Top panel: rather extreme example of the ripples, with a period of 150 spectral pixels or about 0.03 μm, that can be seen in the spectra of individual spatial pixels if the seeing is so good that the star is spatially undersampled. Middle panel: spatially integrated spectrum extracted from a different IFU which does not show the ripple effect. Bottom panel: ratio of the 2 spectra to show the ripples more clearly. This example is for cubic spline interpolation in 0.35′′ seeing (as measured in the H-band on the star itself). For nearest neighbour reconstruction, the effect is more severe and can be manifested as saw-tooth discontinuities. In either case, it can only be avoided by better sampling, which is one of the things provided by the multi-reconstruct recipe (described in Sect. 4.2).
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