For a ground-based Cherenkov radiation detection, what are the theoretical/practical constraints on the gamma-ray energies we can detect? Simply speaking, if we have something like an IACT observatory, how do we know it's low and high energy detection limits? Is the higher one due to Cherenkov radiation shifting away from optical/UV region as incident gamma-ray is more energetic (meaning that such a bust still can be detected by X-ray observatories)? What about the lower one? For IACT, it's typically about 50-100 GeV, any particular reason for that? And do we encounter pair-production mechanisms other than γ → β- + β+? Also, what happens to the positron? Does it produce Charenkov radiation or is it annihilated too fast for that? Just to be clear, I'm asking about photon-caused Cherenkov radiation only, it'll be fun to hear any of these neutrinos/protons/electrons/whatever else scenarios too.