Wavelength zero-point offset for a point-source as a function of wavelength and target displacement from the micro-shutter centre in the dispersion direction. For extreme offsets, the core of the point-spread function progressively moves out of the shutter opening and the position of the photocentre becomes sensitive to secondary features like Airy rings. This explains the non-monotonic behaviour of the wavelength zero-point offset at short wavelengths and for large source offsets.