Schematic presentation of the subgrid sampling of the slit illumination function L and the corresponding structure of the ω tensor. On the top is an example of a slit illumination function aligned for column x. L is set on a subpixel scale v (shown at the bottom panel) that may shift relative to the detector pixels from one column to the next. For three pixels in this column centred at row y we show the structure of the corresponding section of . The fraction of each subpixel filled with blue is proportional to the value of the corresponding element of ω.