Neptune

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geckzilla
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Neptune

Post by geckzilla » Sun Dec 17, 2017 2:25 am

I was studying the latest Neptune imagery from Hubble, which is already free to the public due to being part of the Director's Discretionary programs, when I was surprised to see limb brightening in the red end of the visible spectrum. I am aware that the methane bands in near-infrared exhibit this phenomenon, but had no idea that it could possibly be human-eye-visible.

The reason for this being that no other Neptune image claiming to be "natural" (I dislike this word in this context) showed anything other than a blue to cyan sphere with whiteish clouds. Some of them showed reddish or greenish limbs, but I figured those either used infrared or were maybe just bad processing.

After seeing this in the Hubble archive, I decided to get into the Voyager archive, which I really thought had been gone over by now with many fine toothed combs. My understanding is that Voyager 2's visible filters went like this: violet, blue, green, orange. Most of the processors use the "orange" filter in place of red, so I had assumed Voyager 2 couldn't see much red.

I noticed the "green" filter exhibited a certain kind of limb brightening that reminded me of the Hubble dataset. Why did this "green" filter fit the pattern of a red one? I decided to check the filter plot and there it was. That green filter was something I would rather use for red. Furthermore, the violet and blue filters were ones I'd rather use for blue and green, respectively. Heck, there is visible RGB data in the Voyager 2 archive for Neptune.

Lo and behold, when I put the filters together like that, there was that lovely pink limb, most prominent at the north pole. Anyway, just happy to have found this, and thought I would share. I had no idea Neptune was so colorful.
Neptune1989-08-17T0530.png
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geckzilla
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Re: Neptune

Post by geckzilla » Sun Dec 17, 2017 4:01 am

Here's a saturation-enhanced version. Sometimes I forget what I think of as colorful is pretty much grayscale to a lot of people.
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neufer
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Re: Neptune

Post by neufer » Sun Dec 17, 2017 4:30 am

As I understand it, methane's red absorption makes
both a front lit Neptune and a back lit Pluto blue.

Uranus is also bluish due to Methane but it is more greenish than Neptune... for no known reason.
Art Neuendorffer

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geckzilla
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Re: Neptune

Post by geckzilla » Sun Dec 17, 2017 5:02 am

I think they may well have very similar compositions, but due to the generous axial tilt of Uranus, different things go on in the atmosphere. I'm guessing the weather scatters light differently. I can't say I have a very informed opinion on the matter, though. Looking at the two in infrared, Uranus has more banding. Neptune also has much more dramatic variation in its high clouds. This year and last the northern hemisphere has seen some stunning formations. The ones Voyager saw were little babies.
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Ann
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Re: Neptune

Post by Ann » Sun Dec 17, 2017 7:05 am

neufer wrote:As I understand it, methane's red absorption makes
both a front lit Neptune and a back lit Pluto blue.

Uranus is also bluish due to Methane but it is more greenish than Neptune... for no known reason.
Are we sure of that? Because I'm not.

How do we know that Neptune is really decidedly bluer than Uranus? Isn't it because Voyager 2 showed them that way?
Uranus and Neptune.
Look at these Voyager 2 portraits of Uranus and Neptune. The source of the composite image may be The Verge.

I don't believe in the color contrast seen in these two images. I think that the NASA people felt that the general public was disappointed in their bland, pale portrait of Uranus. Neptune obviously offered more drama with its great dark spot and light-colored cirrus clouds, but personally I believe that NASA decided to make Neptune look even more interesting by tweaking and enhancing its overall color, making it look dramatically blue.

I searched for other portraits of Neptune and found, among others, this green-looking version by NASA, this pair of differently mapped and enhanced ones, this "pale and faded and partly yellow one" posted by wikimedia commons and this rather elegant one, which is pale and not strikingly blue at all.
And look here what I found when I googled for images of Uranus and Neptune! The pictures at right were taken by Hubble, not by Voyager 2. Or so I believe anyway.

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Derek9531

Re: Neptune

Post by Derek9531 » Sun Dec 17, 2017 12:24 pm

If Voyager has four filters, and we choose to use exactly three, then there are four ways to show Neptune. I'd be interested to see the other three (or other two, since NASA chose one combo and you chose another).

If instead we choose to use blends of two adjacent filters instead of omitting one completely, then the options become not four, but a continuous line of options. I'd like to see a YouTube video cycling through the options, or better still, an app with a slider allowing you to move throu the options at will.

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Re: Neptune

Post by Chris Peterson » Sun Dec 17, 2017 2:41 pm

Ann wrote:I don't believe in the color contrast seen in these two images. I think that the NASA people felt that the general public was disappointed in their bland, pale portrait of Uranus. Neptune obviously offered more drama with its great dark spot and light-colored cirrus clouds, but personally I believe that NASA decided to make Neptune look even more interesting by tweaking and enhancing its overall color, making it look dramatically blue.
I don't think it has anything to do with blandness or drama. It has to do with which processing shows us the most information, and that is obviously the one that uses more color and has more contrast. That's why virtually every astronomical image has its color saturation increased and its contrast stretched between white and black: to take advantage of the range of vision our eyes are capable of in order to maximize information content.
Chris

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geckzilla
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Re: Neptune

Post by geckzilla » Sun Dec 17, 2017 4:05 pm

Ann wrote:And look here what I found when I googled for images of Uranus and Neptune! The pictures at right were taken by Hubble, not by Voyager 2. Or so I believe anyway.

Ann
The Neptune one is probably wrong. Two perspectives I gained from conversations on Twitter: Björn Jónsson analyzed the spectrum of Neptune and concluded its average global color was a moderately saturated cyan. H. B. Hammel shared her personal memory of Neptune's color from direct visual observation atop Mauna Kea, relating to me that its color was a deep, dark blue "star" without any discrete details. In the end I decided that they may both be right, because if you take a cyan circle and view it as a darkened point source, it can definitely lean more toward the blue side in appearance to the human eye. It is also clear to me that Neptune is not the same color all around.

Uranus, being closer, and having a somewhat larger apparent diameter for Earth observers, has more direct visual descriptions available. Its portrayal as a pale cyan sphere may well be correct, but I need to study it more, and ask around.
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geckzilla
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Re: Neptune

Post by geckzilla » Sun Dec 17, 2017 4:15 pm

Derek9531 wrote:If Voyager has four filters, and we choose to use exactly three, then there are four ways to show Neptune. I'd be interested to see the other three (or other two, since NASA chose one combo and you chose another).

If instead we choose to use blends of two adjacent filters instead of omitting one completely, then the options become not four, but a continuous line of options. I'd like to see a YouTube video cycling through the options, or better still, an app with a slider allowing you to move throu the options at will.
It has more than four. The difficulty with this is that the clouds move significantly across the planet in each shot as Neptune rotates, requiring extra time to get them aligned. I used Photoshop's puppet warp to do this. While it didn't take much extra time, it takes enough that it precludes mass production of the entire dataset. Furthermore, the same filters aren't available for each and every imaging sequence. There were UV, visible, and infrared filters... From my understanding, sometimes the spacecraft did other science instead of gathering pictures, too. CPU power wasn't too good back when the craft was built.
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Ann
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Re: Neptune

Post by Ann » Sun Dec 17, 2017 6:14 pm

geckzilla wrote:
Ann wrote:And look here what I found when I googled for images of Uranus and Neptune! The pictures at right were taken by Hubble, not by Voyager 2. Or so I believe anyway.

Ann
The Neptune one is probably wrong. Two perspectives I gained from conversations on Twitter: Björn Jónsson analyzed the spectrum of Neptune and concluded its average global color was a moderately saturated cyan. H. B. Hammel shared her personal memory of Neptune's color from direct visual observation atop Mauna Kea, relating to me that its color was a deep, dark blue "star" without any discrete details. In the end I decided that they may both be right, because if you take a cyan circle and view it as a darkened point source, it can definitely lean more toward the blue side in appearance to the human eye. It is also clear to me that Neptune is not the same color all around.

Uranus, being closer, and having a somewhat larger apparent diameter for Earth observers, has more direct visual descriptions available. Its portrayal as a pale cyan sphere may well be correct, but I need to study it more, and ask around.
Thanks, Geck, very interesting!

Ann
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