Detection of gain instability in an OTF scan using a rolling standard deviation over the integrated intensity of an OTF scan. Left panel: integrated intensity variation over a 25 s OTF scan with 84 spectra. The variation for LFAV6 is stronger than that in LFAH6. Center panel: rolling standing deviation over five samples. A rolling standard deviation removes the sky signal variation during the OTF scan and leaves just the OTF sample to sample variations. Right panel: distribution of rolling standard deviations. The unstable pixel, LFAV6, shows a larger mean standard deviation than pixel LFAH6. This property can be used to detect gain instability in a given pixel.