Historically, the "pixels" are mostly round photomultiplier tubes. So the natural way to place those is a hexagonal grid. o o o o o o o The

Historically, the "pixels" are mostly round photomultiplier tubes. So the natural way to place those is a hexagonal grid. o o o o o o o The pixels are very sensitive, we can register single photons with a time resolution of half a nano second. Quite recently, some telescopes started using silicon photo multipliers, which are mostly square by design. But: The first telescope to use those also used light concentrators to increase the detector area and these have hexagonal entrance windows, so we also have hexagonal pixels. Physicists like symmetries, and light concentrators are more effective when round than square, hexagonal is a good middle ground.