Gain calibration and nonlinearity analysis in single photon sensitivity Skipper CCD

The Skipper CCD allows pixel measurements with sub-electron noise levels. The gain of the CCD, that relates the number of electrons to the number of analog to digital units (ADU) is almost linear but with some degree of nonlinearity. This article uses the charge quantization allowed by the Skipper CCD to estimate the gain with different strategies. Linear and quadratic models are analyzed and differential and integral nonlinearity (DNL and INL) metrics are computed. Experimental results in a range of charge between 0 and 700 electrons shows that the linear model achieves an INL of 0.8 e⁻, while the second order model reaches an INL of 0.09 e⁻.