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

Yes Don. What I pointed out from the beginning (you shold read the papers they are "illuminating") is that studies show that *there are* color perception, also in scotopic conditions. But in scotopic conditions they are false colors related to brightness levels (brightest lights are "cold" colors, green-blue-gray) and medium lights are "warm" colors, pink-red-orangish). Many people (but not all) have these kind of perceptions context-related and variable among the individuals and sometimes richer that color perception in mesopic condition (curiously those people that see gray are those who have the least inaccurate color perception).

So here is the point: there are two theories.

Theory 1 is based on visual-neuroscience experiments and say that:
a) - at light levels in the low mesopic condition (down to ~17.5 mpsas) there still is a remnant of *real* color perception based on rods-L-cones interaction. There are basically two colors: in the Lab colorspace they are a*=-5, b*=3 (a greenish gray) and b*=5, a=-1* (a reddish gray). Which one happens depends on the ratio between rods and L-cones stimulation (a completely different point is if and how much L-cones may stimulated by H-aplha: I strongly doubt).
see:
Change of Color Appearance in Photopic, Mesopic and Scotopic Vision
Jae Chul SHIN, Hirohisa YAGUCHI 1 and Satoshi SHIOIRI1
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
OPTICAL REVIEW Vol. 11, No. 4 (2004) 265–271
# 2004 The Optical Society of Japan
b) - at light levels further below, vision is mediated by cones and there still are color perception (in contextual conditions if there are different light levels). The brightest lights are perceived as "cold" (many shades from blue to gray and sometimes even yellow). The medium lights are perceived as "warm" (many shades from pink to red). This kind of perception is probably related to the Purkinje effect and is in continuty with the rudimentary perception at low mesopic conditions (the reds are darker than the blues and the brain thus interprets medium grays as reds - our brain does a marvelous job in guessing and interpreting missing data, always!). If the same medium lights are alone (no brightest lights) they are seen as greenish (in fact they are the brightest).

Theory 2 is based on the reports of expert observers who say thay have seen colors (which is true) and that colors "must be true" (which is questionable).

So, let us see how the two theories explains some observations, and if they survive to experimental data:

M42. This nebula is very bright. The central part (with more OIII) near the trapezium shines at 14 mpsas. That is in the middle of the mesopic region, bright enough for real perception. The farther parts (richer in H-alpha) are at 17-18 and below, at the limit of mesopic perception for a "normal color" and probably far below for H-aplha.

Now Theory 1 predicts green at the brightest part (1.a, the real color) and reddish elsewhere (1.b, reddish contextual perception). That is what many people, me included see. Thus reports on M42 do not conflict with theory 1.
What about theory 2? Theory 2 predicts green at the brightes part (the real color by OIII) and red for the fainter parts (supposed to be the real H-alpha at 18 mpsas and below!). Again reports on M42 do not conflict with theory 2.
Thus we can conclude that both theories predict what the observer reports (with different explanations but both predict the same effects). M42 *is not* a case able to discriminate between the two theories.

So, let us consider another case: M27. It is not difficult to find that the bow-tie is where there are strong emissions of both H-aplha and OIII. In fact the "real colors" of the bow-tie are part red and part green (the ESO and HST photos are in false colors, buy you my find here some more realistic colors http://www.astrodon.com/oldsite/ColorExposure.html and you may easily find that the bow-tie is where H-aplha may be found. The bow-tie shines at 18.4 mpsas. The faniter part of the nebula is OIII only. Thus the real color of the fainter part of the nebula is green.

Now theory 1 predicts "cold" for the bow-tie (1.b) and "warm" for the fainter part (1.b). This is in agreement with reported colors.
Theory 2 predicts green and red for the bow-tie and green for the fainter parts. This conflicts with the reports in both parts. If we mitigate theory 2 (assuming that H-aplha is not seen) we should conclude that the bow-tie is green (seen only OIII) and that the fainter part is green (there only is OIII). Thus the red in the fainter part of M27 still remains unexplained. This is enough to seriously question theory 2.

Note that if we patch theory 2 assuming that H-alpha is not seen (in the effort to match at least the reported color for the bow-tie) then we are no longer able to report justify the rported red in M42. This patch produces a bug elsewhere! Usually when things like these happens a theory is wrong.

Edited by Mauro Da Lio (11/26/07 05:30 PM)