My Ganymede mosaic, Made by hand, around 1999 Options

[EDG]

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.

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[EDG]

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?)

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[EDG]

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?

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[ugordan]

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?

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.

[tedstryk]

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

[tedstryk]

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).

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

[EDG]

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

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.

[ugordan]

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.

[EDG]

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.

[ugordan]

Are you referring to what looks like some of the component images near the terminator being different brightness/contrast to the others?

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.

[tedstryk]

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.

[EDG]

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.

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.

[tedstryk]

I didn't notice that problem.

[EDG]

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 .

Attached File(s)

 globalblue.zip ( 5.88K ) Number of downloads: 60
 globalgreen.zip ( 4.5K ) Number of downloads: 48
 globalorange.zip ( 4.75K ) Number of downloads: 49
 globalUV.zip ( 3.8K ) Number of downloads: 65
 globalviolet.zip ( 5.9K ) Number of downloads: 57
 randcolour.zip ( 6.31K ) Number of downloads: 70

 

[ngunn]

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.