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Fig. 3.

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Density plot of the pipeline-measured sky noise in the Hα filter as a function of sky level. Both are in units of counts per pixel. The coloured data shown, and plotted on a logarithmic density scale, are drawn from the IGAPS uniformly calibrated set. The solid black line is the fit discussed in the text, given as Eq. (4), that shows that the sky noise increases mainly as the square root of the sky level. The lighter grey data (underneath) are from images with zero points that were not passed through uniform photometric calibration.

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