Keck II Telescope Peers into Uranus’ Depths Using Adaptive-Optics


High signal-to-noise images of Uranus from Keck II. Two specially processed sets of images of Uranus taken from the Keck telescope on July 25 and 26, 2012, reveal astounding level of detail in Uranus’ clouds. NASA / ESA / L. A. Sromovsky / P. M. Fry / H. B. Hammel / I. de Pater / K. A. Rages.

These images showcase the fine-scale structure of the atmosphere of Uranus, and contain vastly more detail than other images. The images were taken by the adaptive-optics-equipped telescope, the Keck II, which employed its NIRC2 camera.

Near the equator, there is a rhythmic wave structure in a cloud belt. Near the north pole, the mottled texture of high clouds is juxtaposed with atmospheric holes that look similar to IR views of Saturn’s pole. Several image processing techniques were required to tease out these details, which are usually very subtle but coherent.


This image of Saturn’s north pole was taken by Cassini’s VIMS spectrometer at a mid-infrared wavelength of 5 microns. It was winter at Saturn’s north pole; all illumination is thermal radiation (heat) welling up from Saturn’s depths. Some of the heat radiation is blocked by clouds floating in Saturn’s atmosphere at about 75 kilometers below the cloud tops that can be seen in visible wavelengths. The pressure at that level is about three times Earth’s atmospheric pressure. The patterns in the image are created by alternating cloudy and clear areas. The image has been contrast-reversed so that the glouds show up as bright spots, while open areas appear dark. NASA / JPL / U. Arizona

It’s common in astronomy to take large numbers of images at a rapid rate, and then stack them up in order to bring out details. This stacking involves carefully aligning the images, and then averaging them. This helps reduce the effects of random noise and sharpens them.

Since Uranus rotates fairly rapidly, the astronomers had to “reproject” the images onto flat maps, and then convert each image from an orthographic to a cylindrical map projection. The images were then shifted to compensate for Uranus’ rotation. Uranus’ surface is a hydrodynamically circulating atmosphere, which has winds that move at different rates at different latitudes, so the astronomers had to further shift the images.

After this elaborate process, the images were stacked. Most of the variation in pixel values in their stacked composite was from broad-scale atmospheric banding. They applied a high-pass filter to remove the broad features, leaving only smaller, more local features.


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