Thanks for your feedback Andrew, and Robin as well. By normalising the relative flux near the centre of the V-band filter, I wondered if the changes in the relative flux in the near IR could then be monitored in a useful way compared to changes in the observed V magnitude (essentially a way of quantifying changes in the shape of the spectrum as the visual magnitude varies). But I can see some of the dangers, not least the initial process of corrrecting the shape of the spectrum for atmospheric effects. In the BAA database, Robin has a number of Alpy 600 spectra of Betelgeuse on which I made the same analysis:
There is an offset between our two methods of atmospheric correction. However the difference should just be a constant factor which can be best estimated from our almost simultaneous observations on JD 2458867.4 (2020 Jan 18), marked as point 1 in the adjusted chart below which shows all of the points from Robin’s data adjusted by the same factor. Once adjusted both our data follows a consistent trend. Point 2 is very interesting. It was my last observation of Betelgeuse on JD 2458954.3 (2020 Apr 14) before it disappeared into twilight and shows a significant increase in the relative intensity in the near IR even though the V mag was also continuing to increase:
