Is anyone here familiar (more or less) with the new type of telescope called IACT, Imaging Air Cherenkov Telescope. Apparently TeV gammarays come to us from Active Galactic Nuclei (AGN) which are massive accreting black holes. Individual TeV photons leave blue or UV trails in the atmosphere, like 10 kilometers altitude. You can tell approximately where the original gamma photon is coming from by capturing an image of the trail. And you can estimate its energy. There is a lot of interesting particle physics here. We should discuss this. 1. How can the AGN black hole make such energetic gamma? 2. What reactions occur when the gamma photon comes into our atmosphere, like at 10-20 kilometers? 3. What makes the Cherenkov radiation? Presumably it is some secondary or tertiary particle shower containing thousands or even millions of particles all set in motion by the original TeV photon, and traveling at speeds near the speed of light in vacuo, faster than the speed of light in air. One of these telescopes is called MAGIC. I saw some photos of the parabolic dish. It is over 10 meters diameter, perhaps the biggest optical parabolic dish on earth currently. The idea is you need a lot of light gathering power in order to see these blue Cherenkov trails in the atmosphere.