If anyone's interested in other unusual (i.e. other than an optical imaging telescope) detectors, have a look at Imaging Atmospheric Cherenkov Telescopes [0]. More details can be found e.g. in [1].

TL;DR: how to detect a very high-energy gamma-ray photon? Either with an orbital observatory (but these have limited size); or let the photon hit upper atmosphere (at a few tens of km), create (together with some atmospheric nucleus) a fast electron-positron pair, which through additional interactions eventually causes a narrow cone of Cherenkov light to shine all the way down to the ground (having ~100 m diameter there), and then gather this light with an array of a few large reflectors. Finally, perform some tricky processing to recover the initial photon's direction and energy.

[1] https://en.wikipedia.org/wiki/IACT [2] https://arxiv.org/pdf/1510.05675