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When Charge Coupled Devices are used for scientific observations, their darksignal is a hindrance. In their pristine state, most CCD pixels are `cool';they exhibit low, quasi uniform dark current, which can be estimated andcorrected for. In space, after having been hit by an energetic particle, pixelscan turn `hot'. They start delivering excessive, less predictable, darkcurrent. The hot pixels need therefore to be flagged so that subsequentanalysis may ignore them. The image data of the PICARD SODISM solar telescope(Meftah et al. 2013) require dark signal correction and hot pixelidentification. Its frame transfer E2V 42-80 CCD operates at -7{\deg}C. Bothimage and memory zones thus accumulate dark current during, respectively,integration and readout time. These two components must be separated toestimate the dark signal for any observation. This is the purpose of the DarkSignal Model presented in this paper. The dark signal time series of everypixel is processed by the Unbalanced Haar Technique (Fryzlewicz 2007) in orderto timestamp when its dark signal is expected to change. In-between thoseinstants, both components are assumed constant and a robust linear regressionvs. integration time provides first estimates and a quality coefficient. Thelatter serves to assign definitive estimates. Our model is part of the SODISMLevel 1 data production scheme. To check its reliability, we verify on darkframes that it leaves a negligible residual bias (5 e-), and generates a smallRMS error (25 e- rms). The cool pixel level is found to be 4 e-/pxl/s, inagreement with the predicted value. The emergence rate of hot pixels isinvestigated too. It legitimates a threshold criterion at 50 e-/pxl/s. Thegrowth rate is found to be 4% of the frame area per year. Aspects of the method(adaptation of the Unbalanced Haar Technique, dedicated robust linearregression) have a generic interest.

Dark signal correction for a lukecold frame transfer CCD. Application to the SODISM solar telescope on board the PICARD space mission