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

Real color perception variance is probably similar to visual sensitivity to stimuli, which, in my experience, covers about a magnitude and is partially due to differences in visual acuity. As is the case with stellar images, defocusing often makes colors more apparent. Since doing so spreads the light out, reducing unit intensity, size appears to be a bigger factor than intensity.
As an aside, the small % of women (only women) who have tetrachromatic vision (4 different color receptors) would see colors differently than the rest of us, but, since they use the same language, we'd never know how.

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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).

Therein lies a problem. I would rather the test be done with monochromatic light (say H-Alpha) in which the intensity is reduced to the point where no color perception is possible. I would guess different individuals would lose color perception at different luminosities. Judging from the studies on perception of grayscale brightness differences, it is likely that the differences between individuals could easily exceed 5 magnitudes. That no two individuals see a five-magnitude difference through the scope does not invalidate the point that there could be a 5 magnitude difference in color perception. The cited studies do not address this issue, but do discuss false perception of color. There would be a fine line between real and false at some brightness levels. It is quite possible that some individuals would be seeing "real" color when others are seeing "false". If they see the same color, how could you tell? A "threshold" test might help.

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There are areas of M42 that shine at 17 and below. Here is were rudimentary real color perception ends. The remaining are artificial colors.

That statement is implied by the studies, but not verified to my satisfaction. Some individuals see color in the zodiacal light and even Gegenschein, yet these are both fainter than the mag.17 figure. What is interesting is that the colors seen are verified by photometry. Could it bee that the threshold of true color perception varies a lot among humans?

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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 to study. Small objects should also be subject to false color perceptions. Are they? Unknown.

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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.

Stellar visibility and extended object visibility are not the same thing because of the radically different sized areas of the retina engaged.

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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).

This is an assumption based on a small "N" and under conditions in which full dark adaptation might not have been applied. We know that rods grow in sensitivity by a factor of 90,000 times. We also know that cones turn off below a certain intensity of light. What we don't know (I've read hundreds of studies) is the response of the cones under low light conditions (except for the blue shift) and under conditions of maximal dark adaptation. I know that dark adaptation varies HUGELY in the human population--way more than 5 magnitudes--but we do not know about variances in the perception of different intensities of red (see my above comments). The cited studies only show what happens below the threshold for broadband-illuminated, large-scale, areas.

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Another point I did not notice is 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.

Which would explain why a lot of the double star viewers in the past saw such vivid contrasting colors. Bear in mind that the perception of color in stars varies hugely with some individuals seeing more red than others.
I would like to see a red-light stimulus that is gradually raised in intensity from a very low level (say mag.24-25) which checks first where the stimulus is first seen (a test of dark adaptation and rod sensitivity) to the point where color is first perceived. Then, the identical test with a green light. If the cited studies are valid, the faint green light would first be seen as red and then turn greenish at some light intensity.
Starting out below the threshold of visibility would be critical in determining the variance in the visible threshold for light in general, and also for specific colors. If the blue shift holds, the threshold for blue light should be much fainter.
It would be an incredibly hard test to administer, since staring at a computer screen would ruin dark adaptation. The subjects would first have to sit in nearly-complete darkness for 45 minutes, and then look at a black screen on which a small square was projected. Looking at the light source or a computer screen would immediately invalidate the results. Because of huge variation, the projected images would have to start quite dim, say below 26th magnitude per square arc second.
This is similar to the hearing tests in which I participated. Not only the range was large, but the minimum threshold varied by a factor of over 1000 among our subjects. We even had some individuals who could hear the pulse of each heartbeat as the blood moved through the ear's blood vessels.
We have no reason to believe the threshold of perception in vision does not have a wide range. The cited studies purport to measure the perception of color at low intensities. I would immediately suspect results that show a 1-magnitude range in visual perception of color. Why? Because I routinely see large variations in the ability to see at all: acuity, faint images, colors in stars, limiting magnitude (which we know varies more than 4 magnitudes--see Schaefer).
Further studies are needed. That doesn't invalidate the cited studies and they are interesting.

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Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie

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