Placement of the PN on the NISP focal plane of 16 detectors, for the 4° rotated position of one of the red grisms, in the NISP R_MOSAIC detector coordinate system. The dots show the positions of the 0th order, and the lines the location of the 1st order, colour-coded for easier association. The triangles mark the short-wavelength end of the 1st orders. The pattern is compressed for the top detector row, because we must measure both the 0th and 1st orders simultaneously to determine the dispersion laws. In the absence of the grism, the images of the PN would appear within the first order. Since the dispersion laws vary slowly, this pattern compression does not bias the result.