  We describe a system that builds a high dynamic-range and wide-angle image of
the night sky by combining a large set of input images. The method makes use of
pixel-rank information in the individual input images to improve a "consensus"
pixel rank in the combined image. Because it only makes use of ranks and the
complexity of the algorithm is linear in the number of images, the method is
useful for large sets of uncalibrated images that might have undergone unknown
non-linear tone mapping transformations for visualization or aesthetic reasons.
We apply the method to images of the night sky (of unknown provenance)
discovered on the Web. The method permits discovery of astronomical objects or
features that are not visible in any of the input images taken individually.
More importantly, however, it permits scientific exploitation of a huge source
of astronomical images that would not be available to astronomical research
without our automatic system.
