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Fig. 6


Representative flat fields for Gr#11, 13 and 16, taken in August 2013. Each panel shows a cut along column 450 of the two-dimensional flat-field, corresponding to the approximate pixel coordinates of the target. The black lines show the intrinsic shape of the flat fields, while the red lines show the normalised flat field after fitting with a high-order polynomial (all spectra are normalised to 1, and offset for clarity). For Gr#13, the flat field with the GG495 order blocking filter (as used when correcting for second-order contamination) is also shown. For Gr#16, both internal flats and dome flats are shown; in the former the absorptions due to H2O vapour are visible in the un-normalised flat. This absorption feature is fit by the high-order polynomial during normalisation and hence is removed in the normalised flat.

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