The image is a mosaic of "postage stamp" regions taken during the engineering test run this spring by NASA's Kepler spacecraft operating in the two-wheel K2 mode. Highlighted in the image are two open clusters Messier 35 (top) and NGC 2158 (bottom).
The shape of the image is due to the fact that K2, like Kepler, only brings back "postage stamp" regions surrounding targets of interest. For single stars, the postage stamps are small, but for clusters, it is often the case that large super apertures are used and placed together to collect light in a large grouping of pixels, often of odd rectangular shapes, made to fit the sizes of the clusters.
Learn more about the K2 mission in the Kepler mission manager update.
Credit: NASA Ames/T Barclay/W Stenzel