Hi Dongdong, hi all!
It looks like Betelgeuse has reached the plateau (but we'll see), so I decided to catch an evidence of the historic minimum too. Spectroscopic observations were carried out on 23/01/2020 with Lhires III and 600 l/mm grating. The profiles cover the wavelength range from about 4200A to 9400A in six spectral intervals picked up in sequence. Spectra above 7000A are obtained with a blocking filter (Wratten 25) in the optical path to avoid the 2nd order contamination.
The combined profile and an attempt to remove the atmospheric absorption bands in the interval from 620 to 940 nm. I derived the bands profile from the spectrum of Bellatrix (reference, B2V) observed with the same setup while it was at similar height on the horizon (the same night).
Comparison with alf Ori Miles spectrum (V mag in the range ~0.4-0.6 in 2000-2001 years). Differences are obvious but it should understand if the the response curve calculation, using Bellatrix spectrum and a couple of Pickles library profiles, is precise enough (I have a little experience with NIR spectra).
Finally, a visual comparison. In 2014 I took a picture of Orion constellation with a DSLR out of focus to emphasise the color of the stars. From AAVSO light curve, Betelgeuse magnitude was about V 0.5 at the time. On january 23th 2020 I repeated the photo shoot. The comparison shows quite well the extreme dimming of the red supergiant.
Paolo
