within a window 250x150 pixel chosen close to the gap region between the 2 CCDs to minimize the illumination gradient. This is only a nominal 'efficiency' actually measuring the stability of exposure level (influenced by e.g. aging of lamps etc.).

Fixed-Pattern Noise: The fixed-pattern noise is computed in the following way: The difference of two 100x100 sub-frames of the same master flat field image is determined. These two sub-frames are are shifted with respect to each other by 10 pixels in X- and Y-direction. The population standard deviation of the difference frame, divided by the square root of 2, minus the photon noise (geometrically subtracted) yields the fixed-pattern noise, which is then divided by the median of the respective region. The fixed-pattern noise is dominated by the wave like pattern shown by the CCDs, that is particularly strong in U and is due to the CCDs thinning procedure.

Conversion factor (1/gain): f the exposure time of the first two raw screen flat fields in the input set of frames is the same (within 4%), the difference frame is computed. Then the whole readout port is divided into a chessboard of 16x16 boxes. For each one of these boxes, the median signal level from the first raw frame is divided by the variance in the difference frame scaled by 2. The median value of the 16x16 values obtained is the accepted value for the conversion factor. CONAD QC parameters are trended per readout mode for the R_BESS filter only. The CONAD is measured in e^-/ADU.

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