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Old 04-10-2020, 09:12 AM
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Tulloch (Andrew)
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Tulloch is offline
Join Date: Sep 2019
Location: Melbourne
Posts: 368
Originally Posted by pmrid View Post
displaying my ignorance here, but I would think that the spectra would largely be sunlight being reflected back at us.
Well, what is the colour of an object, other than the spectrum reflected off the object by sunlight, then interpreted by our eyes and brain?

We see lots of images of the planets, which are usually "prettied up" by NASA or others for the general population to "ooh" and "aah" at. The worst of these are from Hubble, with the images from its extremely narrow band filters designed to capture far distant nebulae, hacked together to produce something approaching human vision with the contrast blown out.

My interest in all this started when I stopped capturing images of the planets with my DSLR (with the white balance set to "Daylight") to a dedicated planetary camera, the ASI224MC, and innocently asked the community, "What white balance settings do I use on my new camera?" When the answer came back, "Well, here are some settings provided by the manufacturer, but you need to modify the colour balance afterwards to make them images look nice", I was less than impressed.

This motivated me to embark on a quest for Colour Truth, where I investigated monitor calibration equipment, Macbeth colour charts, camera calibration matricies, G2V star measurements, atmospheric propagation effects, the CIE 1931 standard on tristimulous colour coordinates, and measured spectral data of the planets. If you have a spare few hours you can read about my quest here (along with everyone else's objections to why it won't work ).

Where I eventually ended up was a NASA report by Erich Karkoschka in which he publishes the actual spectroscopic data from his experiments on the planets, which can be used to convert to the colour coordinates XYZ, L*a*b* and finally RGB for (calibrated) computer monitors. This data is an "average" colour of the planets, from which I calculated red/green and blue/green ratios for each planet, then used these values as a crude tool for "white balancing" my own images. It's an order of magnitude calculation at best, however it is at least something that could be used to provide a slightly more accurate image.


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