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Voyager and Galileo Images of Ganymede, The Ganymede images and mosaics thread
Bjorn Jonsson
post Jun 9 2016, 06:12 PM
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Awesome mosaics. The huge global mosaic is probably a bit distorted though but is a great overview of Ganymede's anti-Jupiter hemisphere. It's easy to see from these mosaics that the Voyager 2 Ganymede coverage is really good and the data quality is much higher than in the Voyager 1 data set (in particular there are no smeared images). The only 'missing' thing is really hi-res images - Voyager 2 got no closer to Ganymede than about 60,000 km, corresponding to a resolution of 600 m/pixel.

Here is a new 4000x3800 pixel color mosaic of a large area around Osiris from Voyager 2 images:

Attached Image

North is up and the color should be fairly accurate - possibly a bit too reddish and saturated but Ganymede's color from Voyager images is a complicated subject and I'm still working on it. This mosaic is composed of 21 clear filter images obtained by Voyager 2 at a range of 91,000 to 118,000 km (resolution 0.9 to 1.2 km/pixel). Its northern half has been colorized using a three frame color mosaic obtained at a range of a little over 300,000 km (resolution 3 km/pixel). The southern half (starting from just south of Osiris) has been colorized using a single wide angle color composite obtained when Voyager 2 was ~75,000 km from Ganymede (resolution ~5.6 km/pixel). Some of the lower resolution color coverage is included in the mosaic outside of the area covered by the higher-res clear filter images. I mosaicked everything in simple cylindrical projection and also did all of the color processing there and then rendered the resulting map using the viewing geometry Voyager 2 had when it obtained its best image of Osiris (image C2063759).

Ganymede exhibits big albedo differences and this can complicate the image processing. I took great care not to overexpose/saturate any parts of the brightest terrain (Osiris in particular). This preserves every detail but makes the terminator area a bit dark.

A huge variety of interesting terrain can be seen in the mosaic. With the benefit of hindsight I can safely say that there are clear hints of the extreme roughness of some terrain types covered by some of the higher resolution Galileo images but there is also terrain that looks smooth in the Voyager mosaic. I look forward to seeing Osiris in hi-res JUICE images and also the bright smooth-looking terrain around the bright crater SSW of Osiris.

I'm going to end this with yet another version of a well known global Voyager 2 image. This version should be of slightly higher quality than my earlier versions of it.

Attached Image

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Ian R
post Jun 9 2016, 07:01 PM
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Stunning Bjorn! ohmy.gif Can't believe this imagery came from a TV camera!

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post Mar 6 2017, 03:42 AM
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QUOTE (4throck @ May 27 2016, 06:43 AM) *
That darker detail, so well centered on the disk, seems a bit like a processing artifact. Or your secondary mirror ;-)
Extreme processing on images so small (compared to the theoretical resolution) can be tricky.
The limb's contrast with the dark background can be enough to create a "doughnut" or ring effect.

But if you can get more images and do a rotation movie, I'd be convinced ;-)

This challenge sat in the back of my mind for 9 months, until Jupiter opposition came back around. I had good seeing three nights in a row last week, when Galileo Regio happened to rotate into and out of view My animation, paired with Solar System Simulator plots, is attached.

These are images taken with my 150mm NexStar 6se and a broad blue filter, stacking 1500 frames. Images taken with a green filter look quite similar. Images taken with a red filter, however, do not show Galileo Regio. I've now noticed this on four different dates – one last year and three this year. After examining Cassini images of Ganymede, I find no reason for that in terms of Ganymede's actual color/terrain. Here's my conclusion: I have just enough resolution to capture Galileo Regio in shorter wavelengths. In red wavelengths (~40% worse diffraction-limited resolution), I can't capture it, so my color image shows Galileo Regio as red, which is a total artifact.

Nevertheless, I feel assured now that I have imaged Galileo Regio, which pretty much maxes out the resolution I can hope for with my gear.
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post Mar 6 2017, 08:59 PM
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QUOTE (JRehling @ Mar 5 2017, 09:42 PM) *
...Nevertheless, I feel assured now that I have imaged Galileo Regio.

Resolving Galileo Regio with an amateur scope is surprisingly feasible. Back in the mid-80s, on a dark West Texas night with amazing seeing, I resolved Ganymede as a disk with a 6" reflector with added Barlow lens (680X is popping into my head-- pretty ridiculous magnification) . I noticed a smudge on one portion of the disk, and sketched it, then later verified based upon Voyager maps and Ganymede's orientation that the smudge was Galileo Regio. Just about my single toughest observation. I've recently seen some rather amazing astrophotography of it with amateur scopes, but I haven't done anything serious myself since 1990.

What you've shown here is quite persuasive.
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