Telescope Reviews: Is it possible to see the nebula with colour?

All your points are mentioned and cared in the "experimental procedure". Studies on dim ligth perception are of paramount importance in a large number of fileds (from military, road safety etc.). The authors do that professionally and there is a vast community who did similar studies (among which the peer reviewers of the paper). But there is a simple check... go to the library again and get that paper too and read "with your own eyes" how the experiments were carried out (BTW that is helpful to understand that person to person variation si not that large as long as people with normal sight are of concerns . BTW BTW all test people passed standard tests on visual acuity, color blindness (including minor blindeness which usually goes undetected).

Don... I think you are doing the devil's advocate too much... (although it is correct to question every detail).

So let me try to summarize:

1) State of the art knowledge (the beliefs) says there is mesopic and scotopic vision. In mesopic range there is real color perception which becomes rudimentary at the limit with scotopic. In scotopic there are illusion color perceptions that resembles the rudimentary perception in mesopic levels (and thus are very difficult to distinguish from real perception- on the other hands illusions arise from the fact that brain extract color information from luminance levels , which often corresponds to warm and cold colors).
2) Reported experience conflicts with knowledge only if we pretend that real colors are percieved below the threshold of cones. So far the common figure for this threshold stands between 17 and 18 (the latter meaning the ability to see the color of the sky in a full moon night).

Open points are:
a) which is the exact threshold and how much does it vary from person to person and from color to color.
b1) are there nebulae (or parts) that shine in the mesopic range for OIII?
b2) are there nebulae (or parts) that shine in the mesopic range for H-aplha?

As for point a) there are these elements: - studies hint little variation of thresholds between the test persons, (the studies were not carried out on large numbers of test persons but sufficient to give a hint of the standard deviation (all testers had normal sight according to state of the art) - a similar attitude like limiting magnitude varies of 0.2-0.4 among people. I know you report more but these are my numbers and also thos reported by Brian Skiff http://www.astropix.com/HTML/L_STORY/SKYBRITE.HTM :

"Is it possible for someone with extremely unusual vision, like Steve O'Meara, to see the color blue in the night sky background?

Nope. Steve and I (and John Bortle) once made mag limit checks at the Texas Star Party using a photometric sequence around M3 in a 7-inch Starfire refractor. All three of us saw to the same limit---none of us has unusual vision, just a lot of experience. Steve's vision is unusual only in that he knows how to use it!"

As for point b1) there are few nebuale that shines above the threshold for OIII, Not many (the same M27 is 18.4 peak, M97 is 21.6).

As for point b2) one has to consider that eye sensitivity to H-alpha is about 2.5 magnitudes less than to OIII. Thus the point is: are there parts so bright in H-alpha to stimulate L-cones? 2.5 mags brighter than OIII?). The most correct test here is the H-aplha filter. You have only to check if you see something or otherwise. The OIII with red leak filter demands not to check is something is seen but wether something "red" is "really" seen (this short-circuit and inavalidate the test). With H-alpha filter you use rods/cones to detect wheter something is seen, that knowoing that only H-alpha is passed you knowk it has been detected.
You said something is seen though the H-alpha in M42. If this happens it does not yet mean that red can be seen because the perception of red (at cognitive level) depends on the ratio between stimulation of rods to L-cones. If the same region emits a lot in OIII (which might be he case of the huygenian region) then the color is percived as green (the real color) despite the L-cones sees some H-alpha (but not strong enough to prevail). There are regions of M42 where H-alpha prevails but they are at surface magnitude 17+ (actually in V-band), which with the handicap of 2.5 mags less sensitivity of the eye are very unlikely to reach L-cones thresholds. So You shoold look wheter you see something with H-alpha filter in those regions.