Example of the effects of time varying sky background in two different datasets of WFC3 IR in the F105W band. Left panel: median background flux of ib5x15vrq (yellow circles) and ibp338xzq (red triangles). While the dataset ib5x15vrq shows a nearly constant flux on all the different readouts (∼0.50 counts s⁻¹), ibp338xzq is clearly affected by time variation of the sky background, increasing from ∼0.50 counts s⁻¹ to ∼1.3 counts s⁻¹ after 400 s from the start of the exposure. Middle panel: sky-corrected ibp338xzqflt.fits image without time-varying sky background correction applied. Right panel: sky-corrected ibp338xzqflt.fits image accounting for the time-varying sky background correction. Both images are set to the same colour scale.