Panel a): an intensity image of NOAA 12049 recorded with GRIS close to disk center. White and gray contours outline the umbra and penumbra. Black contours encircle regions that are not taken into account in the calculation of an average QS profile. The white arrow points to the center of the solar disk. In panel b) we compare the average QS spectrum (black) with the FTS atlas (red). The regions shaded in gray are used in the analysis of the residual fringe pattern, see Sect. 3. Panel c): intensity variation in the continuum of the GRIS spectrum (black dots) and a polynomial fit (red) used for correction.
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