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:Click to view attachment
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:Click to view attachment
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.Click to view attachment
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.