[X] My Assistant

[DDAVIS]

http://www.donaldedavis.com/2002_addons/SSYCOLRS.html

A new set of color samples at the bottom of the page. links to other planetary color related sites as well as to my updated pages concerning Mars.
Any corrections, reactions, etc. welcome!

Don

[4th rock from th...]

Very nice and informative, specially the Mars pages. I also really like the planetary palette and the fact that albedo is taken into account in the comparison images.

One interesting thing is that the overall color balance seems, to my eye and on 3 different monitors, a little strong towards the green. This is most noticeable in Jupiter, for example.
This is interesting because my own crude attempts to generate average RGB values for several planets based on their spectra gave similar results.

[JRehling]

Great stuff, as always.

The upper left Mars image from Mt. Wilson pops out as just what I see through my 3" scope.

A calculation I once did, which is perhaps relevant, is that the illumination of the planets' surfaces are, in most cases, brighter than a typical monitor can imitate, and this makes any screen portrayal inherently different from "true color". The Purkinje Effect is just an extreme example of how it is impossible to have the same colors at different illumination. ("Separate but equal"?) It turns out that Uranus, Neptune, etc., are dimly-lit enough that they *can* be imitated, to within epsilon, of "true color". Eg, true illumination as well as true color. But for everything else, there's a need to compensate.

I think the Moon also makes a great example of the importance of the degree of dark-adaptation the viewer has undergone. A full Moon on a winter night can be dazzling. It just short of hurts to look at it. But I've noticed that when a day time Moon, in a place with dry air, is visually located near snow or cumulus clouds, that it's suddenly noticeable how tan it is.

[DDAVIS]

[quote name='JRehling' date='Jul 23 2008, 04:00 AM' post='121231']
>Great stuff, as always.

Thanks!

>The upper left Mars image from Mt. Wilson pops out as just what I see through my 3" scope.

It is a rather dazzling predominately yellow orange at first, then as that part of your retina gets 'saturated' something of the color range can be seen to better advantage.

>A calculation I once did, which is perhaps relevant, is that the illumination of the planets' surfaces are, in most cases, brighter than a typical monitor can imitate, and this makes any screen portrayal inherently different from "true color".

Well, yes, but it seems to me I can look at my photos made at Monument Valley and judge which ones have a more realistic color balance, etc. even though the monitor is far removed from solar illumination brightness levels. Are you saying I would need a carefully color balanced transparency, placed on a white 'light box' with enough brightness to register the same on a light meter as a white paper in daylight would, to resonably accurately show the colors of a sunlit surface? Or perhaps a projector as bright and color balanced as sunlight?

Don

[JRehling]

Well, yes, but it seems to me I can look at my photos made at Monument Valley and judge which ones have a more realistic color balance, etc. even though the monitor is far removed from solar illumination brightness levels. Are you saying I would need a carefully color balanced transparency, placed on a white 'light box' with enough brightness to register the same on a light meter as a white paper in daylight would, to resonably accurately show the colors of a sunlit surface? Or perhaps a projector as bright and color balanced as sunlight?

It's levels-on-levels complex!

In principle, any change in illumination is a deviation from accuracy in and of itself. But certainly we look at photos all the time, on photo paper, in books, and on monitors, that don't utilize sunlight-brightness, and still look OK.

The two biggest added variables are:

a) Perceived color balance is a function of illumination, even if the color mix of the light source is held absolutely constant.

Dark/light adaptation, which is something the retina and the pupil are both involved in.

I guess on one level, you can say that the complexity of light/dark adaptation makes it an easier problem to solve because you can say you did it if you even match one of the real situations.

And I think the other problem, perceived color balance vs. illumination, can basically always be solved for with *some* tweaking of the actual balances in the display. If dimmer screens make things seem bluer, then decreasing the blue should compensate for it and give you the "same" perceived hues with less illumination. I guess the question is how to arrive at that. Just to show how dastardly the whole situation is, I'm certain that the solution would be different for the fovea than for the perifoveal field of vision. So a 60-pixel image of Mars might actually call for different balance than a 600-pixel image.

As much as we all love science, it might really be just as well to tweak it to the approval of people who've seen it with their own eye. Although if we lock down a specific situation we want to capture (eg, an eyepiece view of Mars), then there ought to be a way to derive the correct balance with the right human performance information, trying to produce the same response ratios for all of the cones.

[NGC3314]

For telescopic observations, I've tried various things with groups at our campus observatory. Many people see more vivid color on Jupiter if I leave the white dome lights on - apparently that puts typical adaptation in a much better place instead of having the eyes try to adjust for some kind of mean brightness including lots of dark space, and get dazzled. This fit with a comment that (IIRC) T.W. Webb made in his "Celestial Objects for Common Telescopes" about 130 years back - that the level of naked-eye detail on the Moon is enhanced if there is a lamp in the field of view at the same time. I concur, using street lights for the purpose - plus it gets more contrast than the daytime view. When it's just at the sunrise line, there are times when you can just about convince yourself you can pick out Clavius as a dimple in the terminator this way.

[JRehling]

I've got 20/13 vision or so, and I have definitely seen the evidence of individual craters on the Moon. Promontorium Laplace also casts an impressive profile which I believe I can see in the right circumstances. That is, you can really notice when it is sticking out into the darkness.

http://farm1.static.flickr.com/109/2609560...00dd14c.jpg?v=0

Keeping both eyes open while observing planets has also been my way of determining the apparent size of the object. I've drawn circles onto pieces of paper that correspond to the image my eye in the eyepiece is seeing. A smarter way, now that I think about it, would be to pre-draw many circles with precision and make find the best match.

If someone wanted to get really crazy, they could make a jpg with the same little Mars illuminated with many color balances, gammas, etc., look in the eyepiece with the right eye and simultaneously scan a laptop monitor with the left eye looking for the best match. Make sure that the graphics program commands the whole screen so that white menu bars aren't disturbing things. Then it would probably be best to switch eyes and see if the same match exists -- it's possible for the two retinas to be at very different levels of dark adaptation at the same time.