Evidence for self-interaction of charge distribution in charge-coupled devices

TL;DR

This paper provides evidence that charge distributions in CCD sensors interact via Coulomb forces, causing a brighter-fatter effect that impacts flatfield statistics and star shape measurements, with implications for astronomical data accuracy.

Contribution

It introduces a physical model linking charge self-interaction to the brighter-fatter effect and validates it across multiple CCD types using flatfield correlations.

Findings

Charge distributions broaden linearly with brightness.

Flatfield variance grows less rapidly than Poisson expectations.

Neighboring pixel covariances increase with flux.

Abstract

Charge-coupled devices (CCDs) are widely used in astronomy to carry out a variety of measurements, such as for flux or shape of astrophysical objects. The data reduction procedures almost always assume that ther esponse of a given pixel to illumination is independent of the content of the neighboring pixels. We show evidence that this simple picture is not exact for several CCD sensors. Namely, we provide evidence that localized distributions of charges (resulting from star illumination or laboratory luminous spots) tend to broaden linearly with increasing brightness by up to a few percent over the whole dynamic range. We propose a physical explanation for this "brighter-fatter" effect, which implies that flatfields do not exactly follow Poisson statistics: the variance of flatfields grows less rapidly than their average, and neighboring pixels show covariances, which increase similarly to the square of the flatfield average. These covariances decay rapidly with pixel separation. We observe the expected departure from Poisson statistics of flatfields on CCD devices and show that the observed effects are compatible with Coulomb forces induced by stored charges that deflect forthcoming charges. We extract the strength of the deflections from the correlations of flatfield images and derive the evolution of star shapes with increasing flux. We show for three types of sensors that within statistical uncertainties,our proposed method properly bridges statistical properties of flatfields and the brighter-fatter effect.