Moons and rings in color

[mmcirvin]

Two moons (Mimas and, I think, Pandora) and the A and F rings:





Making the color picture here is actually a bit nontrivial because the relative position of the moons and rings varied between the red, green and blue shots. I put the moons at their positions in the green frame. But I didn't adjust the image positions well enough to completely eliminate fringing, and the F ring is also whirling around fast enough to make some color fringing there impossible to get rid of. I thought it was pretty, in any event.

Comments

Thanks for the pretties!

[tomscud.livejournal.com]

No real comment here, just thanks for posting all the pretty Cassini-related pictures.

Re: Thanks for the pretties!

[mmcirvin.livejournal.com]

You're welcome. I actually think that the JPL people do pretty well at getting information out rapidly, and despite the usual carping, I even think ESA did better than usual and better than expected with Huygens. But one thing that we've had less of than we could is color pictures.

Color is, I think, important to catch people's attention, and Cassini's usually taken the shots necessary to make nice color pictures. But actually putting them together takes some manual labor (which doesn't really have a direct scientific payoff), and they're often too busy to spend a lot of time on that.

Also, the colors of Saturn's moons other than Titan are often extremely subtle, and getting them wrong without saying so can terribly confuse people. (Sometimes they get confused anyway; I saw a Washington Post article that claimed that methane gives Titan "a greenish cast", an idea the reporter obviously gathered from looking at false-color infrared pictures, possibly from the imaging spectrometer.)

The great thing about the Internet and readily-available photo editing technology, though, is that amateurs can actually step in and fill the gap to some extent.

Re: Thanks for the pretties!

[iayork.livejournal.com]

Thanks for the pretties from me, too. Not only do I like to see them (and if you didn't blog them, I'd likely forget to check the primary sites), I've been showing them to Wlliam as part of his SCIENCE IS K00L indoctrination.

Re: Thanks for the pretties!

[mmcirvin.livejournal.com]

But, wait, I've been told that this stuff is insufferably boring (http://www.sfgate.com/cgi-bin/article.cgi?file=/chronicle/archive/2005/01/19/DDG5HARL9969.DTL)!

(I know, it's a cheap shot: subtract the general anti-intellectual tone and the guy touches on some actual problems with public science outreach here and there. Maybe I'll write about that someday.)

[tongodeon.livejournal.com]

Where exactly are you pulling the source data and what's your actual role in this? I can't imagine that you're out in the backyard with a digital SLR and a telescope.

[mmcirvin.livejournal.com]

I'm just a nerd with a Web browser and a copy of GIMP.

The data comes from here (http://saturn.jpl.nasa.gov/multimedia/images/raw/index.cfm). The Cassini team decided to follow the lead of the Mars Exploration Rovers and publicly release uncalibrated images from their VIMS camera quickly auto-converted into JPEGs. It satisfies hardcore geeks' hunger for pictures without putting too much pressure on the people who spend their time assembling the pretty pictures for general public consumption. It also means that if you know how to use an image editor, you can colorize them yourself.

The VIMS camera has a 1-megapixel square monochrome CCD that is sensitive to a decent range of infrared, visible and UV wavelengths, and a couple of filter wheels that have several kinds of filters on them that can be applied in pairs (one from each wheel. so they can do things like combining a color filter and a polarizer). The raw image archive listings say what filters were used if you know how to read them. Frequently they'll shoot red-green-blue separations so that realistic color images can be constructed from the results. In other cases they do the IR/green/UV thing to bring out subtle color variations in gray landscapes.

[mmcirvin.livejournal.com]

Brain fart: in the above comment, I was referring to Cassini's ISS (Imaging Science Subsystem), not VIMS (Visual and Infrared Mapping Spectrometer, a different instrument that produces low-res spectrographic images).

..It's an interesting question how accurate these colors are. The Mars rover pictures are usually harder to colorize in this way, because they meter the exposures to use as much of the dynamic range as possible in each color channel. So you have to renormalize the channels with some sort of calibration reference if you want the colors to look anywhere close to right. My experience with the Cassini pictures suggests that with Cassini ISS they don't do that so much, so if you just assume that the RGB channels are close to equally normalized you'll get something that looks moderately correct.

I suspect that the colors in these pictures, when they're not intentionally fake, are considerably better than the ones returned by Voyager 1 and 2. The Voyagers had a vidicon-based imager with a slightly peculiar assortment of filters (orange, green and violet were usually used to make color pictures), and the publicly released pictures, as wonderful as they were, were often wildly oversaturated, making Io look like a red pizza and Titan like a painted supermarket orange. The real colors are somewhat subtler.