Normalized monochromatic laser flat-fields in logarithmic scale. The dispersion direction is from bottom to top. Left: standard wavelength calibration image for the extended (YH) mode using all four lasers simultaneously. This is provided by ESO as part of the standard calibration sequence and is used by the ESO pipeline. Right: similar image, but with only the 1123.7 nm laser on. Images at this and three other wavelengths were taken and used to construct our lenslet PSF library (Sect. 3.4).