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Pluto Surface Observations 3: NH Post-Encounter Phase, 1 Feb 2016- TBD
MarcF
post Jul 17 2018, 09:42 AM
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The image processing techniques also progressed a lot and I'm convinced that the Voyager images still hide some "secrets". A nice example is in the second paper I've mentioned earlier which compares the topography of Charon's Oz Terra with the surface of Ariel, moon of Uranus, both showing similar disrupted crustal blocks (Fig. 19 & 20).
The reprocessed Voyager images are really impressive.
Marc.
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JRehling
post Aug 16 2018, 05:10 PM
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There was some discussion here of true color pictures of Pluto from NH. What I can promise here is the lowest resolution true color picture of Pluto. From the "I'll do it my damned self" files I (after a few less successful attempts) took a true color image of Pluto on August 6.

This was taken with a C 9.25", ASI-1600mm, and RGB filters, I took 40 exposures x 5 seconds in each of RGB on an object that I had verified to move between August 5 and August 6. Then after combining the color frames, I subtracted the background light pollution RGB from the entire image. And voila, Pluto in color. It is the beige dot below the white dot (star) near center.

By a very lucky bit of happenstance, it is flanked on either side by very deeply (and opposite) colored stars of comparable brightness, one red and one blue, which gives some assurance that there wasn't a bias that made all dim objects appear to be one color or another.

The center of my Pluto has an average RGB of about 68, 67, 59. For comparison, a "true color" image from New Horizons, blurred and set to similar brightness has an RGB of about 83, 68, 55. So we're only talking about a couple of bytes of information in this photo, and that's being generous, but at least I can tell you, this is straight from the real thing.
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Holder of the Tw...
post Aug 16 2018, 06:59 PM
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Doesn't that also contain the color of Charon mixed in?
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fredk
post Aug 16 2018, 08:15 PM
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QUOTE (JRehling @ Aug 16 2018, 06:10 PM) *
There was some discussion here of true color pictures of Pluto from NH. What I can promise here is the lowest resolution true color picture of Pluto. From the "I'll do it my damned self" files I (after a few less successful attempts) took a true color image of Pluto on August 6.

Nice.

How is the white balance set? A simple test would be to image some daylight target (eg white cloud) and see if you get equal R, G, B.
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JRehling
post Aug 16 2018, 09:08 PM
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Yes, Charon is included, so this should be roughly 20% Charon and 80% Pluto.

My RGB filters are from Baader and are rotated into the optical path of a monochrome ASI-1600 camera. Because the images are taken sequentially over a period of minutes, one possible source of noise is that a wispy cloud could have passed through the line of sight and altered the gain of one or two of the channels differentially. That's not highly likely, but not impossible. I've often used this same configuration to image Venus, for example, and I could use that data (it comes out quite white) to validate the color balance of the gear.

I think the lucky inclusion of quite red and blue stars flanking it close by help constrain the error, because stars only get so red and stars only get so blue, and a skew in either direction should have whitened one of those stars. I'm not sure if I can find any data on those specific stars that could be used to ground my measurements. There are brighter stars in the image (this was cropped from a 16 megapixel image and we're looking right into the plane of the Milky Way) so I could try to see if any of the stars in the image have a spectrum on record.
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fredk
post Aug 16 2018, 10:52 PM
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QUOTE (JRehling @ Aug 16 2018, 10:08 PM) *
I've often used this same configuration to image Venus, for example, and I could use that data (it comes out quite white) to validate the color balance of the gear.

That's reassuring - I've never noticed Venus to appear other than white by eye (at least when it's high enough that reddening from Rayleigh scattering is negligible).

In principle there will be white balance correction factors that the RGB channels must be multiplied by, which will depend on the bandpass of the filters and response of the sensor.
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