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Neptunian System Imaging
MarcF
post Oct 22 2012, 08:11 PM
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I found even a nicer movie:
http://www.youtube.com/watch?v=_BPY1aGsrPs
Best regards,
Marc.
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Paolo
post Jul 15 2013, 04:59 PM
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New Neptunian moon discovered
http://www.skyandtelescope.com/news/home/N...-215535121.html
I was wondering: being so close to the planet, any chances that it was already present in Voyager imagery?
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Explorer1
post Jul 15 2013, 06:56 PM
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The article says the discoverer couldn't find it in them.
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Paolo
post Jul 15 2013, 06:57 PM
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QUOTE (Explorer1 @ Jul 15 2013, 08:56 PM) *
The article says the discoverer couldn't find it in them.


they seem to have updated it since I posted my comment...
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elakdawalla
post Jul 15 2013, 07:05 PM
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I had the same question and asked Mark. He replied:
QUOTE
I did extrapolate the orbit back to the Voyager era and search the images that were supposed to have captured it. Apparently it is just too small for the Voyager cameras. I seem to recall that their quoted detection limit was comparable to the size of Naiad, the smallest moon that they did find. This object is quite a bit smaller than Naiad.

By the way, the Uranian moon Cupid, discovered in Hubble images in 2003, was also too small for Voyager to detect.


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Ian R
post Jul 15 2013, 10:47 PM
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Is my memory deceiving me, or has Naiad eluded all attempts at its recovery since the Voyager encounter?


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Hungry4info
post Jul 16 2013, 12:22 AM
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AFAIK, that's correct.


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Hungry4info
post Oct 9 2013, 12:12 PM
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And on that topic, Naiad has now been recovered.
http://www.seti.org/seti-institute/press-r...lost-inner-moon

http://www.universetoday.com/105361/a-tale...-and-its-rings/
Attached thumbnail(s)
Attached Image
 


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Bjorn Jonsson
post Aug 24 2014, 09:54 PM
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A few hours from now, on August 25, 2014 at 03:56 UTC, there are exactly 25 years from Voyager 2's closest approach to Neptune.

I have noticed that mosaics of Neptune are rare, the only mosaics I remember seeing are two global cylindrical maps that first appeared in Science back in 1989 and also the mosaics posted by machi earlier in this thread. So I decided to do some anniversary mosaics. The goal was a global (or near-global) color mosaic and the higher the resolution the better. This resulted in the highest resolution global mosaics of Neptune I have seen (but see the processing description below - maybe it can be argued that these images are in the gray area between mosaics and computer generated images/simulations):

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There are two versions of each mosaic, an approximately true color/contrast version and a version where contrast has been greatly exaggerated and the effects of global illumintaion removed.

Some notes on the image processing: Voyager 2 usually had to transmit everything to Earth in real time. At the low bit rates possible from Neptune's distance from Earth this meant a long time between successive images. This makes assembling mosaics of Neptune difficult since the images that must be used are obtained over a period of many hours (or even a few tens of hours for big mosaics) and Neptune rotates fast. The best way for assembling big Neptune mosaics is to reproject the images to simple cylindrical projection as I typically do when doing mosaics of Jupiter or Saturn. But in addition I had to remove the effects of global illumination using an inverse photometric function when reprojecting the images, otherwise seams in the map can't be removed with acceptable result. In contrast, this is not the case when dealing with Jupiter or Saturn. I then assembled a mosaic in simple cylindrical projection and then rendered images of Neptune using the appropriate photometric function. This results in some photometric innaccuracies (the photometric function isn't 100% accurate) but despite this the resulting global images are much better than what one gets by using the source images directly since the illumination conditions change fast due to Neptune's fast rotation. All of this was done in grayscale as only green and clear filter images were available. I used mainly green images since they had better contrast than the clear filter images.

In addition to the complications mentioned above, some of Neptune's atmospheric features change very fast (especially the bright clouds) and also the rotation period (or zonal winds) varies a lot with latitude. Major features can drift over 60° in longitude during one rotation of the planet. Because of this the relative longitudinal positions of the cloud features are only approximate in the mosaics above.

The final step was to colorize the grayscale renders. For that I used an earlier global true color image I assembled back in 2009 - it can be seen here.

With all of the caveats above, the result is the highest resolution global mosaics of Neptune that I have seen.

Most of the source images I used were obtained about 2 to 3 days before closest approach when Voyager 2 was 3 to 4 million km from Neptune. I used the calibrated and geometrically rectified images available at the PDS Planetary Rings Node. I used about 20 images.
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Ian R
post Aug 25 2014, 03:05 AM
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Bjorn, I am in awe of what you've achieved here. Simply magnificent: a real masterclass in working with old yet valuable data. ohmy.gif


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machi
post Aug 25 2014, 10:46 AM
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Excellent as always Björn!
I think that those images are the best global mosaics of Neptune ever made.


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tedstryk
post Aug 25 2014, 04:37 PM
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Amazing. Just amazing.


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jccwrt
post Mar 22 2015, 10:51 PM
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The other day I found an image of Despina's shadow transiting across Neptune in Voyager 2 images taken on August 17. I've seen the wonderful pictures that Ted Stryk and Daniel Macháček have made with moon shadows transiting, but I wasn't sure how much searching they had done for those. So I decided to do a thorough, systematic search through Voyager 2's approach images to see what turned up.

My first step was to download all of the c112xx images taken through Voyager's narrow-angle camera, and search visually through the images for potential moon shadows. Despina's shadows were easy to find, but I also came across some smaller spots that were only about 1 px across, but appeared to be features I could follow from image to image. I kept a spreadsheet of all potential shadows and the images they were found in, then cross-checked them with the PDS Neptune Viewer to see if any of the moons were in positions where they could cast shadows on the planet. Sure enough, a few of those iffy identifications coincided with Naiad and Thalassa approaching the limb of the planet.

Once I had identified a few shadow transits of Despina, Thalassa, and Naiad, I generated a table to predict the times that their shadows might fall on the planet. To do this I simply found an image where the moon's shadow was close to 2nd or 3rd contact, then multiplied by the moon's orbital period. Table in hand, I went through and checked those times against what Voyager was taking pictures of to see if the camera was pointed at the planet during a transit period, and if so, if it got a picture of the moons' shadows tracking across Neptune. I expanded this second pass through the data to include wide-angle camera images starting on the 16th. During this second pass I turned up a few more transits.

Here's a sampling of what I turned up:

Despina's shadow, followed about 10 minutes later by Thalassa's shadow. The gif is enlarged by 2x for better visibility. Here I've applied a 3px unsharp mask to increase contrast and bring out some subtle detail in the clouds:

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Thalassa and its shadow on the planet at the same time. The inset is 2x the original resolution. I just stretched the histogram here, unsharp masking didn't really bring out any additional detail:

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Naiad's shadow on August 24 through the wide-angle camera. The inset is also at 2x, and has had a 3px unsharp mask applied to increase contrast between the shadow and the clouds.

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In addition I was able to identify the spots on Daniel Macháček's photo in the OP as the shadows of Despina and Thalassa, with a guest appearance by Thalassa itself.

My final tally:

18 transits of Despina, with the first observable on August 8. There were 6 more transit events between August 7 and August 24 where Voyager was observing Neptune when Despina's shadow was crossing the disk, but the images were either blurry or the shadow wasn't detectable. I may have seen a shadow in two of these events, but it was only present in a couple of images and may just be noise. The image sequence that Ted Stryk discovered, c1138023-56, appears to be the only set that captured Despina or its shadow through the narrow-angle camera within a day before Voyager 2's closest approach.

9 transits of Thalassa, with the first observable on August 16. There were 4 additional events between August 15 and August 24 where Voyager was looking at the right time, but 3 of these were taken through the wide-field camera on August 22-23 and I wasn't able to see Thalassa's shadow in any of the images taken during this time. No Thalassa transits are visible through the narrow-angle imager within a day before Voyager 2's closest approach, but frustratingly a narrow-angle image, c1136125, would have captured a high resolution image of a Thalassa shadow transit had it been taken about 10 minutes later.

9 transits of Naiad, with the first observable on August 17. There were 4 additional events between August 15 and August 24 with Voyager looking at the right place at the right time. It appears that Naiad's shadow was just below the detection limit in three of these, and the fourth was only visible when Voyager was taking UV images, which have a really low S/N ratio. No Naiad transits are visible through the narrow-angle imager within a day before Voyager 2's closest approach, although it does appear twice in wide-angle images taken on the 24th. The highest resolution views may be the last narrow-angle images to see Naiad's shadow, c1124332-58, back on August 20.

Finally, I was unable to find any shadow transits of Galatea visually, and without that I was unable to generate table where there were windows of opportunity for Voyager to see its shadow. I did search around in Neptune Viewer time periods between August 15 and 18 when Galatea was approaching the limb of the planet and was in position to cast its shadow, but didn't turn up anything. It seems that the shadow either grazed the northern limb of the planet and lasted less time than the gap between images (usually 9 minutes) or missed the planet entirely. No ring shadows were seen, and as Ted Stryk suggested in his conference paper, they are likely well below Voyager's resolution limits.

Not sure where to take this from here. The shadows are much too small and fuzzy to really do much to improve the shape model of these moons, although I suppose they might be usable in refining the moons' orbits if they had some precise astrometry done. Mostly just enjoyed the challenge of finding these things as far back in Voyager's encounter as possible.
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Ian R
post Mar 23 2015, 12:39 AM
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Fantastic work!


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tedstryk
post Mar 23 2015, 01:16 AM
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I worked through these images with Mark Showalter after finding the intitial set. The images are certainly interesting, although given the long baseline with Hubble, they unfortunately didn't prove very useful.

Nice work with the images.


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