Hopefully this will be my last posting on this subject. I have slogged my way through the process for colour calibration of my monitor, using Registax's auto-balance feature, using G2V stars as colour calibration targets, measuring the colour coordinates of standard Macbeth colour charts, finding the "best" values for the AS224MC camera for correct white balance settings, measuring the effect of planet elevation on colour cast and using measured spectral data as measures of "colour truth" for calibrating image data.
While the last link pointed to a plot showing measured albedos (reflectances) of the planets as a function of wavelength, it wasn't until recently that I was able to source the original paper by Erich Karkoschka in which he publishes the actual data from his experiments (available here) and also the colour tri-stimulus values for the full disc albedo measurements of Jupiter, Saturn, Uranus and Neptune (plus a few moons and Saturn's ring). Unfortunately the format Karkoschka used to describe the colours in the CIE 1931 format of (albedo, spectral purity, dominant wavelength) is rarely used today, and this required more than a little investigation to find the method of converting these values to the more common XYZ, Yxy and (eventually) RGB formats.
Karkoschka's calculated tri-stimulus values for the planets are shown below, together with the corresponding RGB values in D65 colour space shown as the first set of RGB values. Also shown are the RGB values I calculated by feeding the measured reflectance data from Karkoschka into an Excel spreadsheet published here, showing good correlation between the RGB values calculated using the two methods.
Albedo Purity Wavelength R G B R G B
Jupiter 0.50 0.10 572 190 188 172 190 187 170
Saturn 0.48 0.21 577 198 183 153 196 182 151
Uranus 0.50 0.12 490 155 195 198 162 197 200
Neptune 0.43 0.19 485 138 182 199 140 185 200
So therefore, we can assume that the values shown above should represent the average full disc colour coordinates for the planets (at least at the ESO in 1993 when the measurements were taken). The next question is; what does this look like in reality?
To answer this question, I took some of my own data and used Photoshop to measure the median R, G and B values of the full disc after processing. I found that what I thought were a set of pleasing images to the eye were in the main quite different from the values shown above, especially for Uranus and Neptune. Rather than trying to match the actual R, G and B values, I instead used Photoshop to apply a linear shift to the colour data of the images so that the Red/Green and Blue/Green ratios in the images matched the data from Karkoschka. This method is shown in the first image below, this may not be the correct way of doing it, if there is a better way then please let me know.
So without any further ado, here are the "colour corrected" images for (in order) Jupiter (shown 100% captured size), Saturn (150% captured size), Uranus (150% captured size) and Neptune (200% captured size). I am still working on Mars, I have some data but it's incomplete below 500nm.
Edited by Tulloch, 12 September 2020 - 12:30 AM.