Bj�rn J�nsson's Voyager 1 Jupiter animation, new and improved
Posted by Emily Lakdawalla
16-05-2011 11:32 CDT
Late last year I posted an amazing video of Jupiter's moving clouds, an animation made from images that Voyager 1 took as it approached. Below is a new and improved version of that animation. The first one was based on 16 Voyager color photos; this one covers a much longer period of time, and includes 58 images.
The description of this movie is basically the same as the earlier version: Every second in this movie corresponds to about 4 Jupiter days or about 40 Earth hours. This movie began as a 58-frame animation of Voyager 1 images of Jupiter. Each of the 58 images was composed by Björn Jónsson from three Voyager frames taken through orange, green, and blue filters, which he reprojected into a cylindrical map, aligned, and then projected back into the slightly squashed spherical shape of Jupiter. (This reprojection step is necessary because around two minutes elapsed between each of the component images, which, because of Jupiter's fast rotation, would result in color ghosting if they were simply overlaid.) Jónsson selected sets of images featuring the Great Red Spot near the center of Jupiter's disk, one per Jupiter day, and reprojected them to maintain a constant position for the Spot. By holding the GRS still he highlights the motions of the clouds that happen from Jupiter day to Jupiter day.
Jónsson took the 58-frame animation and "tweened" it, using software to compute seven frames to fill in the time between each of the original 58 images. The result is a smooth animation of the motion of Jupiter's clouds.
Jupiter may appear slightly "washed out." Jónsson has attempted to reconstruct Jupiter's color as it would actually appear to the human eye, without exaggerating the colors. However, it should be noted that the Voyager camera systems were not sensitive to light in red wavelengths (the longest wavelength they could detect is in a region we'd call "orange"). Since Jupiter is colorful in red wavelengths, attempting to produce "true color" images from Voyager data results in slightly less colorful views than we can see with modern CCDs or our own eyes.
Producing this video required an incredible amount of work. Björn writes: "Because I didn't have any information on the viewing geometry and camera pointing I had to 'reverse engineer' all of that information. I was unable to do it to perfect accuracy but I still think the resulting geometric parameters are fairly accurate. Another source of errors is that the images had to be corrected for geometric distortion and that correction probably isn't perfect, especially where Jupiter is close to the edge of a frame (or even partially outside the frame due to bad pointing). These errors manifest themselves as lots of features that appear to 'wobble' a bit and I'm pretty sure that motion is not a real feature. In fact this looks a bit like an earth-based telescopic video of Jupiter except that Jupiter appears very sharp.
"Some minor but spurious color variations from frame to frame may be apparent. This is because in many cases the source frames had data dropouts and I had to use partially synthetic color. In some cases Jupiter was partially outside the frame due to pointing errors and in one case the pointing was so bad that none of the three images (OGB) could be used. To keep the apparent frame rate constant everywhere I used Sqirlz Morph to generate a totally synthetic frame from the two adjacent frames where this happened. This worked remarkably well and I think it may be almost impossible to spot where I did this."
Fantastic work as usual, Björn!