It will be another week before I am at a dark site to test the H-Alpha view of M42. However, I have invenstigated the spectrum of M42, and it has more energy at that frequency than at any other. The second strongest emission is at 500.7nm (O-III). All other emissions are too low a level to have any relevance in this discussion. I will also tell you what the SQM reading is during the observation, as this may be relevant for dark adaptation. I will test first with O-III having zero red transmission and report the colors I see, then with H-Alpha to tell you what of the nebula is visible. I suspect the central Huygenian region to be faintly visible. Other than that, it remains to be seen. However, I would point out one caveat: a typical H-alpha filter transmits about 90% at that frequency. That ten percent could represent a threshold brightness factor that cannot be eliminated, so NOT seeing a part of the nebula with the Ha filter doesn't PROVE that a reddish color is not sensed WITHOUT the filter in place. However, since the Huygenian region is so bright, I suspect there is reduced rod sensitivity when viewing the nebula, and increased cone sensitivity. Unfortunately, my test will not tell me much about the threshold of red sensitivity, but it will allow me to see if illusory colors are visible. If I still see faint reds and pinks with the contemporary O-III filter in place, that will prove to me that the colors are illusory. This will be fun, regardless of what it doesn't and does prove. I typically observe with others, so I'll see if others would be interested in participating. Since exit pupil would have an effect, I will first stick to a large exit pupil (over 5mm) and then go smaller to see if certain aspects disappear at higher magnifications (perhaps crossing a threshold in surface brightness?).
-------------------- Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
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Re: Is it possible to see the nebula with colour?
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