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

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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