Residuals of the two different calibrations for Ord 119. The red points are those obtained after dividing the CCD by 8 different chunks of 512 pixels along the dispersion direction and fitted with a polynomial of 3rd degree. The black squares are those obtained by treating the CCD as one made by pixels of equal size and same order polynomial. In the latter calibration the residuals are greater in correspondence of the CCD stitching borders.