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HST images of Uranus and Neptune
titanicrivers
post Feb 14 2019, 09:16 PM
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Amazing new images from Hubble Space Telescope (HST) of Uranus and Neptune! https://earthsky.org/space/hubble-space-tel...-uranus-neptune
Here are the Hubble site images: http://hubblesite.org/images/news/release/2019-06
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antipode
post Feb 15 2019, 06:56 AM
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Uranus especially continues to surprise - Voyager IIs flyby timing and its camera's filterset certainly misled people about how dynamic its atmosphere can be.
In the continuing absence of the followup probes the two ice giants so badly deserve, I look forward to what the ELTs and next gen adaptive optics will achieve.
GMT arrival = 6 years with any luck.

P

Actually, considering the large portion of the visible globe of Uranus covered by that seemingly bright cloud (I presume this is a near IR image), has it had any
effect on the planet's magnitude in the visible?
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dolphin
post Feb 17 2019, 06:30 AM
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QUOTE (antipode @ Feb 15 2019, 07:56 AM) *
Uranus especially continues to surprise - Voyager IIs flyby timing and its camera's filterset certainly misled people about how dynamic its atmosphere can be.
In the continuing absence of the followup probes the two ice giants so badly deserve, I look forward to what the ELTs and next gen adaptive optics will achieve.
GMT arrival = 6 years with any luck.

P

Actually, considering the large portion of the visible globe of Uranus covered by that seemingly bright cloud (I presume this is a near IR image), has it had any
effect on the planet's magnitude in the visible?



That was my question. Does Uranus have such complexity in the visible range? If so, that would be markedly different from the uniformity we saw during the Voyager flyby.
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titanicrivers
post Feb 17 2019, 06:28 PM
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Excellent questions Antipode and Dolphin and further inquiry into details regarding Uranus’s north polar cloud and Neptune’s new dark spot have been ongoing. Two abstracts gleaned from Hubble’s OPAL (Outer Planet Atmospheres Legacy) website https://archive.stsci.edu/prepds/opal/ are interesting in this regard. For the north polar hood of Uranus https://agupubs.onlinelibrary.wiley.com/doi...29/2018GL077654 and for Neptune http://adsabs.harvard.edu/abs/2018AJ....155..117W .
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JRehling
post Feb 18 2019, 06:26 PM
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QUOTE (dolphin @ Feb 16 2019, 11:30 PM) *
That was my question. Does Uranus have such complexity in the visible range? If so, that would be markedly different from the uniformity we saw during the Voyager flyby.


I think Uranus may have been cheated a bit by the design decisions of Voyager.

The color filters on the Voyager cameras are described in wonderful detail here:
http://www.planetary.org/multimedia/space-...r_response.html

There was no red. The orange filter zeroed out at 650 nm, with a peak around 580nm.

I've taken many multispectral photos of Uranus including B, G, R, and near IR. I can expect easily to resolve the polar hood if nothing else. It is blank as can be in B and G, but the polar hood is easily visible in IR (>685 nm) and dimly in R (595 – 680 nm).

It's a near certitude that an orange filter missed out on a lot of the needed sensitivity for seeing this feature, if it were the same in 1986 as it has been 2016-2018.
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JRehling
post Feb 18 2019, 07:38 PM
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Here are pictures I personally took with a 235mm Schmidt–Cassegrain and the aforementioned filters on October 18, 2018. The contrast is evident in the IR image, discernible but subtle in R, and completely absent in the G+B (it is absent in both of those, of course, for it to be absent in the sum of them).

At right is a composite where I used the luminance of the IR image, then colored it with the RGB image but with the red plane doubled. It's a nice appearance, but a bit past the bounds of "true color." The fact that both the hue and the brightness of the cap are different from the rest of the planet speaks to the fact that both IR and R show the hood.

There is also a thin equatorial belt that other observers captured but I did not.

But green does not. So we should expect the Voyager camera, were it taking an image of Uranus today, to turn up something blank because of the lack of a proper red filter.
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titanicrivers
post Feb 18 2019, 08:45 PM
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From the OPAL site the images of Uranus are taken with the WF3-UVIS filter camera on Hubble and are not IR images. There are 63 UVIS(ultra violet-visual) filters http://www.stsci.edu/hst/wfc3/ins_performa...ponents/filters. My guess is that the images published were from the F845 filter which shows the N polar cap AND the compact methane ice cloud at the polar cap edge https://solarsystem.nasa.gov/news/839/hubbl...uranus-neptune/
As the compact cloud is considered an upper atmospheric phenomenon (sometimes visible in amateur telescopes) and the large polar cap is not a high troposphere or stratospheric phenomenon, that a visible light view of Uranus might still appear relatively bland! With the north polar cap of Uranus facing the sun however one wonders whether it might become visible as the planet’s summer progresses.
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JRehling
post Feb 18 2019, 09:41 PM
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The F845 filter is certainly what I'd call infrared, whatever bin they happened to organize it into. There may be no exact sharp cutoff of the longest wavelength a human eye can see, but I don't think anyone considers >800nm to be red as opposed to IR.
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titanicrivers
post Feb 18 2019, 10:41 PM
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QUOTE (JRehling @ Feb 18 2019, 03:41 PM) *
The F845 filter is certainly what I'd call infrared, whatever bin they happened to organize it into. There may be no exact sharp cutoff of the longest wavelength a human eye can see, but I don't think anyone considers >800nm to be red as opposed to IR.

Excellent point! Looking a bit further that filter spans the 8K to 9K bandwidth; might be better in the IR bin! Will have to look further as to exactly how that image was created. Does your colorized image in your figure above approximate what the human eye would see?
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JRehling
post Feb 18 2019, 11:40 PM
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No, I see absolutely no features in Uranus with the naked eye, and I'm not sure that anybody ever has with any telescope. There are certainly bright white storms that appear at times, but they push the limits of resolution. While the gradient in red can in principle be seen with the human eye, it is drowned out by overwhelmingly higher levels of brightness in blue and green. When I add the red plane to the blue and green in Photoshop, I can hardly even notice its presence (which is why I doubled it to make that image above).

I think at the limits of terrestrial telescope, Uranus is a blank disc, and I've seen it many times. It's a very appealing color, but that's all one sees. From a vantage point inside the Uranus system, the enormous increase of resolution might make the slight contrast in color emerge.

Here is a conventional RGB color image that I took in 2016. This is basically what it looks like to my eye.

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titanicrivers
post Feb 20 2019, 09:01 PM
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Appreciate your expert commentary JRehling! While visible spectrum images of Uranus may remain bland we are fortunate to have the capabilities of Hubble and large ground based telescopes (Keck, Gemini at present) to image Uranus in the near IR and methane/hydrogen filter views. It is humbling (for myself at least) to review the literature of the last 20 years and discover papers, such as https://www.sciencedirect.com/science/artic...0320?via%3Dihub of Sromovsky et al providing images spanning almost a quarter of Uranus’s 84 year sol revolution (see first figure below) revealing changes in the atmosphere we might not otherwise be aware of. (as in Fig. 3 below from that paper).
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Fig. 3. H-band (1.6-μm) images of Uranus from 1997 through 2015, from observatories/instruments given in the legends. The bright south polar region seen in 1997
(A), 10 years before equinox, is similar to the bright north polar region seen in 2015 (F), eight years after equinox. Images taken during the 2007 equinox year ©
found that neither polar region was bright. The longitude and planetographic latitude grid lines are at 30° and 15° intervals respectively.
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titanicrivers
post Mar 26 2019, 04:47 PM
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A new publication in Geophysical Research Letters concerning Neptune's new Great Dark Spot can be found here: https://agupubs.onlinelibrary.wiley.com/doi...29/2019GL081961
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