Quote: 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.
This witness is close to what I expected.
H-alpha is strong enough to be seen ONLY in the Huygenian region. However there is also a strong emission of OIII there. Since our eye is more sensitive to OIII the color that prevails is green (unless we use the filter). H-alpha does not prevail there (that is whay Nils Olof Carling reports seeing H-alpha where the nebula is not red). The other parts, where H-alpha is stronger than OIII, are below the threshold for cones and H-alpha is not detect. There remain a faint perception of OIII by the rods... the rest in my opinion is the well described contrast/color constancy effect.
BTW. your witness is consistent: H-alpha is perceived only by L-cones. Rods have no sensitivity at all to H-aplha. Cones are most concentrated in the fovea so to perceive red you need the red to be visibile with direct vision in the fovea. It is exactly the opposite than adverted vision when we try tu use the region of the retina which has the highest concentration of rods. To see red (if a hope exists) we have to use the region with the highest concentration of L-cones.
I think this severely questions any claim of seeing *real* red in faint objects like M97 and the Veil. For M42 perhaps Don has a better sensistivity. I still wait the reports by Don. Perhaps he might be able to se H-alpha somewhere else (somewhere where he sees red). In my oionion real red is very very rare, if not impossible at all.
"Antares is nearly 50 times brighter than its companion star. The companion star is blue-white. However, it is close enough to white that you perceive it as red’s complementary color: green. Yes, the blue-white star looks green! Some references designed for professional astronomers list the correct color, but many written for amateur astronomers list “green” as the companion star’s color, since that is what it looks like to the eye. But, if you take a photograph, the companion is definitely not green in a photograph (actually, it is really tough to take a photo of such a dim star next to such a bright one)."
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