I’ve continued to follow the evolution of the spectrum of Betelgeuse, taking advantage of this quite incredible run of clear weather. My last observation was on Thursday evening with Betelgeuse at an altitude of about 30deg as it just about got dark enough – I think this could be my last reliable observation before it disapppears into twilight.
The deep fade in January corresponded with a significant increase in relative brightness in the near infrared which is now declining back to the state in my spectrum of 2016 Mar 5 when the V mag was also near the current ~0.5. To ensure reliable response correction in the shape of the spectra I captured the nearby B5V star HD 36267 from Francois Teyssier’s Reference Star Finder; it has a low E(B-V) extinction of 0.03 and so the B5-7V Pickles reference spectrum can be used to give an accurately response corrected Betelgeuse spectrum right out to 7740Å. To avoid any scintillation effects I defocused the image of Betelgeuse on the spectroscope slit to allow 30sec exposures without oversaturating the spectrum image in the camera. It’s been an interesting episode for spectroscopy.
Hugh
P.S. I’ve also added the spectra comparison as an attachment for legibility