PSF fitting test with a variable PSF and a constant background. Upper left: point-source subtracted image after use of a single, constant PSF. The circle in the lower right shows a region in which we measured the sum of the squared residual, which is 70.6σ2 for this region. Upper right: like upper left, but after using a variable PSF. The sum of the squared residual in the circle is 5.6σ2. The grey scales are expressed in terms of σ deviations from the noise image. Bottom left: differences between the measured magnitudes of stars and their input magnitudes when a single, constant PSF is used. Bottom centre: differences between the measured magnitudes of stars and their input magnitudes when a variable PSF is used. Bottom right: plot of background, after point-source subtraction, as a function of distance from Sgr A*. The background is the median in rings around Sgr A*. The blue data are for the case of a single PSF and the black data for the case of a variable PSF.
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