Empty aperture noise measurements for the GTO NB118 data. The rectangles in panel a) show the regions used to analyse the empty aperture measurements. The detector(s) contributing data in the given region are listed, and parentheses indicate regions that have half the exposure time of the other regions. Panels b) and c) give the 2′′ diameter 5σ AB mag values for the NB118 TERAPIX stack (Sect. 3.4) and our NB118 test stack based on the CASU sky subtraction (which has about 0.1 mag lower noise, but which still may not be the best possible version of our data; Sect. 3.5), respectively. The aperture correction to total magnitude of 0.34 mag has not been subtracted.