Optimally-extracted S/N spectrum (top panel) and measured FWHM (bottom panel) in echelle order 10 (UVB arm) for HD 38237. The downward S/N spike at 4938 Å arises from a cold spot on the CCD and demonstrates the quality of our optimal extraction procedure. The FWHM shows variations on a five percent level with a minimum close to the blaze wavelength.