def saturation_sat_at_xy1024(x1024, y1024, vignette_at_1024, adu_saturation):
    """ Return estimated saturation ADU limit from aperture's distances from image center."""
    dist2 = x1024 ** 2 + y1024 ** 2
    fraction_decreased = vignette_at_1024 * (dist2 / 1024) ** 2
    return adu_saturation * (1.0 - fraction_decreased)