Panel a: example for a patch of an exposure which only shows background. The rectangle marks the area which is masked during the residual background fit, this is where the star would usually be located in a star image. This image was smoothed with a Gaussian filter to highlight the residual background structure. Panel b: “true” residual background when the principal components are fitted to the whole background image. Panel c: reconstructed background when the principal components are only fitted to the area outside the mask. Panel d: difference between (b) and (c). The colourbar in (a) is valid for (a), (b) and (c).