Characterization and correction of charge-induced pixel shifts in DECam

TL;DR

This paper models and corrects charge-induced pixel shifts in DECam CCDs, which affect image quality and bias shape measurements, by fitting a charge-dependent shift model to flat-field noise correlations and applying corrections during processing.

Contribution

It introduces a detailed charge shift model based on flat-field correlations and demonstrates how to mitigate charge-induced biases in astronomical images.

Findings

Flat-field noise correlations follow a power-law decay with pixel separation.

Charge shifts cause biases in shape measurements exceeding science requirements.

Applying reverse charge shifts during processing reduces stellar size biases.

Abstract

Interaction of charges in CCDs with the already accumulated charge distribution causes both a flux dependence of the point-spread function (an increase of observed size with flux, also known as the brighter/fatter effect) and pixel-to-pixel correlations of the Poissonian noise in flat fields. We describe these effects in the Dark Energy Camera (DECam) with charge dependent shifts of effective pixel borders, i.e. the Antilogus et al. (2014) model, which we fit to measurements of flat-field Poissonian noise correlations. The latter fall off approximately as a power-law r^-2.5 with pixel separation r, are isotropic except for an asymmetry in the direct neighbors along rows and columns, are stable in time, and are weakly dependent on wavelength. They show variations from chip to chip at the 20% level that correlate with the silicon resistivity. The charge shifts predicted by the model cause biased shape measurements, primarily due to their effect on bright stars, at levels exceeding weak lensing science requirements. We measure the flux dependence of star images and show that the effect can be mitigated by applying the reverse charge shifts at the pixel level during image processing. Differences in stellar size, however, remain significant due to residuals at larger distance from the centroid.