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The Planetary Society Blog
By Emily Lakdawalla
Infrared map of Jupiter
Mar. 27, 2007 | 09:36 PDT | 16:36 UTC
Most of the images that we've seen of Jupiter from New Horizons have come from the LORRI camera. These images are produced in a way that's very easy for the layman to understand: the spacecraft frames a target and takes a picture on a square CCD in a process that's not fundamentally different from the way anybody on Earth takes a photo with a point-and-shoot camera. Obviously it's a little more challenging to frame an image from a fast-moving spacecraft with a 0.3-degree field of view than it is to snap a photo with your cell phone, but the basic idea is the same. Point, click, and here's your photo. Jupiter, Io, and GanymedeOn January 24, 2007 at 04:41 UT, As New Horizons approached for its encounter with Jupiter, it snapped this photo of the giant planet with two of its attendant moons, volcanic Io (left) and darker Ganymede (right). Ganymede casts a shadow on Jupiter's disk. The image was taken from 57 million kilometers (35 million miles) away. Credit: NASA / JHUAPL / SwRI |
The process is usually not the same for imaging spectrometers, which take data in two spatial dimensions plus one spectral dimension, acquiring a "cube" of data rather than just a rectangular photo. I thought I understood how these things work pretty well. Most imaging spectrometers also use CCDs, just like cameras. Since there's no such thing as a cubical CCD that could "snap" a photo in the two spatial and one spectral dimensions simultaneously, imaging spectrometers have to work differently. What they usually do is have a field of view that is in the shape of a slit, a slit that's as long as one spatial dimension on the CCD but only one pixel wide. The light that comes in to that slit is fanned out -- think of the prism on the cover of Pink Floyd's Dark Side of the Moon album -- so that the "rainbow" it produces is projected onto the other spatial dimension of the CCD. In other words, a single "image" on the CCD records the full spectrum simultaneously for every point along a one-pixel-wide view of the target. In order to build up an image that's more than one pixel wide, the imaging spectrometer's viewing slit is swept across a target like a pushbroom; every time the slit moves one pixel in distance, another spatial-spectral "image" is taken. When you're done taking images, you can assemble all the photos into a cube, so that all those one-pixel-wide lines are stacked next to each other like books on a shelf; then you look at all the spines of the books to see an image in one wavelength. You can take a slice through the cube to get a snapshot of the target in a different wavelength. You can take three slices in different wavelengths and merge the three into a pretty color picture. That works, because for any given point in the image, all the spectral (color) information at that point was gathered at once, in one spatial-spectral photo.
So when I saw this image release from the LEISA imaging spectrometer, my first instinct was to try to take those three pictures and merge them into a psychadelic color view.Jupiter in the infraredIn addition to its LORRI camera, New Horizons also carries an imaging spectrometer called Ralph. Ralph has two components, one of which, called LEISA, was used to acquire these three views of Jupiter on February 24, 2007. Each view, taken at a different wavelength, penetrates to a different depth in Jupiter's cloudy upper atmosphere. Light at 1.53 microns (center) comes from the highest levels; bright cloud structures in that image reach to the highest altitudes. One such structure is Oval BA, also known as the Little Red Spot, at lower left. Credit: NASA / JHUAPL / SwRI |
I expected to be able to produce a picture that looked something akin to this Gemini photo, which is composed of three images taken in different infrared wavelengths; the Great Red Spot and Little Red Spot are bright because they're at high elevations, the same reason that Little Red is bright in the LEISA image.Jupiter's two red spots approachThe adaptive optics-equipped Gemini telescope snapped this view of Jupiter on the night of July 13, 2006. The images were taken in near-infrared wavelengths, in which the "red spots" show up as white. For several months, the smaller of the two red spots (known as Oval BA) has been approaching the Great Red Spot, and is here passing it to the south. The fact that Oval BA is nearly as bright as the Great Red Spot in infrared wavelengths suggests that its elevation approaches that of the Great Red Spot, whose clouds soar 8 kilometers (5 miles) above neighboring cloud bands. Credit: Travis Rector, Chad Trujillo, and the Gemini ALTAIR adaptive optics team |
Try as I might, though, I could not get the three different-wavelength images from LEISA to line up to make one pretty image. There seemed to be a twisting of Jupiter from one wavelength band to the next; I was pretty sure that I was seeing Jupiter rotate between wavelengths. But if that were true, that would mean that all the color information for each pixel was not acquired simultaneously, which means that LEISA does not operate in the way that I just described. Scratching my head, I finally had to send an email off to the LEISA principal investigator, Dennis Reuter, asking him why the pictures didn't line up. Here's what he told me:LEISA is a wedged filter spectrometer, which means that different spectral elements are taken at different times. In a wedged filter spectral imager, each row of the detector array effectively corresponds to a different wavelength, and the spectrum at any point is formed by combining data from several frames (in this case, ~250). In our detailed processing, we remove the spacecraft drift and planetary rotation, but the images in this initial release have not been fully processed, so there is relative motion present. The whole LEISA observation took about half an hour, plenty of time for Jupiter to rotate quite a bit underneath LEISA as it built up its spectral information. So in order to make pretty pictures from LEISA, the team first has to account for how New Horizons moves over time, and then has to account for how Jupiter rotates over time -- and, on top of that, Jupiter's clouds and storms are moving relative to each other over time! LEISA will have a much easier time producing pretty color postcards of targets that are not quite so dynamic.
I looked around to see if there were any other spacecraft flying a wedged-filter spectrometer. I couldn't find any active ones, but I noticed that the HySI instrument planned for Chandrayaan-1 uses that design.
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