Illustration of how CAAPR determines the aperture noise in map, by placement of randomly positioned sky apertures, for our GALEX NUV map of NGC 5147 (full 0.5° × 0.5° cutout shown). Thick dashed green ellipse around NGC 5147 is master aperture, within which random sky apertures may not be placed. Thin pink ellipses show all of the candidate randomly positioned sky apertures generated by CAAPR; as can be seen, they are biased towards being located closer to the target source, with distribution thinning out at greater distances as per a Gaussian distribution. Thick blue ellipses are accepted random sky apertures used to calculate aperture noise.