Instrument calibrations, C(λ), for 23 observations of GD71 from the same highly non-photometric night (top panel). Relative error of each instrument calibration with respect to the mean value (bottom panel). The mean rms over the whole wavelength range is    0.007 and the features are due to spectral resolution issues between the spectra and their reference. Poorer accuracy due the water telluric band in the region redward 8900 Å is noticeable.