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I've always seen M42 as blue-ish-green...almost identical as teal. Of course because its so big I can see alot more of it, but this is pretty much the same color that I see when observing some planetaries as well. It's funny that many of us see it so differently. I'd like to know the science behind this as well.

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I only see M42 as a gray-green cloud in my 25". Bright, high surface brightness planetary nebuale, like NGC 7009, NGC 6543, etc, appear as a very emerald green though (at low power).

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I also see certain planetaries as an emerald green as well at low power in my 8-inch dob. I'm assuming this is the same teal-ish color I'm seeing in M42, but since the area of light is much smaller and focused, it makes the color appear brighter thus making it the emerald green color?

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Best,
Dain
Adirondack Mountains (my true dark sky site)
@ Cedar River Flow



Local Site


Clear Skies?

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I also see certain planetaries as an emerald green as well at low power in my 8-inch dob. I'm assuming this is the same teal-ish color I'm seeing in M42, but since the area of light is much smaller and focused, it makes the color appear brighter thus making it the emerald green color?

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I don't see color in anything pretty much. Ring nebula looks grey, dumbell looks grey, m42 looks grey, blue snowball looks grey, saturn nebula looks grey, cat's eye looks grey.

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I'm also one of those who rarely see color in DSOs, although M42 does appear to have a slight greenish hue, and there are a few PNs which appear bluish to me...Ghost of Jupiter, Catseye, Saturn Nebula, Blue Snowball.

But everything else is just gray to me.

One odd thing for me...in my C8, Uranus and Neptune are definitely blue (or blue/green), but in my Z12 they actually appear yellowish, which made me doublecheck my charts to make sure I was looking at the right object. Uranus is easily a disk, Neptune a little harder to discern as a disk, but once confirmed, I was surprised at the color.

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Rocco

Zhumell Z12
Super C8 (1984 vintage)
Celestron 102 f/5
and a cheap pair of binoculars

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I always saw Orion Nebula with a bluish-gray with a greenish tint.

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There was a post in the beginner forum, a person using an XT8 scope just like mine who commented that he saw M42 as a grey cloud. I observed M42 quite a bit last winter and while my 80mm refractor always showed it as a grey cloud the 8" showed it as a green cloud, so I made the comment that he should have seen green in M42 when to my surprise some other posters mentioned that with even bigger scopes and darker skies couldnt see any color.
Why is this? I feel kinda stupid for making the statement I did in the begginer forum but to be honest I always see it as a green cloud,not on any special night but always. My wife, my buddy and even my kids have seen it as well.
Can anyone explain to me why this is? I want to understand this.
Keep in mind I live in red zone and still see it this way.



Thanks

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The color sensitivity of people's eyes at low light levels varies considerably. Some can see faint colors fairly easily, while others report no significant color at all. To me, in my 10 inch Newtonian at from 40x to around 100x, the brighter parts of M42 appear mostly a pale bluish or bluish-white color except for the very central "Huygenian" region, which is a notable bluish-green color. The very outermost regions show no color, as they aren't bright enough to activate my color sensitivity. With a narrow-band filer, the brighter segments appear bluish-green, while the outer regions show no color at all. The exception to this is the view with the DGM Optics NPB filter. It gives a somewhat less greenish tone to the nebula, and occasionally allows me to glimpse faint reds along the edges of the inner "wings" of the nebula. Clear skies to you.

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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org

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So I'm still trying to understand something here, I saw my neighbor this morning as she has viewed Orion through my scope on three or four occasions and she confirmed that she saw a green cloud as well.
Thats five people who have looked through my scope and every one of them saw green.
Does this mean that the majority see M42 as green and the minority see it otherwise? because otherwise I would not expect all five people to share the same observation.if the split was more even then at least one person looking through my scope should have seen something else.
I'm still a bit confused, sorry if I'm being slow about this

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Most people who can see any color in deep-sky objects when they look at the Orion nebula usually report either a greenish hue or a bluish-green coloration. The emission lines which give us most of the light we can detect with our eyes under low light conditions are the somewhat greenish Oxygen III lines (4959 and 5007 angstroms), and the Hydrogen-Beta line (4861 angstroms). These lines are in the blue-green part of the spectrum, so depending on the visual response of the person viewing, bluish-green or greenish hues are the colors that are frequently reported. Clear skies to you.

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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org

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I propose a test to be objective about color, given the subjective nature of color vision on dim objects.

Get a common set of filters that only see red, orange, yellow, green, blue - that are exclusive in frequency range, and undistinguished by tactile perception.

Shuffle them randomly, then successively view M42, ranking them laterally into a line from what you think is longest to shortest in frequency (color), possibly by sorting between nearest neighbors in the line. Then once the ordering appears stable, turn on a red flash light, record the filter sequence in a log book. Turn off the light and repeat this a dozen times.

It could simply be that due to lack of cognitive cues, ones vision system doesn't associate color correctly and so perceives color differently. I'd be willing to believe that those who correctly sequence the filters, come to the same agreement on the color of the object.

It may be that at low light levels, a difference in intensity is being seen as a difference in color arbitrarily mislearned, because there is no color scale in this light level to keep it "honest". Perhaps having a dimly illuminated spectrum present would allow reregistration of color independent of intensity by enforcing a pattern order otherwise unassociated.

I wonder if many of us see gray or peculiar color not because our rods/cones don't have the information, but that because we don't use the ability our visual systems map it out as "noise".

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Man , that sounds like a lot of work !
It's way over my head anyway .

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LXD75 AR5
LXD75 SN8
Series 4000 Plossls
Misc. other stuff


Fort Rock , Az .

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Man , that sounds like a lot of work !
It's way over my head anyway .

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You're absolutely right. My dad always told me I talked too much! Guess I got contaminated too much at college.

But I sure wish color and DSO's was a lot easier than it is

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The argument is about how scoptic color is perceived. My suggestion is that it requires a context that is absent, and like other vision system illusions, it invents one.

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By definition, scotopic vision is incapable of distinguishing between colors. If somebody wants to claim seeing imaginary colors under scotopic conditions, that's their prerogative. It's very rare that an observer actually achieves true scotopic vision. Regular and repeated exposure to the night sky--even a pristine, dark sky--is enough to stimulate the color-detecting cone cells. Vision is mesopic under such conditions. We have enhanced light sensitivity due to the active rod cells but are still able to detect color in stars, planets and bright deep-sky objects. It is only after prolonged isolation from ambient light that the transition to scotopic vision completes. Hence, the benefit of devoting 20 to 30 minutes at the eyepiece to study a faint planetary or distant galaxy cluster.

Bill in Flag

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Grand Canyon Adventure
Lowering the Threshold

18" Obsession
4.5" Meade 4500
10x50 Swift Audubon

Cosmic Voyage

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I don't see color on faint objects. I don't even see color on the bright objects all the time. But when I do see color (and lots of other observers are, too, on the same night), and those colors correspond to the colors seen in photographs, I'm inclined to believe I am seeing some color.

I.e.if the object emits a strong signal at that wavelength, and I see it, there is a chance what I am seeing is real.

And since the colors repeat on other nights, and are always in the same places on the same objects, and because I have seen the exact same colors in much larger scopes on the same night (only with a much higher intensity), then I think that for those bright objects the colors are real.

We went through this same discussion on another thread, and the only unresolved issue was the threshold of mesopic vision. One poster thought mesopic vision started at a much higher brightness than anything in the sky other than the planets, while others weren't so sure. The argument swirled around M27, where red and green visual identifications are often the reverse of what shows in color photos.

However, we are talking about M42 here, which has a MUCH higher surface brightness than M27. I won't even look at M42 if I am going after faint targets that night. M42 noticeably damages night vision when viewed through 32cm under dark skies.

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I see people using a red light that is so bright it hurts my eyes and they are observing after using a light that bright.

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Its funny about all the talk of different kinds of vision and seeing color. A blind man could have seen intense color on that perfect night I had and that was through a 6" scope. I have my bullet proof vest on so go ahead. All you need is a perfect night.

Mark Kaupas
20"F5

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If you really let your eyes dark adapt completely, the light of the sky is sufficient to read headlines in a newspaper (and pages in a star atlas). It isn't a matter of making all the lights around you red, it's a matter of making the red lights DIM.

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If you really let your eyes dark adapt completely, the light of the sky is sufficient to read headlines in a newspaper (and pages in a star atlas). It isn't a matter of making all the lights around you red, it's a matter of making the red lights DIM.

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Words of wisdom. The eye has a wonderful power to adapt, if you let it.

One more piece of advice: don't get old. Twenty-five years ago my 10" showed M42 as vivid green. Now it's blue-grey.

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Sounds like new mirror coatings are needed.

Mark Kaupas
20"F5

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I wonder if color can be somehow induced by an effect similar to this: http://en.wikipedia.org/wiki/McCollough_effect

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But the blue cones in our eyes are much more sensitive than the ... greens and can activate in much dimmer light.

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But the blue cones in our eyes are much more sensitive than the ... greens and can activate in much dimmer light.

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I thought our sensitivity peaks above 500nm, which is more green than it is blue. Regardless of whether someone sees green or blue in M42, they are seeing O III, which is blue-green.

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Well, actually you are seeing several species with M42. The 5007 and 4959 angstrom (bluish-green) OIII lines are pretty strong, but so is H-Beta at 4861 angstroms. In addition, there is some reflection nebulosity from light scattered off of gas and dust originating in the brighter stars in the nebula, so some of the pale bluish coloration often reported visually can come from that as well. I can see that difference when I use a narrowband filter that only transmits the OIII and H-Beta emission lines, as then, most of the nebulosity tends to be a bluish-green color rather than the more pale bluish or bluish-white color I sometimes see without a filter. Everyone sees pretty much what they see with color in M42, and with the variation in color sensitivity between individuals, it is no wonder there sometimes isn't a lot of agreement in what is being seen. Clear skies to you.

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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org

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The sensitivity of color vision peaks at 550nm, not 500.

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I don't see anything there that suggests I should be seeing blue in M42. It just seems to say that our blue sensitivity is "comparable" to our green and red despite such a low number of blue cones.

With how green M42 looks to me it's difficult for me to grasp how people see it as blue. I notice very little change in the hue when I use a UHC, it just turns to a richer looking green. When I tilt the filter the nebula becomes red, so it is even bright enough to active the red cones.

Perhaps it's my young eyes, I've had people say it is blue or colorless through my telescope while I was seeing a vivid green.

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The sensitivity of color vision peaks at 550nm, not 500.

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It's closer to 500 when dark adapted, though I don't know the exact wavelength.

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Yeah, but the 500nm figure is for detecting light, not color.

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The sensitivity of color vision peaks at 550nm, not 500.

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It's closer to 500 when dark adapted, though I don't know the exact wavelength.

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Yeah, but the 500nm figure is for detecting light, not color.

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Ahem, peak sensitivity drifts downward from 550nm to 500nm as our eyes dark adapt.
Here is the description of the well-known Purkinje effect:
http://en.wikipedia.org/wiki/Purkinje_effect
It means that peak sensitivity is close to the blue-green wavelengths of the O-III emissions from nebulae in our perceptions of light intensity, which is to our advantage in seeing faint nebulae.
H-Beta emission is typically seen as more blue.

Possibility 1: Planetary nebulae which appear distinctly blue and not blue-green have stronger H-Beta emission than O-III (there are some that do).
Possibility 2: The colors seen are "filled in" by the brain and are not real representations of the emitted colors.
Possibility 3: The Purkinje effect says the bluer parts of the spectrum tend to dominate as our eyes dark-adapt. The 500nm peak sensitivity is an average and there is a little variation from observer to observer.
Possibility 4: the linguistic identification of color varies from person to person, with one person's blue-green being another's blue.
Possibility 5: the yellowing of the lens (not cornea) of the eye filters out blues, making blues appear more green.

There is undoubtedly a tiny bit of all 5, but the green/blue debate usually is a bit age-related, pointing to #5 as a likely candidate for explanation. People who have cataract surgery often report seeing a lot more blue in their environments and through a telescope also.

Where M42 is concerned, though, after H-alpha is subtracted, by far the strongest emissions are at the O-III lines, so the central region, if color is perceived, should appear distinctly blue-green.

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Don Pensack
12.5" Truss Dob, 5" Maksutov, Fujinon Binos
Sustaining Lifetime IDA member


Edited by Starman1 (11/15/09 02:23 AM)

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It drifts down, as far as I know, because we switch from A to B.
But if we limit ourselves to describing color vision only, and not light intensity, we can consider it a constant 550nm.

Curiously enough, it'd mean that our bias on white-light objects should be towards yellow colors. That's definitely a color I haven't heard a DSO been called before. In fact, the only non-double star context I know of to have been called yellow is the milky way in sagittarius.

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I'm 20 years old with healthy eyes, and the nebula is distinctly green. It sounds as if I should be on the other side of this debate; I don't think my lenses have had enough time to turn yellow yet...

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A fully dark adapted person whose vision is scotopic won't see a faint light source as having any color at all--real or imagined. This has consistently been the result for scotopic observers in laboratory tests under controlled settings. They don't see colors and cannot distinguish between colors.

In light of this, the notion that deep sky observers achieve full dark adaptation and still see nebulae as having color is highly questionable and probably just plain wrong. The more likely scenario is that their vision is mesopic. The environment is dark enough that rods play a role in visual perception but bright enough to stimulate the cones. We know the environment beneath a pristine dark sky is bright enough to stimulate cone response. The colors of stars and planets still can be accurately detected with direct vision under a such a sky. So, if an environment where a light level of 21.5 to 22 magnitudes per square arcsecond is bright enough to stimulate the cones, certainly the substantially brighter M42 can stimulate cone response and allow for color detection.

But if you observe at high magnifications (2mm or smaller exit pupil) for an extended period of time (30 minutes or more) without exposing yourself to the ambient light environment or artificial lights, you can achieve the scotopic vision that comes with full dark adaptation. While I've often seen stars, planets and bright nebulae as colorful under mesopic conditions, I've never imagined--and certainly never detected--color when my vision has fully adapted in a scotopic environment.

Bill in Flag

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