The brighter-fatter effect and pixel correlations in CCD sensors

Abstract

We present evidence that spots imaged using astronomical CCDs do not exactly scale with flux: bright spots tend to be broader than faint ones, using the same illumination pattern. We measure that the linear size of spots or stars, of typical size 3 to 4 pixels FWHM, increase linearly with their flux by up to 2 % over the full CCD dynamic range. This brighter-fatter effect affects both deep-depleted and thinned CCD sensors. We propose that this effect is a direct consequence of the distortions of the drift electric field sourced by charges accumulated within the CCD during the exposure and experienced by forthcoming light-induced charges in the same exposure. The pixel boundaries then become slightly dynamical: overfilled pixels become increasingly smaller than their neighbors, so that bright star sizes, measured in number of pixels, appear larger than those of faint stars. This interpretation of the brighter-fatter effect implies that pixels in flat-fields should exhibit statistical correlations, sourced by Poisson fluctuations, that we indeed directly detect. We propose to use the measured correlations in flat-fields to derive how pixel boundaries shift under the influence of a given charge pattern, which allows us in turn to predict how star shapes evolve with flux. We show that, within the precision of our tests, we are able to quantitatively relate the correlations of flat-field pixels and the broadening of stars with flux. This physical model of the brighter-fatter effect also explains the commonly observed phenomenon that the spatial variance of CCD flat-fields increases less rapidly than their average.

The first-year shear catalog of the Subaru Hyper Suprime-Cam Subaru Strategic Program Survey

“…We also apply a correction to account for the brighter-fatter effect (Antilogus et al 2014;Gruen et al 2015). Charge that enters the detector is deflected by a lateral electric field due to accumulated charge in the pixel.…”

Postirradiation behavior of p-channel charge-coupled devices irradiated at 153 K

“…The cause of this effect is likely to do with the preferential movement of charge towards a region of increased potential resulting in the redistribution of charge and the formation of light and dark bands at the edge of the irradiated region, a cartoon of the process is shown in Figure 12. The process is the same as that which occurs during point spread function measurements commonly referred to as the brighter-fatter effect 28 , however in this case the description of charge redistribution 29 is more appropriate.…”

Measuring a charge-coupled device point spread function

“…Antilogus et al 2014;Rasmussen 2014, and references therein). This effect has been dubbed as "brighter-fatter" and is often identified from pairwise flat field data showing that a photon transfer curve deviates from the shot noise prediction.…”