Thursday, August 27, 2009

Processing an image versus reconstructing a view: An example from Galileo

Anyone familiar with my image processing works knows my love of working with the worst of datasets in order to try to create something presentable.  In some cases, the images can be cleaned to the point that the image resembles images that were created from much better data.  In some cases, missing data is found or callibration problems are solved.  In these cases, the data really can be brought up to a higher standard.  At other times, the image, while still limited, looks fine to the eye but has clear limitations under closer inspection - certain areas damaged by smearing or perhaps certain areas missing color data which has to be filled in.

However, there are times when an image is simply a cool or unique view that has such serious shortcomings that any product made from it isn't going to be pretty or even interpretable without using extreme and unreliable techniques to fill in gaps and then tweeking it based on other images of the target.  The result is useless scientifically (the raw image might have some basic value, but with heavy reconstruction it would be dangerous to try to base any interpretations on the reconstruction).  The sole purpose of such images is to get an idea of what the camera was "seeing" when it took the picture in a remote corner of the solar system.

An extreme example of this is a dataset from September 13, 2000, when Galileo was millions of kilometers from Jupiter in its long 28th orbit that would send it to the heart of the Jovian system during the Cassini spacecraft's distant flyby in order to do some synchonized studies and pass close to the Jovian moon Ganymede.   Just before Cassini started its far encounter imaging of Jupiter, Galileo snapped some images of Jupiter and Ganymede.  Galileo, its main antenna having never opened, was forced to spend its entire mission communicating through a small antenna that was only there for emergency situations and for the early days of the mission before the planned antenna opening.  It communicated at a paltry rate of  10 bits/second (up to 100/bits per second when antennae were arrayed).  For comparison, the infamously slow 2400 "baud" modems had an effective data rate of 2400/bits per second (although there are some technical differences between baud and bits per second that are irrelevant here).  In other words, it was ssslllooowww.  A compression algorithm wsa developed so that each navigation image, or "OPNAV," would contain just enough data to locate, for example, the position of a moon or the planet versus a star, for example, and nothing more, although two images had a small "truth window" near the center of the disk just to the left of the terminator.  Here is one of the images in its raw form (please note that to see anything but the "truth window," you will need to click on the image and view it at full size):

Not much there.  Using "maximum" filtering one som of the existing pixels on the terminator and limb, and them patching them based on changing solar illumination to fill in the area inbetween, most of the disk could be reconstructed.

 The missing lower tip was filled in based on the upper part of the disk and a transitional area between that and the nearest available data. Ganymede's tiny disk was only missing a few pixels, which could be interpolated (they were not in the same field of view, so it doesn't appear in the above images).  In the example below, it has also been brightened. 

Back to Jupiter, I removed the artifacts that stretched off the disk, making a smooth limb, and smoothed it to get rid of the blocky look.  The banded appearance looked remarkably good considering the source (it was produced using a combination of all the images, not just the one shown above) and combined with color data from a telescopic photo taken around the same time.  I independently process the the "truth windows" and it fit in nicely in its proper place on the reconstructed disk before adding the color data.   Here is the "truth window" mosaic, shown at 2.5x its actual size to aid visibility and showing some banded structure.  The original data was noisy and only 50x50 pixels per image. 

When finished, I added some artificial noise in order to make the reconstructed image look less cartoonish. I also reduced the entire image to 22 percent of its original size (that was the largest size at which it looked somewhat decent).  However, Ganymede suffered, as you can tell by comparing it to the image I posted of Ganymede alone.  The image is extremely limited, but if one put together a photo-essay on the Galileo-Cassini "Millenium Flyby" of Jupiter, it would be nice to give an idea of the how Galileo appraoched the planet as Cassini approached the daylit hemisphere.  There are similar OPNAVS showing Callisto at about the same size as the disk of Jupiter in this dataset, but since its surface has no convenient banded structure and is covered with discrete craters, the only thing one could do is to fill in the entire image was data from other images, which could be done just as well without even incorporating the OPNAV image.  Jupiter  has cloud bands running east and west, meaning that data along the edges of the disk could be used to fill in the gaps to some degree.   So, here it is, the final version,  The "truth window" is impossible to spot because each "window" is only 11x11 pixels at this size.

Processed images Copyright Ted Stryk, Raw Data Courtesy NASA/JPL

Wednesday, August 26, 2009

Flotsam and Jetsam

In addition to Triton, Neptune has many smaller moons.  In the compilation below, the image on the upper left depicts the moon Larissa and the rest of the images depict Proteus, Neptune's second largest moon.  Proteus is one of the largest worlds in the solar system to not be fairly round, despite the fact that it is similar in size to Saturn's volcanically active moon Enceladus.

The image to the right depicts Nereid, Neptune's third largest moon, which orbits very far from Neptune and therefore was not seen close up.  On the right is tiny Despina, a moon that orbits close to Neptune and is only about 150 km across.  
Finally, here is Despina, transiting the disk of Neptune.  This is a montage showing Despina and its shadow at four positions, each taken nine minutes apart.  The last one shows Despina itself against the clouds of Neptune.

Finally, here is a version of the transit image with Despina artificially brightened to aid visibility.

Processed images Copyright Ted Stryk, Raw Data Courtesy NASA/JPL

More Triton Images

Below I have posted the closest images from the Voyager flyby.  The first image shows where they are located on the disk. Please excuse the "halo" around the top of the image - by mistake I used a very early version of the crescent image to chart out where the images were located. The images range from about 700 to about 450 km/pixel, although in many cases the smearing due low light levels and spacecraft motion significantly limits what can be seen. 

I will finish with a close-up shot from the outbound leg of Voyager's Triton encounter.  While this set is of much poorer quality than the set above, it provides a glipse at a new region of a largely unexplored world.
It will receive a dedicated post of its own.

Processed images Copyright Ted Stryk, Raw Data Courtesy NASA/JPL
So it has been several years since I abandoned this blog. Today is a major day in the history of space exploration, the twentieth anniversary of the Voyager Neptune flyby. While the planet itself was fascination, perhaps the most fascinating part of the flyby was Voyager's visit to Neptune's largest moon, Triton. It was an exhilarating feeling seeing the Neptunian system come to life, a feeling that will perhaps be replicated when New Horizons completes the initial reconnaissance of the solar system at Pluto (regardless of what you think of Pluto's status, when we set out to explore the solar system it was considered a planet, so to claim that our initial reconnaissance is complete because of its demotion feels hollow). I will post some views of Triton today, and will be posting more material today and in the coming days.

This is a sequence showing the changing view as Voyager flew by Triton. The upper tip of the last view is reconstructed from other images because all the images were cut off.

   This is a mosaic using some of the imagery obtained during the closest portion of the encounter.

A wide angle shot showing one of Triton's geysers in the upper left.

Processed images Copyright Ted Stryk, Raw Data Courtesy NASA/JPL