URANUS

Uranus is a much more difficult subject than most of the other planets. It's very small in the telescope (maximum diameter 3,9") and quite dim (mag 5,9 at best). It's barely discernable with the naked eye under a correctly dark sky if one know exactly where to find it (I've seen it in 1997). The view at the eyepiece is nonetheless pleasing, where a small disk of clear planetary aspect, lightely colored. The color can be hard to see ; in my 7" newtonian a pale violet prevails, not really the correct blue-green hue ; as seen in a C14 it really looks greenish.

Does Uranus offer some details to the amateur ? This is a highly discussed topic. My conviction is that in the coming years Uranus will show details on amateur's images, thanks to the improved technics (especially webcams) but also because it's finally going to climb higher above the southern horizon for those who live in the N. Earth hemisphere. Also the conditions will be at best as the planet will be observable in the second part of the night during summer monthes when good seeing is usually found.

My images of 2004 are a first attempt at detecting a detail on Uranus. For this very delicate work a serious preliminary thought has been necessary ; one shouldn't go at it without any idea of what could be seen and how it could be seen ! Various points have been considered.

1) In the previous years, the images taken by professionals means have shown that the south polar region (SPR) of Uranus was very bright, in respect to the rest of the globe, in near infrared (700-1000 nm) but also in red light (600-700 nm)

- Some images taken by the HST in 2003 those last ones seem to proves that the SPR is also bright in red light (R component of RGB image).

- Other images by Hubble in 2004 (along with Neptune). The SPR doesn't look especially bright in R.

2) This detail is the only one that can be reasonably considered to be within reach of amateur means :

First... because we are certain that it does exist ! This forms one incertitude less. Also because it's quite contrasted if one observes in red or near IR light. Last, its relative surface is quite important, and so theorically within the resolution limit of small instrument. Its average size looks to be something like 1,5" x 2,5".

But, Uranus's orientation is puzzling and it small diameter causes a great risk, that is to be unsure, once the image is obtained, of what is the orientation of the image ! Uranus's position angle of the north pole was 256° in 2004 july, "at 4 H" if the celestial north is up. When observing the direction of north must be known.

If the relative surface of Uranus's SPR is important, the principal problem will be the low brightness of the planet. Thus the B&W webcam (ATK-1HS) is chosen.

The observation must be done in red/near IR light. A filter is then mandatory. The planet is already dim, but moreover, its albedo in longer wavelenghts is further reduced (because of methane absorption, mainly). Two filters looked interesting :

RG 610 : this filter passes both R and I bands, then it will provide a good amount of light. This one will be the "ideal" tool.

IR 700 : this very broad infrared filter could give a better contrast, while still allowing a reasonable amount of light to pass. And the seeing will be better than with the RG610. It coud be used with exposures of 0,5 or 1 seconds with the ATK.

The celestial south will be placed at the upper part of the image, so the location of Uranus's SPR will be known.

At first I had of course elected the C14 of my astro club to carry out the observation. While it didn't produce sharp images usually, it should make the contrast difference clear, at least. Nonetheless I did make some experiences with my 7" newtonian.

The second half of summer night can provide excellent conditions if a high-pressure is present. I have then waited for such a night and the first occasion occured on june 28th, 2004, and then again on july 6th and 24th. The seeing each time was good, while not excellent.

I should have rotated the webcam at least once. If real the brightening of the SPR would then have rotated also ! It's quite a shame that I didn't thought about it.

I should make a few remarks about the processing of the images. The very little size of planets like Uranus and Neptune, along with the questionable nature of what can be detected, could lead one to realize a strong and long processing of the images. This is a mistake that should be avoided. Many images seen on the web of these planets have beared various filters under non-astronomical softwares (PSP, Photoshop, Neat Image...) with tremendous enlargments up to 1000 % !! This kind of processing is to be rejected absolutely. No one would accept the results obtained on more classical planets after those kind of processings. There is no reason why they could give bad results on some planets and good on others. Such images are not credible.

Then we'll note that the following images have been processed like any other planets : a simple wavelet analisis (here with Registax). Once again, the quality of the image is obtained during the observation, and NOT during the processing

28 JUNE 2004

Here are the results obtained with the C14 of my astro club, with the two filters cited above. The results look favorables : an albedo difference is detected on the SPR, which looks brighter than the rest of the disk.

6 JULY 2004

Those images have been obtained with my 7" newtonian. The instrument is not really powerful, but it often gave better images than the C14. Moreover the size of the bright SPR is largely greater than the resolving power of the instrument.

Now, the SPR again looks brighter in R+IR but also in R alone. Note that the possible details doesn't show in green and blue light : it has a good reaction to the changing wavelenght.

A color image has been built but finding the adecuate balance is not easy....

24 JULY 2004

The raising fog didn't allowed me to take images in near IR. But, the brightening looks still present in the red component of the image (not shown here).

The results look favorable as the data answers in a positive way to every technical requirement that had been elaborated. Nonetheless, the low quality of the whole globe in the images can legitimately raise some doubts, that's why we'd need further results to confirm these, or not.

Unfortunately, Uranus's SPR is going to be less and less tilted toward Earth as the years go by. 2005 may be the last confortable chance to catch the phenomenon...