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I've been around way too long to know or not to know what I'm seeing.

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The pinkish color I reported last year in my 8" refractor on a superb night was seen not only by me, but other observing partners who are quite skilled observers.

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... many skilled observers have reported pinkish color. So we should at least believe that it is quite possible.

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1) Colors are real because reliable observers have seen them. This sentence does not proof anything

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2) There are large variations between persons, thus color is visible.

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3) A scope large enugh will show colors.

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Again, you don't seem to be reading or understanding what people have said here. First, a person must have the sensitivity to the various colors in the first place. A color blind person will never see much in the way of color in nebulae regardless of the aperture, and more than a few people have very little color sensitivity at the low light levels found in astronomical objects. Other people *do* have greater color sensitivity and will be able to see colors in bright nebulae fairly easily (people like this *do* exist).

Second, larger aperture will *help* significantly with revealing color in bright nebulae, as long as the eye is sensitive enough in the first place. Most of your earlier discussion centered on claims for faint reds in M42 which you have been trying to debunk. Given sufficient aperture and a person with the right color sensitivity, color can be seen in many nebulae. I do not see very much color in NGC 6543 in my 60mm f/11.7 refractor, yet I have no trouble at all seeing its bluish-green color in my 8 inch Newtonian at 47x. It is even more stunning in a friend of mine's 30 inch Obsession, where it shows a brilliant almost lime-green hue with stunning interior detail at over 400x. The faint reds in M42 are definitely easier to see in a 30 inch Obsession than they are in my 10 inch Newtonian, although they still tend to be somewhat pastel even in a 30 inch aperture.

Please carefully consider the wording of your statements before they are made. Some of your statements about what has been said here are not at all what was really written, and other statements run counter to accepted facts known in the amateur astronomy community.

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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info

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Lots of eye-brain-mind stuff going on here.....

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In my 25" I have seen color lots of times, in lots of objects, including the brighter emission and reflection nebula scattered across the sky. I would have bet a lot of money that the colors I saw were real.

However, after seeing several off those illusion examples, I can no longer swear that they were real. I mean, that illusion of the 2 rubic-cubes, for example, I swear that the squares on the left are blue and the squares on the right are yellow, but the mask makes it clear that both are an identicle shade of gray. After this experience, how can I believe anything I see at the eypiece anymore?

Well, if these colors are due to a set of circumstances that trick my brain into believing they are real, or whether I really am seeing these colors, it matters not at all to me; I will continue to enjoy them nonetheless.

I assume that the scope of this thread is limited to seeing colors in bright nebula, correct? I have seen lots of different colors in lots of planetary nebulae, in lots of telescopes, including the 82" at McDonald Observatory in Texas, where several objects were simply kaleidoscopic; words could not describe the colors I saw in NGC 3242, the Ghost of Jupiter.

Real or imagined, seeing colors in nebulae is still a measure of the quality of the instrument and the quality of that particular night's sky.

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Different persons "perceive" this thread in different ways . I do not think the point here is whether colors are visible in M42 (which part? M42 brightness goes from 14 to 22!) or whether they are visible in a given specific bright nebula or planetary.
The point is: *are all color seen by experts real, or are there limits to real perception* (only after that we can try to make the list of real and false perceptions).

So I see only two options:
a) colors reported by expert observers are always real colors, there is something wrong with the state of the art of vision science;
b) there is a threshold for real color percetion, below it there are illusory color perceptions;

My point is that most of the colors are perceptions (I suspect that all reds are percetions).
The case of M27 seen green in place of red and red in place of green is emblematic. M27 shines at 18.4 mpsas. False color perception in M27 is exactly what the theory predicts. Thus we should conclude that at 18+ the likelyhood of real color percetion is "faint" (again in agreement with the theoretical thresholds). There are lots of things that agree if we accept the idea that starting from a given point (not perfectly known but in between say 17 and 19, maybe maybe maybe somewhat more for a "pure" color) colors are simply guessed by the brain (sometimes correctly and other times incorrectly). I think that some people whish option a were true and that every time they see colors those colors must be real. There are people reporting colors (different shades of pink) in the Veil nebula, and green the Owl nebula (only to mention two cases that I recall). These are explained by the theory but they pretend the theory to be wrong and the colors to be real.
This is the real point. Vague statements, exceptional cases and conditions are invoked to allege that colors may indeed be *alway* real.

Colors (real colors) needs at least two photoreceptor to be working. I wrote that colors can be seen really only if the brightness is above a given threshd. This is a necessary not sufficient condition. There are other conditions (size of the area, which also means large scopes, and saturation of color). We know there are parts of nebuale that are bright enough in V band. That only means that OIII *may* be seen. We do not actually know if there are part of nebuale which are bright enough in H-alpha (V-band tells little about the intensity of H.alpha since it wheights only marginally). The only test is to look with a H-alpha filter. Nils Olof Carlin (I now recall of this http://tech.groups.yahoo.com/group/bigdob/message/16646 ) have found he can see H-alpha in the brightest parts of M42 with a H-alpha filter. Unfortunately those parts are not red. Why? Because there also is a lot of OIII emission and thus the brightest green produces a green color. To see the real red we need a region H-alpha without OIII. Since the phorecptors of H-halpa are the L-cones which are 120 times less sensitive than rods (the photoreceptors of OIII) and since L-cones sensitivity to H-aplha is only 10% of peak L-cones sensitivity we definitely need a region with very strong H-alpha emission and a very faint OIII emission to see it as red. Does this region exist? This is what I expect to know from Don. He must be able to see (a region) with something in H-aplha and nothing in OIII (or much faniter in OIII). That region, if found, is a good candidate for real red perception. In fact that region (Don we are all waiting for you) will be able to stimulate L-cones without stimulating rods. Theory tells that in this case real red perception should be achieved.

Edited by Mauro Da Lio (12/02/07 11:36 AM)

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Experienced observers do know about the little tricks the eye can play

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First, a person must have the sensitivity to the various colors in the first place. A color blind person will never see much in the way of color in nebulae regardless of the aperture, and more than a few people have very little color sensitivity at the low light levels found in astronomical objects.

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I have always seen M42 as green or blue-green. When I started in this hobby, I noticed that the nebula in Orion had a green color in 7x50 binoculars. No other object in the sky had this color. Galaxies like M31 and M33 were white or gray, without any hint of color.

When I later got my 17.5” Dobsonian, M42 became one of favourite Deep Sky objects – very bright, spectacular details and an intense green color. I might add that I have not seen any suggestion of a red or pink coloration in M42. Perhaps it could have been done under darker sky conditions; I don’t know. The 17.5” is no longer in use, so I can’t verify this.

But when in use, I did try the see the red H-alpha light in M42 with a filter and I succeeded.

I used a Kodak Wratten 92 deep red filter. It is a color filter of the longpass type with a cut-on wavelength of 640nm (50% transmission) and 80-90% transmission at 656nm. A filter with a similar transmission characteristic today would be a Lumicon H-Alpha Pass filter or Kenko R-64 filter. Both are used for H-alpha photography.

Using the 17.5” at 63x magnification (7mm exit pupil for maximum brightness) plus a head cover for blocking ambient light, the central region in M42 were visible as a red glow with the same color as a red stoplight. I could see the Huyghenian nebulosity, the dark fish-mouth and the trapezium stars.

I might add that the view through the deep red filter was dark and the rules, which we are using for deep sky observing didn’t apply.

In essence I was blind except for a small region in the center of the eye. It was only possible to see a star or the nebulosity by looking directly at it. Using adverted vision and the objects disappeared and all that were left was darkness. That made it difficult to reacquire an object if lost. Several times I searched the eyepiece field in vain only to reacquire M42 by stumbling upon by chance. Looking directly at it and it was again a relatively easy though not bright target; a very odd experience indeed.

What is the explanation? The fovea in the center of the eye has the largest contraction of cones. They are daylight cells and sensitive to red light. Rods (night vision cells) have a maximum density about 8-20 degrees off the center of vision. They can’t detect color or see red light.

My experience with M42 in the 17.5” and a red filter is only consistent with daylight cones being active and sensitive enough to detect the deep red H-alpha light in M42.

It is possible, but it is a difficult task.

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It will be another week before I am at a dark site to test the H-Alpha view of M42.
However, I have invenstigated the spectrum of M42, and it has more energy at that frequency than at any other. The second strongest emission is at 500.7nm (O-III). All other emissions are too low a level to have any relevance in this discussion.
I will also tell you what the SQM reading is during the observation, as this may be relevant for dark adaptation.
I will test first with O-III having zero red transmission and report the colors I see, then with H-Alpha to tell you what of the nebula is visible. I suspect the central Huygenian region to be faintly visible. Other than that, it remains to be seen.
However, I would point out one caveat: a typical H-alpha filter transmits about 90% at that frequency. That ten percent could represent a threshold brightness factor that cannot be eliminated, so NOT seeing a part of the nebula with the Ha filter doesn't PROVE that a reddish color is not sensed WITHOUT the filter in place.
However, since the Huygenian region is so bright, I suspect there is reduced rod sensitivity when viewing the nebula, and increased cone sensitivity. Unfortunately, my test will not tell me much about the threshold of red sensitivity, but it will allow me to see if illusory colors are visible. If I still see faint reds and pinks with the contemporary O-III filter in place, that will prove to me that the colors are illusory.
This will be fun, regardless of what it doesn't and does prove. I typically observe with others, so I'll see if others would be interested in participating.
Since exit pupil would have an effect, I will first stick to a large exit pupil (over 5mm) and then go smaller to see if certain aspects disappear at higher magnifications (perhaps crossing a threshold in surface brightness?).

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Experienced observers do know about the little tricks the eye can play



Sure? I think that most of the illusions I posted were unknown.

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First, a person must have the sensitivity to the various colors in the first place. A color blind person will never see much in the way of color in nebulae regardless of the aperture, and more than a few people have very little color sensitivity at the low light levels found in astronomical objects.



Without numbers this sentence is so vague that might be either true or false.

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It is true:

^ Neitz, Jay & Jacobs, Gerald H. (1986). "Polymorphism of the long-wavelength cone in normal human colour vision." Nature. 323, 623-625.

As for numbers, it won't take a long search of available literature to bring up some study, but suffice it to say that there is some variation in the color sensitivity of different people. In any case, there is enough evidence to consider the reports of sightings of color in bright emission nebulae (even red) to be possible.

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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info

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These color illusions are fairly common, but do not necessarily explain the observations of those who report faint red or pinkish colorations in M42 (nor should they have to, as it is quite possible that red from H-alpha is being seen).

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Neitz, Jay & Jacobs, Gerald H. (1986). "Polymorphism of the long-wavelength cone in normal human colour vision." Nature. 323, 623-625.
As for numbers, it won't take a long search of available literature to bring up some study, but suffice it to say that there is some variation in the color sensitivity of different people. In any case, there is enough evidence to consider the reports of sightings of color in bright emission nebulae (even red) to be possible.

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No please bring up the numbers (I did). What are the variations? "some variation" is still vague. How could you tell if "some" is "enough". How much is that "some"?

If you tell the numbers (and they are what I know) I will try to show you that they are definitely not enough (and you will try to show that maybe stretiching etc etc... perhaps maybe^3... they are hopefully... almost nearly enough ). Tell the numbers so that we have a firm point. (also for those people who cannot access the paper).


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PS let me show you an example.

Premises:

- people on average run 100 meters in 20 seconds (say):
- somebody runs faster than others;

So, let us try to answer these questions:
a) is there *enough evidence* that somebody can run 100 meters in less than 20 seconds?
b) is there *enough evidence* that somebody can run 100 meters in 10 seconds?

Edited by Mauro Da Lio (12/03/07 02:44 PM)

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Spectrum of M42:
translated from the Japanese
with slit transmission and graphs further down the page.
Looks like O-III emission only about 60% as bright as Ha.

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Ha and O-III pics of M42

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Boy, the B/W H-Beta image towards the end really shows why I consider M43 to be an "H-Beta Object", as it is nearly as good as the H-alpha shot but noticeably better than the OIII filtered image. The secondary red filament next to the south "wing" also shows up best in the H-alpha image, and that jives with that area being faintly reddish visually. Clear skies to you.

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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info

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Ha pic of M42

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I do think that we do see some real colour contrast in M42 - in my 16" it's very apparent that I lose the perception of redness when I lower the exit pupil, so it's dependent on the absolute surface brightness (rather than the contrast with the surroundings).

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