Well, tonight, I ran into my friend with the extended red sensitivity. It turns out that from his old physics undergraduate lab notes, he had actually been also using a Xenon emission lamp. With the spectrometer he was doing the lab with, he had been seeing way out to the 7548 angstrom, 7642 angstrom, and 7671 angstrom lines. Needless to say, his red sensitivity is somewhat better than most (and better than mine). With his red sensitivity, I decided to sort of "blind" test him on M42 using his 8 inch f/5 Newtonian from my home location (ZLM 5.5). M42 was near the meridian, so there would be little atmospheric reddening and as much transparency as possible.
Without filters, the skyglow made it difficult to see much color other than a faint sky bluish with hints of greenish colorations in the Huygenian region. I put in the DGM Optics NPB filter and he immediately commented that he saw faint red at both 32x and 51x. The central core was somewhat bluish with maybe just a hint of green, but any green color was somewhat subdued to him. The reds he reported were somewhat farther out. I put in my OIII filter (the one *without* the red passband), and he said that he no longer saw any reds. I also tried my new Lumicon UHC (which also has no red passband) and he again reported no red. I tried my new 2" Lumicon H-Beta (which also has high transmission in the red), and he said the reds were back and a little more visible, although the nebula was somewhat smaller than with the NPB filter. We increased the power, and at or beyond 101x, he said the reds were somewhat marginal, although he still loved the almost 3D effect the NPB filter gave him. I had a look and I could also see faint reddish coloration with the NPB and H-Beta filters in M42, confined mostly to the areas a bit away from the really bright central region and at powers of less than 101x.
With these results, I have little doubt that the red he saw was indeed real and not just some illusion. Clear skies to you.
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