Intensity, photocenter, and FWHM spectra of GU CMa from top to bottom for the parallel (left) and perpendicular (right) slit directions. The left panels plot the individual spectra for the two parallel positions and the averaged spectra (see legend). The continuum-corrected photocenter shift is indicated with the black dashed line in the middle left panel. The right panels show the averaged spectra for the perpendicular orientation (PA = 103° and 283°) with the original spectral resolution (dotted lines) and after rebinning (solid lines). The corresponding ±1σ levels are shown in the middle and bottom right panels with the horizontal normal and bold dashed lines, indicating ±2.1, ±1.1, and ±4.8, ±2.2 mas for the photocenter and FWHM accuracies.