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

There are many studies in the scientific literuture concernig color vision.
These two are among the best I found:

Change of Color Appearance in Photopic, Mesopic and Scotopic Vision
Jae Chul SHIN, Hirohisa YAGUCHI 1 and Satoshi SHIOIRI 1
Graduate School of Science and Technology, Chiba University, 1-33 Yoyoicho, Inage-ku, Chiba 263-8522, Japan
1 Department of Information and Image Sciences, Chiba University, 1-33 Yoyoicho, Inage-ku, Chiba 263-8522, Japan
(Received October 6, 2003; Accepted February 12, 2004)
OPTICAL REVIEW Vol. 11, No. 4 (2004) 265–271
# 2004 The Optical Society of Japan

The color of night: Surface color perception under dim illuminations
JOEL POKORNY,1 MARGARET LUTZE,1 DINGCAI CAO,1,2 and ANDREW J. ZELE1
1 Depar tment of Ophthalmology and Visual Science, University of Chicago, Chicago, Illinois
2 Depar tment of Health Studies, University of Chicago, Chicago, Illinois
(Received March 16, 2006; Accepted April 7, 2006!
Visual Neuroscience ~2006!, 23, 525–530. Printed in the USA.
Copyright © 2006 Cambridge University Press 0952-52380 06 $16.00
DOI: 10.10170 S0952523806233492

I strongly suggest reading them. Here is a summary.

Color vision is mediated by cones. There are three types of cones. The processing of cone signals is very sophisticated: from conversion to lightness and hue (the r/g and b/y opponents signals) in the retina, to color constancy processes, to contextual interpretation of colors.
Besides cones there are also rods, which are very sensitive to light.

In daylight only cones are active (rods are saturated). Rods become active ad about 0.3 Lux (14 magnitudes per square arc second). As soon as rods provide signals they interfere with the r/g and b/y signals of the cones. At this level there are many mistakes in color recognition.
As the light level further decreases, S cones (those sensitive to blue) become inactive first. Color vision is thereafter mediated by rods and L-cones (those sensitive to red). The number of different hues recognized drops dramatically until, at about 0.01 lux (17.5 mags per square arc seconds) only two hues ("warm", a reddish-gray, and "cold" a blue-green-gray) are discerned. They are only a rough approximation of the real colors. Bright colors, including bright pink, become "grue"-gray, red and dark colors become reddish-gray.
With further decrease of illumination only rods mediate vision, but they use the same nerves -or communication channels- used by cones so that a first source of unreal color perception (illusions) stems from this fact. In addition, the different brightness level are interpreted by the brain as "colors". There is a interesting experiment which shows that any color is seen as "grue"-gray when seen alone, but when diffent levels of brightness are simultaneously seen, the same color (whichever color), if not the brightest, is seen as reddish (not all people have the same amount of illusion).
The second paper is very clear about this.

Bottom line: we can see rudimentary colors in mesopic vision (bright planetaries and maybe the trapezium). I doubt we can see the red at all (L-cones are far little sensitive to H-alpha). We cannot see at all the real colors of the greatest majority of nebulae, but we are often tricked by our visual cognitive system. We interpret moderately bright areas as reddish when close to brighter areas that we see as greenish.

As an example look at the following picture: http://forum.astrofili.org/userpix/1932_orione5_1.jpg
Which color are the wings? Reddish? Wrong. They are gray. This example is an illusion due to the color constancy priocesses that take place in our brain.

Edited by Mauro Da Lio (11/23/07 05:34 PM)

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