Quote: Only if surface brightness is above L-cones threshold (say 17 mpsas) you have a chance to see real colors. Otherwise there are solid scientific studies that show that color perceptions at scotopic levels (say 20) happens but are illusions. But, even if the surface brightness in V band is above the typical figure of 17-18, that does not necessarily mean that the red fraction (H-alpha emission) is strong enough to stimulate L.cones. L-cones are weakly sensitive to H-alpha. Thus it might be that reddish percepition on nebulae that are on "average" above the theoretical limit for cones is still an illusiuon.
A proof might be to look at the nebula with a H-alpha filter. If you cannot see it then your L-cones are not stimulated by the H-aplha emission (rods do not see H.alha at all). You only detect OIII and other lines with rods.
Well, with all due respect to the so-called "scientific" studies, they often do not directly or extensively deal with the exact observation conditions which we as experienced amateur astronomers work with. The studies may be somewhat valid under some conditions but probably not under others. I would have to see any study use specific astronomical targets, instruments, filters, and a large number of experienced observers before I would take its findings as being completely valid.
One particular incident convinced me of the possiblity of seeing reddish colors in some nebulae. Many years ago, Lumicon began producing their first line of Oxygen III filters. The first production run had a single primary passband for the OIII lines in the blue-green part of the spectrum only, but later on during another production run, a red secondary passband was introduced (either accidentally or deliberately). A friend of mine had one of the early OIII's while I got one in the second batch with the "red leak" passband which let through a lot of red light (including the H-alpha line, as I could verify using a spectroscope). We were both observing M42 one evening with our two ten inch Newtonians when I was somewhat startled to see some faint reds in certain parts of the nebula. I had never seen reds in nebulae before in *any* telescope or when using my narrowband filter, but I hadn't tried my OIII out on M42 at the time, as I had only recently purchased it. We could not see any red hues in the nebula using my friend's 10 inch, but in mine, they were faint but definitely present. We then compared our two OIII filters and noted that distant red radio tower beacon lights were visible through my OIII but not in his, so the reason behind the difference in the view was revealed. At the time, Jack Marling was president of Lumicon and we wrote him to tell him what we had uncovered. He wanted the filters back for detailed measurement, so we sent them to him, along with the "recommendation" that the red secondary passband be left in. I had also noted extensive but very faint reddish hues in M8 when I used my OIII. My H-Beta filter also has a red secondary passband, and with it, M42 is not as large as in the OIII filter, but what is visible again shows at least some faint red hues in the outer portions with only the innermost Huygenian region showing much in the way of the bluish color from the H-Beta line. I recently ordered a new 2" Lumicon OIII filter, and the red secondary passband is no longer present, so I don't see reds with it when observing bright nebulae. However, the DGM Optics NPB filter *does* have a nice high-transmission red passband for H-alpha, and with that filter, I once again can see faint reds in some nebulae provided I am using enough aperture and don't kick the power too high.
Some people do have enough red sensitivity to see faint reds while many others don't. I am sort of "in the middle", as I can see reddish hues in some cases but not others. I can, for example, just make out the faint reddish color of Mu Cephei with my unaided eye when it is around 4th magnitude, while some others may not see the color at all. In particular, some children sometimes have a *lot* more red sensitivity. I recall at a dark sky star party when we had an 8 or 9 year old youngster at a 12.5 inch Newtonian looking at M20, and without being prompted or told what he would see, he immediately mentioned the faint reddish color of the primary nebula. Thus, to just dismiss all claims of seeing reddish colors in some nebulae in moderate to large apertures is not only unfair, it runs counter to what repeated observations by some very experienced amateurs have observed. Clear skies to you.
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