The image below shows a measurement of the cross-correlation between DB white dwarfs taken from the SDSS White Dwarf Catalog and far-ultraviolet photons observed by GALEX.
The central source in the image is effectively the average (mean) image of a DB white dwarf in GALEX; its morphology is (roughly) consistent with a GALEX point-spread function. The other sources in the image are residuals created by random bright FUV sources nearby to individual DBs; there weren't enough DBs to make this all average away; none of the nearby sources seen in the image are significant if you ratio this image to the root-variance image obtained by jackknife resampling.
Schiminovich and I spent a long time today discussing the possible effects that might make the same kind of average image not look like a point-spread function when the central sources are at cosmological distances, because of scattering and correlated sources. We also discussed how measurements of the average image as a function of sky position and redshift could constrain transparency.
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