The night sky is not all that dark. Even far from cities, it glows faintly — which hampers astronomers trying to catch weak flashes from distant cosmic rays. Ground-based gamma-ray telescopes, such as MAGIC in the Canary Islands, use large mirrors and sensitive detectors to catch tiny blue flashes — Cherenkov radiation that occurs when ultra-high-energy particles slam into the atmosphere.
Earlier, telescope cameras used photomultipliers — vacuum tubes that amplify faint light. Now silicon photomultipliers (SiPMs) have appeared — semiconductor chips that catch light more efficiently. But they have a quirk: they see red and infrared light better, and the night sky glows more strongly at those wavelengths. So the new sensor collects both more useful signal and more noise.
But there is another path. Instead of amplifying a weak signal, you can count individual photons directly, like a digital camera. Such arrays of many tiny avalanche diodes (SPADs) are already being developed in labs. They promise to squeeze the most out of every photon and, perhaps, make future gamma-ray telescopes even sharper.
The idea is simple: the better a detector distinguishes useful light from night noise, the deeper the telescope can see into the Universe. It is like listening to a quiet voice in a noisy room: you need not just a sensitive microphone but a smart one that filters out the hum.
Silicon photomultipliers are so sensitive that they can detect a single photon, like spotting the light of one match from a hundred kilometers away.