Extended (black, linear scale) and point (red, log scale) source illumination along a detector row. For the point source, the telescope-plus-instrument PSF was split spatially by the integral field unit of the respective spectral channel. Different parts of the PSF were projected onto different parts of the detector in slices that do not neighbor one another on the sky. The numbering of the slices (slice number s) on the detector follows a similar alternating pattern for all channels to that shown for channel 4. The effect of light scattering inside the MRS detectors is clearly visible for the extended source in channels 1 and 2, where the signal between adjacent slices does not drop to zero. Although less visible in channel 2, the same effect is seen for the point source illumination.