Example of a single channel of an Apertif H I emission cube, prior to noise normalisation. The colour scale represents the noise balanced on 0, which is non-uniform and increases towards the edges of the cube. The layout of the Apertif compound beams mosaicked into a hexagonal shape is overlaid in white, following van Cappellen et al. (2022). Note that the cubes were cropped spatially, here indicated by the yellow rectangle, resulting in 1000 × 1600 pixels in dimension, representing the declination and right ascension, respectively.