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Voyager 2 imaging of Triton
post Aug 23 2014, 09:34 PM
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QUOTE (ugordan @ Aug 23 2014, 11:19 AM) *
In my experience, calibration with Voyagers is an issue. Compared to Galileo and Cassini images of Jupiter and Saturn targets, it seems to me the relative filter brightnesses are off for all cameras. Green channel typically ends up being too bright compared to other filters which, along with the O/G/B filter combination, can lead to greenish results. A while back I attempted to use Saturn targets like Mimas, Enceladus, Dione and Iapetus to figure out better correction factors, with mixed results. Here's how a quick-n-dirty Triton O/G/B image turned out using those factors and also corrected for OGB -> sRGB filter wavelengths using linear interpolation:


Source images: C1138639, C1138651, C1138703

that is interesting to hear. I wondered about that but we have no other reliable color data for Triton for comparison. perhaps you can send me some of your estimates for how far off the colors might be. in the meantime, i can look at some saturn data. a difficulty s that the cassini and voyager filters are not identical.

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Bjorn Jonsson
post Aug 24 2014, 05:03 PM
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I have the same experience as Ted and Gordan, I always need to use various correction/fudge factors to get correct color when doing color composites from the Voyager images. As Gordan mentioned, the green channel is usually too bright. I usually use the calibrated and geometrically corrected images available at the PDS Planetary Rings Node but I get similar results if I start with the raw (*.IMQ) images and calibrate them myself.

I haven't done any systematic analysis to see if this problem varies as a function of time, exposure, gain etc. but I suspect that at least time (or mission phase) is significant. For many targets I can use the global spectrum to check the color but this is especially difficult for Triton since its color varies with time. For Neptune I can use its spectrum plus HST images to check the color and determine the correction factors - using the same processing parameters for Triton might work to get correct color.
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post Aug 25 2014, 12:17 PM
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QUOTE (DrShank @ Aug 23 2014, 11:26 PM) *
....can't wait to put the pluto map next to it next year.

Oh hello there Dr Shank. =)

Yes I really would like to see a comparison between those two also to see if they show any signs of having a common origin.
I always have considered Triton to have been captured by Neptune even when the ejection hypothesis were the more popular one.
A little bit OT for this thread yet some expect Charon to show an interesting surface as well I am a bit pessimistic about that idea, but then again our own moon have recently been found to have a molten core even found today despite the fact it got bound rotation so it might be best never to say never to tidal heating. =)
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Phil Stooke
post Aug 26 2014, 01:54 PM
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Fantastic new map of Triton, Paul, thanks! It is nice to see the post-encounter crescent images incorporated into it.


... because the Solar System ain't gonna map itself.
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post Aug 29 2014, 02:54 PM
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QUOTE (Bjorn Jonsson @ Aug 24 2014, 05:03 PM) *
I haven't done any systematic analysis to see if this problem varies as a function of time, exposure, gain etc. but I suspect that at least time (or mission phase) is significant.

Back at the time I was playing with Voyager images I also noticed that. My guess is that the filter's response changed with time. So it's dependent on mission time, craft and camera.

If there are any images of stars taken during the mission (I guess so) perhaps they can be used to infer calibration.
"White" satellites (if we have some spectra for them) would also work on distant images that show more than one (for different brightness), so that you could linearize the filters response.
Just my 2 cents...

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