Brighter-fatter Effect Research Articles

Measurements and simulations of the brighter-fatter effect in CCD sensors

Reduction of images and science analysis from ground-based telescopes such as the LSST requires detailed knowledge of the PSF of the image, which includes components attributable to the instrument as well as components attributable to the atmosphere. Because the atmospheric component is constantly changing, the PSF is typically extracted from each image by measuring the size and shape of star images across the CCD, then building a fitting function over the focal plane which is used to model the PSF for analysis of extended sources such as galaxies. Since the stars in each CCD field have a range of brightnesses, accurate knowledge of the PSF for point sources of varying brightness is essential. It has been found that in thick, fully-depleted CCDs, the electrostatic repulsion of charge stored in the collecting wells gives rise to a larger and slightly more elliptical PSF for brighter stars. This ``brighter-fatter'' effect has been reported in some detail in the literature. In this work, we report direct and indirect measurements of this effect in prototype LSST sensors, and describe a detailed physics-based model of the electrostatics and charge transport within the CCD.

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

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 the response 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.

The brighter-fatter effect and pixel correlations in CCD sensors

We present evidence that spots imaged usingastronomical CCDs do not exactly scale with flux: bright spots tend tobe broader than faint ones, using the same illumination pattern. Wemeasure that the linear size of spots or stars, of typical size 3 to 4pixels FWHM, increase linearly with their flux by up to 2 % over thefull CCD dynamic range. This brighter-fatter effect affects bothdeep-depleted and thinned CCD sensors. We propose that this effect isa direct consequence of the distortions of the drift electric fieldsourced by charges accumulated within the CCD during the exposure andexperienced by forthcoming light-induced charges in the same exposure.The pixel boundaries then become slightly dynamical: overfilled pixelsbecome increasingly smaller than their neighbors, so that bright starsizes, measured in number of pixels, appear larger than those of faintstars. This interpretation of the brighter-fatter effect implies thatpixels in flat-fields should exhibit statistical correlations, sourcedby Poisson fluctuations, that we indeed directly detect. We propose touse the measured correlations in flat-fields to derive how pixelboundaries shift under the influence of a given charge pattern, whichallows us in turn to predict how star shapes evolve with flux. We showthat, within the precision of our tests, we are able to quantitativelyrelate the correlations of flat-field pixels and the broadening ofstars with flux. This physical model of the brighter-fatter effectalso explains the commonly observed phenomenon that the spatial varianceof CCD flat-fields increases less rapidly than their average.