One divides all images with a flat field which has been normalised to unity. A more detailed discussion on how the flat field can be created is given in Section . This will result in photometry that is consistent to the 2% level over the ISAAC field of view. If more accurate photometry is required, then an illumination correction should be applied.
If one was to look at the flat fielded images, one would see that they are far from flat. One would see a jump between the two halves of the array, and one would perhaps see structure at intermediate (5-10 pixels) and large (several hundred pixels) scales. These structures have a variety of causes. The jump in the middle is caused by the fact that we have not removed the zero level offset perfectly. The structures at intermediate scales are probably caused by pupil ghosts and by dust that has moved. The structures at large scales are probably caused by scattered light. Most of these features are additive, so they are removed at the sky subtraction stage.