Signal-to-noise ratio of individual pixels obtained from the coadded ‘cal’ images for NGC 7027 (blue) and Cat’s Eye (red). The circles are the median S/N over each band. The S/N required to reach a fringe peak amplitude below 1% on individual spaxels for extended sources after fringe flat calibration is shown in black; this required S/N is that of the data that would be used to build the fringe flats.