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Uranus is still a planet hard to observe for amateurs. If imaging the planet in infrared is now almost common among them since last year, the experience we have in the visible wavelengths corresponding to the human vision is still weak and controversial. What does the HST show about it ?
True color imaging (RGB or LRGB) of Uranus is not a first choice for amateurs, and there is little successful images. On their side, some visual observers are regularly seeing and drawing belts. In the archives of the Hubble Space Telescope, we can find since 20 years many runs of Uranus imaging in several color bands, some of them taken in visible light. I have worked on a recent set to see if we can find some useful references.
Here are some unprocessed images taken from the run 12894 (if you want to take a look by yourself – for this you can use Fits Liberator, to adjust levels). The filters are numbered with their central wavelength in nanometer (nm), the same scale we use for amateur filters. The details visible on the IR image F763M correspond exactly to those seen on amateur images as well as those taken at the Pic du Midi. The F547M and F467M match more or less the G and B amateur filters, even if the 467 is noticeably narrower.
The problem is how to select the red filter. The different HST filters do not match the amateur red band because they are really narrower; and in red light on Uranus, we already find absorbing bands on the spectrum that are likely to make the details vary quite a lot. The F631N only shows extremely faint belts because it is centered on a band of light weakly absorbed; but the F665N reveals a belt pattern identical to that of the IR filter, and with as much contrast…
Trying some RGB’s
The lack of the well correlated red filter does not allow one to make color images that match exactly the human vision, but it is possible to make some experiences to try to get as close as possible. The images are made with WinJupos.
Left-side image is assembled with the F631N, the closer to the human vision. The belt pattern is just barely visible. After all, most of the light comes from the G and B filters, where the planet is brighter, and the scattering of light is strong in these bands.
However, the F631N is maybe too narrow. With an amateur red filter, It is possible to get details similar to those imaged in IR, although with a noticeable weaker contrast – at left my set from October 30th, 2012, where the equatorial belt is detected with the R Astronomik, a filter whose transmission ends around 670/680 nm, just like the F665N…
This is why I have tried to include the data from F665N (at right on HST set) in the image but with a reduced luminosity (it is centered on an absorption band), combining it with the F631N to try to get a red component closer to that of the Astronomik R filter.
Now we can observe a brighter north pole and equatorial belt. The pinkish hue is maybe not that curious: twice in my 2013 observations I got a pink brightening on the north pole, coming from the red filter.
HST images show that some details do exist in visible light on Uranus, with a contrast accessible to amateurs, but certainly only in red light. However, it does not look really interesting to image Uranus in true colors… infrared is certainly more rewarding.