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L band aurora (Figure 1a from Wavelength Calibration). |
The aurora and sky survey images both showed many spectral features that were well defined, but had no reliable wavelength identifications. By utilizing the polynomial fit wavelength solutions, empirical wavelengths were assigned to these features. By combining the features with known wavelengths and the features with assigned effective wavelengths, the entire detector of each channel was covered with features of known wavelength. To ensure adequate statistics, the images were rebinned into 32 rows and centroids determined for each spectral feature in each row. A spline interpolation along each row yielded a wavelength map which assigned a wavelength for each pixel. In effect, each of the 32 rows had its own local wavelength solution. To assign a correct wavelength to a photon event, the wavelength map was simply subscripted (with sub-pixel interpolation) by its X,Y location. Detector distortions in the dispersion direction were directly eliminated. Integrated spectra of the S band show a spectral resolution (FWHM) of 1.37 Å. For the L band, no unblended features have been observed, but the line width is no more than 3.3 Å at FWHM.
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