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

Hi Don. I am happy you read the paper. Now we can further discuss. The point you raise are all interesting. I have a lot of comments here are a few:

First we are speaking of a very bright nebula. The brightest parts are at magnitude 14 (surface mag). I have a SQM and can assure that 14 is bright. False color percetions at that level are mistakes due to the intervention of rods, but cones are still operative (we are tetrachromatic at that point but we do not see a 4-dimensional color space because apparently rod signals enter the same channels of the cones).
Given the brightness of M42 you have the potential (even theoretical) to see real colors and also saturated colors (not rudimentary). How much saturated you may find in the companion paper.

The experiments were carried out with samples of uniform color which were seen under 8° apparent angle (this apparent angle is important... but again the authors know what they do). If there are regions of different colors compressed into a smaller apparent angle they cannot be recognized, even if the cones see the color. Thus it is not surprising that large scopes split areas of uniform colors in apparent patches large enough to be recognised (I still doub of H-alpha) but colors a 14 are seen provided the coloured areas are large enough (that is why your dark adaptation is disturbed, if the apparent area involve a large fraction of you FOV). If you look at a fractal image at distance (or a planet) you don't see vivid colors, but as soon as the details are resolved you see the color (provided they are large enough).

There are areas of M42 that shine at 17 and below. Here is were rudimentary real color perception ends. The remaining are artificial colors.
Little planetaries are very bright, and thus they are in the *real* mesopic range (for OIII). However the bright planetaries are bright because they are little, so even smaller ar the dim parts (the medium lights that should appear reddish).
The paper demonstrated reddish percepts for medium lights provided that they are large areas. Nothing is said about that effect for smaller areas. Maybe they are simply too small to be noticed (the ansae of M27 are instead large).

Another point you raise is the numerosity of the people who made tests. You are right when you say that the average response is identified but not the variance. However there are people who say seeing color at surface brightness of 22 (veil and M97, or in some regions of M42 that are at 22). From 17 (the average threshold) to 22 there a 5 magnitudes. A factor of 100. I know people that see stars half a magnitude dimmer than others, but nobody who sees 11.5 at the same place and time when the average see 6.5. So even if we do not know the variance (but we have hints it is not large because the thresholds were similar for the small sample of persons who made the tests) it is very unlikely that the threshold is 17+/-5 (what is needed to see reral colors in the faintest parts of M42).

Another point I did not notice si that the tests were carried out with broadband stimuli. Nebuale have line emission spectra. This might have some effect. However a pitfall is hiding here: our visual system uses three receptors in order to reduce as much as possible the phenomenon of metamerism (two different spectra and colors that produce the same stimuli and thus look like the same color). Three receptors are enough for smooth broadband spectra. Not enough for line spectra. L-cones are stimulated by OIII line. The mechanism of color constancy/compensation could produce apparent (metameric) red color.

... more later...

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