6.3.2 CCD cosmetics

The calibration of the CCDs is done using the on-ground measurements taken during the pre-flight tests.

  • Defects: The Gaia CCDs were delivered by the manufacturer (then called e2v, now called Teledyne e2v) with a pre-launch list of column defects. Those classified as either ‘defects in the dark’ or ‘photo-response defects’ were collated and provided to the spectroscopic pipeline. The former could be either hot pixels or hot columns. Because Gaia operates its CCDs in TDI mode, hot pixels appear as hot columns. The defect list contains 26 columns in the 12 RVS CCDs, each with 1966 columns. RVS CCDs have no dead columns in the time covering Gaia DR2.

  • Dark current: Dark current was measured by the spacecraft manufacturer (then called EADS-Astrium, now called AirbusD&S) as the mean CCD dark current at 193 K, measured on the Flight Model CCDs, and then extrapolated to 163 K (Gaia’s operating temperature). This value is 2.80 × 10-4 electrons pixel-1 s-1.

  • Saturation: RVS CCDs have TDI gates but they are used only when acquiring bias non-uniformity calibration data. Because TDI gates are not used in nominal RVS observations (due to lower light levels than the other Gaia instruments), the saturation level of each column does not need to be measured as input for when to trigger TDI gates for bright observations to prevent saturation. Instead, the few brightest stars in the sky saturate pixels within the 10 columns that make up a 2D RVS window. Observations with on-board GRVS <7 mag have 2D windows, where every pixel is read out as a sample. Observations with on-board GRVS >7 mag have 1D windows, where 10 pixels in the AC direction are binned at read out as a sample. Windows including saturated samples are excluded from the CalibrationPreparation and FullExtraction workflows.