Quote: This is a generic statement. I invite you to read the papers I pointed out and then say what is wrong with the procedure to the point that conclusions are false.
A generic statement does not make it necessarily false. However, as others have indicated, under certain conditions and with enough aperture, the light levels found some areas of M42 are high enough to activate the cones and allow colors to be perceived. This is not routinely done (and the colors seen are dim and somewhat pastel rather than vivid), but they are seen.
Quote: This is very interesting. I did not discard the possibility to see red, I said it is more difficult that seeing green (perhaps there are a very few cases). Your case suggests it may possible to see red in Orion Nebula, but there may be other possible explanations
I prefer using Occam's razor and accept the idea that under certain conditions, some of the brighter nebulae are bright enough so that those with sufficient red sensitivity will see at least some reddish hues. The wording you used in the initial statement implied that you doubted any red would be visible at all ever, and that was the problem.
Quote: I looked at the Orion Nebula with a red filter: The trapezium is red (yet I am not convinced I see the real red cominga from H-alpha, for the reason above). NGC 7662 is not seen at all in red. The real test I plan to do is look with a H-alpha filter. If the object is not seen then...
The Trapezium is a group of stars and not nebulosity. A red filter should show them as red. There is also some reflection nebulosity intermixed with the emission nebula in M42, so scattered red continuum light from the stars may also play a slight role (along with H-alpha). The area immediately around the Trapezium (the "Huygenian" region) is somewhat bluish or bluish-green to me, with slight variations in saturation and hue. I have to look much farther away to note the pinkish or faint reddish coloration. One area which shows it to some degree is the southeastern "wing" of the nebula. There is a strip or filament just off the southwest edge of the southeastern wing which is sometimes seen as reddish. The other areas are again subtle pinkish tones which appear in vague patches somewhat outside the Huygenian region but not all the way out to the outermost edges. In scopes larger than 10 inches, on a good night, some subtle pinkish tones are visible, although again, as I pointed out, not everyone will be able to see them. Most people can see the bluish-green core of M42 even in some fairly modest apertures, but often much beyond that, there is usually little distinct coloration visible unless the aperture is considerable.
As for NGC 7662, that object is mostly an OIII emitter, and I don't see any red in it, at least in my 10 inch. As I indicated, the planetary nebulae which tend to show at least a little red are IC 418 and Campbell's Hydrogen Star. Most of the rest tend to have either a bluish-green color or no prominent color at all.
Quote: SO the other hand, however, the sensitivity on cone cells to light is muche less likely to vary from people to peole.
There is little to suggest that the sensitivity does *not* vary and more evidence to suggest that it does vary from person to person. During my days as a Physics student, we were working with spectrometers and a friend of mine was seeing emission lines that extended into the near-infrared (probably close to 7200 Angstroms)! I can't duplicate this (and it is impossible for him to describe the "colors" he sees), but he has seen red when I could not.
While a few people may be mislead into thinking they are seeing color at low light levels when they are not, when two very prominent and experienced amateurs who are well-known (at least in the U.S.) say they saw pinks or reds in M42, I would tend to believe them. Clear skies to you.
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Re: Is it possible to see the nebula with colour?
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