Reprocessing Historical Images, Looking for REALLY big challenges? Options

[tedstryk]

"A crescent Mars... that's one of the most beautiful images you've given us, thank you."

Viking Orbiter 2 got a series of nice crescent views on approach to Mars. One has a nice plume-cloud trailing downwind from Olympus or one of the 3 Tharsis big'uns: Curly, Larry, or Moe.

I have actually been playing around with those. I haven't quite figured out Viking color yet, which is why I haven't done anything with them. But, if we are talking about the same series, it is a somewhat lower phase angle.

[edstrick]

The Viking 1 approach image set (approx 90 deg phase) was incomplete and some data may have been lost. It wasn't complete and maybe not available around 1979 when the "Mars Consortium" was perparing simple-cylindrical map projected global Mars datasets for cross correlation. Also, some image data was significantly overexposed.

The Viking 2 data was better but at higher phase angle, not quite as large as the 2 Mars 3 images. Exposures were good. The calibration files used when they were processed for the Mars Consortium had bad shading corrections and put significant color blotches in some of the data.

Viking orbiter color filters were violet, blue, green, orange <called "red"> minus blue <=yellow> and clear. The vidicon tubes' long wave response went to zero in the middle of what the human eyeball calls "red", resulting in an effective "orange" filter response, with a very large overlap with the passband of the green filter. Violet and green had near zero overlap. Usually, color sequences included orange, green and violet frames (or I think orange, green, clear, and violet). Because of filter wheel step-sequencing, blue was rarely used. The green band can be closely approximated by some weighted average of like 70% orange and 30% violet or 80/20. It contains very little independent color information because of a combination of low color contrast between green and the flanking wavelengths, bad overlap with the orange band, and the 7 bit digitization of the Viking images. If you wanted to make a more true color approximation to "natural" color with VO images, other than adjusting the color balance to darken "green" relative to "orange", you could subtract some green from the orange band to give a more "red" weighted response, then boost the "reddend" band's signal to an appropriate value.

[tedstryk]

Thanks edstrick. That really helps. I couldn't for the life of me figure out what the filters actual response was. The references to the orange filter as "red" really confused me (even with Voyager, the documentation calls orange "orange").

Ted

[tedstryk]

Quoting in her blog Emily Lakdawalla "Ted apparently has a masochistic streak. He seems to delight in working with some of the most ornery data sets. "

Never really thought of it that way, but probably true

[edstrick]

"...I couldn't for the life of me figure out what the filters actual response was. ..."

There is a quasi-published (it was paperbound, at least) Viking Orbiter Camera calibration and information guide. I have a copy somewhere, but digging into the "stacks" with my present overload is more than I can plan at the moment. I'll try to find it maybe this weekend and at least get a title for it.

[Bjorn Jonsson]

Viking orbiter color filters were violet, blue, green, orange <called "red"> minus blue <=yellow> and clear. The vidicon tubes' long wave response went to zero in the middle of what the human eyeball calls "red", resulting in an effective "orange" filter response, with a very large overlap with the passband of the green filter. Violet and green had near zero overlap. Usually, color sequences included orange, green and violet frames (or I think orange, green, clear, and violet).

When working with Voyager and Galileo images I have found that to make the color more realistic I need to use B images instead of V. If they are not available I make synthetic B from G and V images (or even O and V). Galileo didn't carry a B filter so in that case I always need to make synthetic B. Voyager had a B filter but sometimes only V images are available. The color improvement by using B instead of V is somewhat target-dependent. For example you get a big improvement for Io, a significant one for Saturn but not a very big one for Jupiter. See my experimental renderings page for details (scroll down to "Io's color" and "Jupiter's color").

I haven't attempted Viking Orbiter color processing so I don't know how the Viking dataset compares to Voyager and Galileo and how Mars compares to Jupiter, Saturn and Io but I wouldn't be surprised if something similar applied there as well.

[ugordan]

Bjorn: off-topic, but regarding Io's color; I believe that upper left Galileo-based image might actually be very close to "true" color. The only thing that might be slightly off are some areas that are too bluish (essentially the color variation should be smaller). A while ago I did a crude try to use VIMS spectra to get the color for some of the galileans. Its accuracy is questionable given the sheer distance the spectra were taken and also the noise problems of the spectrometer, but the result is distinctively yellow for Io. Europa and Ganymede turned out much less colorful. Here's what I got. Now, this is not to imply Europa and Galileo are whitish, the data quality drowned out their intrinsic color, but Io's yellowness still stands out and that's noteworthy.

Edit: There's also this Cassini-Galileo release that shows Cassini's view also as very yellowish, though I don't recall seeing any Cassini RGB sets for Io in the PDS so it might have been simply adjusting to the Galileo colors.

[Bjorn Jonsson]

I saw your VIMS images some time ago but had forgotten about them. Here's what I get when I blur and resample my Io images:



Interestingly, the images in row 2 are more similar to your image than the images in the top row. The implication is that Io's color should be roughly intermediate between the 'Galileo color' and my 'best color' although this is almost certainly an overinterpretation of the very limited existing data.

I'm not very familiar with New Horizon's instruments but cannot but wonder if any 'VIMS-like' color observations are possible...

[elakdawalla]

They said in the press conference yesterday that their plans included spectroscopy with the "highest spectral resolution ever" on the Galilean satellites, and that they'd also manage to get some imaging coverage at higher resolutions than Galileo in some spots that Galileo missed. I don't know whether that's just infrared spectroscopy or also visible. The Ralph instrument does both, though if I read it correctly there is less spectral resolution in the visible than the infrared wavelengths.

--Emily

[tedstryk]

I have redone the global view taken while the dust storm was still in progress. I think the newer version is much cleaner.

[Phil Stooke]

Going back to an older post on this thread, about nomenclature.

The names of albedo markings are still official for telescopic use, but it's rare (and would be confusing) to combine the albedo feature names with the topographic names on a single map.

However, virtually all of the non-crater features take their names from the old albedo feature names. Acidalia Planitia roughly coincides with Mare Acidalium, for instance. Even where this is not so obvious, careful comparison with old maps will often turn up the corresponding albedo feature. A very good guide to this is Jurgen Blunck's "Mars and its Satellites" (Exposition Press, 1982, second edition). The historical lists of old maps in it are also very useful.

Phil

[dvandorn]

Perhaps actual topographic features and gross albedo markings ought not be on the same maps. But I definitely think that some form of albedo-marking map ought to be officially recognized. As long as amateur skygazers on Earth look at Mars through their 'scopes, they ought to have the pleasure of saying that they saw Syrtis Major, or Sinus Meridiani, when they glanced through the eyepiece. Or, at the very least, have the pleasure of saying that they saw *something* they can name, as opposed to "you know, that big dark triangular feature" or "the big dark thing that looks like a claw-ended arm, right along the equator."

For me, the great reaching claw of Sinus Meridiani has always been the "face" of Mars, the look that defined the planet in pre-space age telescopic images. The fact that one of our plucky little rovers is sitting at the tip of the Meridiani claw is just cool beyond words, to me...

-the other Doug

[Phil Stooke]

Well, just to clarify, I mean the two sets of names are not usually combined on one map, but IAU does have official nomenclature for both albedo and topographic features. Many maps combine albedo and topo features, but they are usually only shown with the topo names. USGS has global maps of topography with and without albedo. And of course ALPO and others routinely map Mars albedo features at every opposition, since they change with time.

Phil

[tedstryk]

I have added a Phobos-2 Thermoscan pan to my site. It is a false color view, combining an "RGB" creation using thermal infrared as red, near infrared/red as blue, and a synthetic green with some VSK color data to give it a "Marsy" look. I also did a lot to correct for the variations in the Thermoscan data caused by the fact that the morning side was much darker than the evening side. I am still tweaking it, but it is nearly a finished product. I removed the shadow of Phobos, which appeared as a streak through the center of both images (because Phobos-2 was in a similar orbit to Phobos at the time), because due to the fact that the intensity of the streak varied based on the amount of time an area had been in eclipse in thermal infrared, while in visible/near infrared it was simply a matter of illumination, the color results were garish. I have posted a small version below, and hyperlinked it to a full-size view.

[Bob Shaw]

Ted:

Delicious! Do you make a point of letting the Russian chaps see this sort of image, or does it just leak out once on the WWW?


Bob Shaw