Mauro, The variation in sensitivity of rods varies substantially more than 0.3 magnitudes. This is to be expected, since the dark adaptation process improves sensitivity by a factor of 90,000x ( I have no idea if that is an average or an extreme). I have, however, participated in many "field" experiments in which the visibility of faint stars in a field with known magnitudes varied by over a full magnitude from individual to individual. Momentary clearings could account for some of that difference, as could experience in simply focusing, but I think that degree of variation among experienced viewers is fairly common. Schaefer's work suggests that inexperienced viewers may lose a couple magnitudes and more. I have seen proof that details of magnitude 25 mpsas are visible in large scopes, despite the fact that this is literally 3 or more magnitudes fainter than the sky brightness at the same site (magnification being the reason, of course). Other people had a hard time seeing details at the magnitude 22 mpsas level on the same object There is reason to suspect the variation in the cones at daylit levels of illumination varies less. That makes sense from the standpoint of survivability in evolutionary terms. What isn't well known from field tests (as opposed to lab tests) is if the threshold of mesopic vision varies by a similar amount. The red-green mix study to which you refer has a flaw in that the threshold for the yellow color of matching may have a large variation (not tested), and the color sensitivity at the lowest possible limit was not tested. Since the eye is so much more sensitive to green light than red light, it's logical to assume that red has to be a lot brighter to be visible to the cones. How deep into the twilight do you see the color red? My own car is red, and I cannot tell it is red under a dark sky (mag.21.5mpsas), but I definitely see it as red (not gray) at a brightness of sky of mag.18mpsas. I will do a test to see at what point the red becomes indeterminate as the twilight deepens. The point is, I expect the threshold for red is indeed brighter than mag.20mpsas. But are your calculations for the nebula correct? I've observed a lot of galaxies where the peak brightness for the galaxy (per the RC3) is magnitude 18 mpsas, and this peak brightness is a lot fainter than 90% of the Orion nebula. I suppose what would help, here, is a drawing of the nebula marked with where each color is seen. Also what would help is a map showing surface brightness at each location in the nebula. Perhaps there is no disagreement at all, and the nebula sections where color is seen are simply above the human threshold.
-------------------- Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
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Re: Is it possible to see the nebula with colour?
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