Because we found some apparent anomalies in the DR16 data, Anna-Christina Eilers (MIT) and I are looking at the noise properties of some of the SDSS-IV APOGEE stellar spectra. It's pretty inside baseball but we are comparing the MAD (my favorite statistic—the median absolute difference in brightness between neighboring spectral pixels) to the reported spectral signal-to-noise ratio and stellar parameters. We find that, as expected, MAD (on a normalized spectrum) decreases with SNR, but increases with metallicity and decreases with temperature (because more metal-rich and cooler stars have stronger lines). But there are outliers, where the SNR measurements appear completely wrong. We're following up now, but first: How to sensitively identify outliers in this space? It won't surprise my loyal reader that we tentatively decided to use nearest neighbors with a kd-tree.
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