VIMOS
Quality Control: detector linearity and gain 



The (non)linearity of the VIMOS detectors is assessed by a series of flat exposures with increasing exposure time. After bias subtraction, the function ADU = A + B * EXPTIME + C * EXPTIME^2 is fitted to the average level of the flats. The coefficients A, B, and C are saved into an output file. They are not used further for data reduction since the VIMOS detectors do not show significant nonlinearity. For monitoring, the QC parameter effective nonlinearity is used. It is the difference between the flux obtained at the reference level 40000 ADU from the polynomial fit and the flux predicted by the linear term only, normalized by the polynomial flux. The mean level of the flats with the highest exposure time must, therefore, be well above 40000 ADU for a correct calculation of effective nonlinearity. QC1 parameters
Trending The effective nonlinearity is monitored for both readout modes (low and high gain). History December 2010 until September 2012: flatfield flux was variable on short time scales which resulted in arbitrary values for the effective nonlinearity
There are two exposures for each exposure time value of the flatfield sequence. The following equation is calculated for each pair: mean(flat1) + mean(flat2)  2 * mean_bias = gain * [sigma^2(flat2flat1)sigma^2(bias2bias1)] . Following the Photon Transfer Curve (PTC) method, the final gain value is calculated from a linear fit to the above equation. Alternatively, the median of the individual gain values can be calculated (method MED). Please note that gain is expressed in electrons per ADU. QC1 parameters
Trending Gain is trended for both readout modes (low gain, 2148x2440; high gain, 2148x4096). History July 2010: new detectors 



