(You may find this website of interest. )
Hi Graeme,
Funny you should mention that: the color pages on that site are the reason I got thinking about the question in the OP in the first place. They are wildly (did I say wildly? WILDLY!) wrong.
To calculate colors in Adobe RGB from spectrum he dot multiplies it with Stiles and Burch 1955 RGB CMFs (the ones with negative portions) and stores the result as-is in an Adobe RGB file. No conversion to XYZ, no adaptation to D65, no projection to Adobe RGB, no gamma (!). For instance, take a look at
Figures 5-8 here, not a single one of those tones is displayed properly (colorimetrically).
It became apparent very quickly in
this thread that Roger Clark does not know what he is doing as far as displaying 'true' color is concerned. His relative pages are an embarrassment and should be corrected or taken down. I offered to help him do them right but so far he has not taken me up on it (:-).
Note that color perception depends on the light level. Natural night time viewing will be in the scotopic or mesopic range, so normal observer spectral sensitivities don't apply. In the scotopic range, vision is monochromatic. So a key question is whether you are trying to reproduce color as we would naturally see it, or whether you are attempting something more synthetic - i.e. how would we see it if the light levels were in the photopic range ?
Yes, that's something else to think about. Since human observers actually do see colors of celestial objects through powerful telescopes I think we are at least in mesopic vision, if not full fledged photopic. But for the sake of this question let's assume that the telescope is capable of collecting enough light (> a few cd/m^2) that the Observer is in the Photopic range where CIE CMFs apply.
So, when converting from spectrum to Adobe RGB, can I assume a starting adaptation of illuminant 'E' once CIE CMFs have landed me in XYZ? Or do I have to take Observer adaptation to the background in the telescope's field of view into consideration? After all, the original CMFs were derived by projecting absolute spectral irradiances (of which the nebula in question is an example), no?
Jack