This is a colour global view of Ganymede, taken using Orange, Blue and Violet images (in the RGB channels respectively) from Voyager 2. I made it using ISIS around 1998 or 1999, it's an orthographic reprojection of a mosaic of the antijovian hemisphere. I don't think I stretched the histograms in the component images at all.

I wish I could remember what specific images I used (I'm reasonably sure that I was using the images that start around C2063105). Annoyingly I can't find my notes on it - I'm lucky I actually managed to find the image itself! . I think there were around 39 images in total (about 13 for each filter)?

I do remember that I had to manually match the images - I didn't have IDL or any of that other fancy stuff, working in a UK university that didn't have the funds to buy it. I looked at each adjacent/overlapping image pair in a given filter, looked for a minimum of three match points between the images, and noted down the pixel locations for all of them in a little black book and in a text file that ISIS would read, and then did that several times for each overlapping image pair - it took weeks to find all the match points, and sometimes ISIS would still just not match the darn thing properly and so I had to go back and find different ones. As it is it nearly broke the computer I used to make it (which of course was great for the time and completely laughable by modern standards), and I think the end mosaic (in original .cub format) was over a gigabyte in size! I'd have to leave the ISIS script to take it from level 0 to level 4 running overnight and hope it all worked. I also used updated SPICE data for Voyager that I got from Tim Colvin at RAND Corporation (that IIRC wasn't in ISIS at the time). The resolution of the mosaic is 2 km/pxl.

Probably not awesomely useful for science, but I'm pretty proud of it considering the effort it took to make it - hope you like it .
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>>I wish I could remember what specific images I used (I'm reasonably sure that I was using the images that start around C2063105). Annoyingly I can't find my notes on it - I'm lucky I actually managed to find the image itself! . I think there were around 39 images in total (about 13 for each filter)?

yep those are the numbers. it was a 3-color 6-frame mosaic (18 total). i dont know why it didnt cover the poles. the basic color patterns of ganymede are in this mosaic: the reddish color of dark terrain, the different colors of the dark rays, even the bluish polar caps in a few spots. i used it in the Atlas to fill certain areas but used lower resolution Galileo color because that included IR data globally.

One of my Ganymede mosaics uses that set with wide angle data for the poles.

Only 18 images? Seemed like a lot more! (maybe I'm thinking of how many overlapping pairs there were, or something).

So barring the slight difference that using OBV filters instead of RGB ones would make, is this actually close to the colours that Ganymede would have if we were there looking at it with our own eyes? There doesn't seem to be much of a colour/brightness difference between "bright" and "dark" terrains (the ejecta around Osiris is much more noticeably brighter though).

It's pretty close, I'd say. FWIW, I ran your image (great mosaic, btw!) through some code to interpolate the entire visible spectrum (linearly between OBV wavelengths) and convert to sRGB:

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The brightness is due to gamma-correction, the actual color is similar to yours, only less reddish/brownish.

Very nice! Thanks for doing that, I've often wondered if there was any software that could interpolate between the filters.

its about as close to natural color (at high resolution) that we can get right now. kind of a pale milk chocolate. The moons not made of green cheese after all. we could sell land parcels on the chocolate moon!

Hooray! I just managed to find all my old Galileo and Voyager image mosaics and notes (I thought I'd lost them all)! I even have the scripts I used to make some of them in ISIS too (complete with matchpoints)!

So I may be posting a few more mosaics here, if they'll fit (and the scripts, if people want them) .

Thanx!!

Since the scripts are just text files and shouldn't take up too much space, I say post 'em. I have fantasies that I'll get back into using ISIS for image processing but I'll have to get myself a Mac first...

Great mosaic, especially when keeping in mind that it was made more than 10 years ago. Reminds me of when I made my first 'real' (and big) planetary map towards the end of the 20th century - it was of Ganymede. My computer was completely swamped, a lot of time was spent on calculation runs and it crashed several times. Now I could make a *much* bigger map with ease and much more quickly.

Needless to say, seeing the scripts would be interesting. There are some ISIS users here.

Yeah, me too (well, the getting back into ISIS part, not the Mac part!). Unfortunately I then remember how horribly painful installing Linux and trying to get ISIS to work with it is... I had a quick look for the (old) versions of Linux that ISIS claims to be compatible with and I can't even find the installers for those anywhere anymore (and it sounds like it has problems running in the newest versions of SuSE).

So, the attached zipfile contains the script for the mosaic I posted. The "colourRAND.txt" in the zip file is the script itself - I'd advise that you have a look through it and check that it's all good, as I'm not sure if it's still compatible with the current version of ISIS. I also do some housekeeping in the script (deleting files) so make sure that's OK too. The zipfile also contains three input.txt files which are used in the "noseam" step of the processing. And there's also the RANDganymede.rmb file, which is a SPICE file that I got from Tim Colvin at RAND used in the "spicelab" step - I'm not sure if this is still necessary, but at the time it was better than the SPICE data in ISIS.

The script will take the original Voyager 2 IMQ format images, turn them into .cub format, update the labels with the SPICE data, remove the reseau marks (lvl 1), reproject them to sinusoidal (lvl 2) centred on 170° longitude with a resolution of 2.3 km, run noseam to make global orange, blue, and violet mosaics, output raw files of the mosaics, and then make global mosaics centred on the leading, trailing, and antijovian hemispheres.

You'll need the following Voyager images, available from http://pds-rings.seti.org/vol/VG_0025/GANYMEDE/C2063XXX/ :

c2063105.imq
c2063113.imq
c2063121.imq
c2063123.imq
c2063131.imq
c2063101.imq
c2063109.imq
c2063111.imq
c2063119.imq
c2063127.imq
c2063129.imq
c2063059.imq
c2063107.imq
c2063115.imq
c2063117.imq
c2063125.imq
c2063133.imq

I'd be interested to see if this works on other peoples' machines

(EDIT: Oh yeah, and make sure all the imqs and the text files are in the same folder, and that you are running the script from the same folder too).

Funnily enough, I still have some email correspondence with you about making those maps .

This is the global mosaic of Ganymede's trailing hemisphere that the script makes (at least I hope it's trailing, I get leading and trailing mixed up!) - note that this doesn't include any other images, it's just a reprojection of the antijovian mosaic:

And here's the leading hemisphere mosaic (huh. Interesting. I thought I did this in a separate post but it seems to have combined them - the one on the left is the trailing hemisphere, the one on the right is leading):

I was going to start a new thread for this but I think for now I'll keep posting my stuff here.

I also made a global colour mosaic of the sub-jovian hemisphere of Ganymede from the Voyager 1 images, but that proved more problematic, largely because some of the blue filter images were blurry. I made global mosaics using orange, green, blue, violet and UV filters, and here's what I came up with (details are all in the captions within the images).

The OBV (Orange/Blue/Violet) is the same filter combination as my Voyager 2 mosaics posted above, but you can see the issue with the blurry blue filters in the top left part of the image.

To correct the blurriness (aesthetically anyway), I cheated a bit - I took the Orange mosaic, fiddled with the brightness/contrast a bit (went into Photoshop > Adjust Brightness/Contrast, and set -10 Brightness, -5 Contrast) so it looked kinda like the Blue one, and put the modifed orange mosaic in the Green channel of the image to replace the blue one that was there. So technically this mosaic is Orange/Modified Orange/Violet in the R/G/B/ channels.

The result is that the blurriness isn't there anymore, but it's no longer accurate in appearance. Still, it doesn't look too bad IMO.

The next attempt is actually as close to true colour as we're going to get here - a colour mosaic using Orange/Green/Blue in the R/G/B channels. Unfortunately I only could find a strip of Green filter images so the central part of Ganymede is the only part that's in (nearly true) colour. I really like how this one looks, it's just a shame that more of Ganymede wasn't covered by all three channels.

All this talk of filters makes me wonder - why did they not include a red filter in the ISS, so at least we could get some true colour pictures? (that said, looking at the filters here - http://pds-rings.seti.org/voyager/iss/inst...a1.html#filters - it looks like Orange extends into the red a bit too. Though "Green" actually seems to be more like yellow?)

And finally, just for kicks I tried an OGU image (Orange/Green/Ultraviolet), which wouldn't really look like true colour at all and doesn't actually have that much overlap between all three filters (the only part that does is a thin strip on the left of the Green, but it blends in somewhat with the yellow overlap so it's hard to make out).

I'll see if I can dig up the scripts for these. Anyone manage to get the Voyager 2 script working at all?

The vidicons were blind to longer wavelengths. Even with the "orange" filter (which does sample some red color), the dropoff curve is most likely driven by detector sensitivity, not filter bandpass. Potentially a bigger problem with using Voyager OGB images directly as RGB is that the "green" filter actually skirts close to orange color (slightly less the case with Cassini). Since most objects are spectrally red, this causes their color in the direct RGB representation to have an excessive green tint if you don't compensate with channel mixing or interpolation. See for example some of the Voyager OGB Jupiter images.

Cool. That looks like the set I used in this one.

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I often try to compensate when possible by mixing green with blue (or, if I must, violet).

Very nice! What filters did you use for that? I thought I'd used all the available images in those filters in mine, but maybe I ran out of steam and missed a few.

EDG, any idea why you have color shifting in the 3-color footprints near the terminator? It looks to me the green and blue filters are darker there than their neighboring footprints, causing some reddening there. For fun I tried coaxing your O+UV color near the left limb to the OGB in the center and I can get them to match more closely than the actual OGB footprints agree.

Are you referring to what looks like some of the component images near the terminator being different brightness/contrast to the others? (e.g. the voyager image on the bottom right of the green mosaic looks darker than the surrounding images)? I'm not sure why that's like that - I think the images might not have been calibrated properly relative to eachother? I'll see if I can find my scripts for this one and see if there's any clue to what's going on in there.

Yes. I assume all the frames were taken at roughly the same time so the terrain illumination as well as phase angle are pretty much constant? It looks like a calibration thing. I guess I didn't expect to see that much calibration uncertainty with ISIS, although I've never really worked with it or Voyager images in general.

See the red strip near the terminator and a yellow-green bit below it - the part of the terminator that looks ok is a synthetic red channel.
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I am trying to find my notes...I did that image three or four years ago. I'm pretty sure I used OG(B+UV) where I could. Where I couldn't, I used whatever was available and shifted it to match the color in wide angle shots.

Yeah, I see that... I wonder if Ted ran into the same issue when he made his mosaic? I don't see the same problem in his image. I'm still looking for my script for this one, maybe that'll reveal an explanation.

I didn't notice that problem.

I've found the scripts for the Voyager 1 mosaics, but they are broken down into individual filters. It also looks like I did it as a two-stage process - I have one script that updates the SPICE labels with the RAND data using spicelab, and then I have the script that makes the mosaic (which refers to the rmb file created in the first part of the process). Each filter's zipfile contains a RAND#.rmb that's already been generated. Does anyone know if this is even necessary anymore, or has the SPICE data been updated to something more accurate in the years since I did this?

So here's the scripts for each filter. The randcolour.zip file is the SPICE label updater but for all the images involved in all the filters that I use here - it just puts the updated data into a single file called RAND.rmb, instead of separate rmb files for each filter. Unfortunately I don't have a single script file that you can edit to make all the mosaics at once.

As before, I'd advise looking through these files and checking that they'll work in your system (e.g. the cd2isis command in RANDcolour.txt script refers to IMQ files mounted locally, so that won't work without being changed). You'll also have to look at the scripts to see what IMQ files you'll need (easiest way is to check the input2.txt files) And use the scripts at your own risk .

Titan has veiled wonders and Io suppurates, but to my eye from a distance Ganymede is just the most beautiful moon in the solar system. Thanks to all for the magnificent images.

Yeah, I quite like how it looks too .

EDG Have you done other moons?

Europa maybe?