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H. P. Jones (NASA/GSFC)
A key problem for the analysis of the spectral-spatial datacubes produced by the many operational and planned solar spectroscopic imagers (e.g., SUMER and CDS on SOHO; the SOLIS Vector Spectromagnetograph (VSM)) is how to obtain a suitable "flat-field" image for fixed-pattern correction of each slice (long-slit spectrum) of the data. We describe here some algorithms developed for flat-fielding NASA/NSO Spectromagnetograph (SPM) and NSO Near-Infrared Magnograph (NIM 1) data both for analysis of He I 1083 nm data and as prototype procedures for the future VSM. The 1083 nm line is a severe test since the line is very weak, and similarly stringent requirements need to be met for precision spectral polarimetry from the VSM. The procedures remove spectrum lines from ``raw'' flat-fields derived by integrating spectra as the solar image is scanned to give an equivalent exposure to each spatial element along the spectrograph slit with the exact instrument configuration that is used in the observations. We find this preferable to other techniques such as moving the grating to nearby continuum or image defocussing, both of which change fringe patterns and/or the way optical impurities such as dust are imaged. An iterative fitting procedure is described which works well on the SPM data but is less satisfactory for NIM 1 because, in the latter case, the detector readout introduces a columnar pattern parallel to the spectrum lines which is removed by the SPM algorithm. The orientation of read-out and spectrum will be the same as NIM-1 for the VSM. We describe initial attempts to use a new least-squares spline technique (Thijsse, Hollanders, and Hendrikse, 1998, Computers in Physics 12, 393) to address this difficulty.
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