Features in the Gallery: Space Frisbees?

This image from 2009-04-27 22:10:22 seems to show a lot of disks in the sky. What’s going on here?

Large Scale ROSAT image

The image is taken from the ROSAT pointed observations. The ROSAT mission comprised two phases. In an initial phase the satellite scanned the entire sky producing the data SkyView shows in the RASS surveys. After the all-sky survey was done, ROSAT made much deeper observations of particular points in the sky. Each observation looked at a circle roughly a degree in radius. These observations are the source for the ROSAT PSPC surveys in SkyView. We took all of the observations and added them up to create all sky tiles. Only about 20% of the sky was covered in total, so a very large scale image, like this one, shows the observations as little disks in the black, unobserved, background.

The GALEX survey is very similar, though the overall sky coverage is substantially higher. However while we created a set of static tiles for ROSAT, with GALEX we add the observations together dynamically when the user makes a request. A big part of the reason is that computers have gotten much faster in the 10 or so years since we built the ROSAT surveys, so its much more feasible to do this kind of dynamic addition of data today.

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