Schematic view of ‘ideal’ monochromatic images of a slit projected onto a detector by a non-Littrow spectrometer. Black squares represent spectrometer pixels. The dashed line traces the centre of a spectral order and the blue boxes show the idealised footprints of slit images. We note that by design the centre of the spectral order and the sides of a slit image intersect at the pixel column boundaries of the detector.