What is this rectangular pattern?


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That’s a place where we only got a single exposure in each filter, so you can see the layout of the 62 CCD chips that make up the Dark Energy Camera. Each of those rectangles is one 2048x4096 chip, and there’s a small space between them because of the limits of how they’re physically installed in the camera.

Here’s a picture of the camera (from this article, https://iopscience.iop.org/article/10.1088/0004-6256/150/5/150)

There are some extra chips in the corners used for focusing and guiding (label “F” and “G” in the figure below); the science ones are in that circular/hexagonal pattern.