Imaging atmospheric Cherenkov telescopes are equipped with the largest optical mirrors in the world and the fastest photodetectors. Their primary goal is the study of the universe at photon energies above few GeV (VHE range). This technique has in fact opened a new astronomical window in this range during the last decades thanks to instruments such as MAGIC. VHE astronomy will experience a new breakthrough with the CTA observatory. But the design of Cherenkov telescopes also enable them as intensity interferometers. In this alternative mode of operation, they detect photons in the visible range but with an angular resolution below 1 miliarcsecond, allowing studies of stars and their surroundings such as winds and disks. I will show how we are applying this second technique in MAGIC and how we propose to extend it to CTA. I will also speculate on future initiatives, which may allow to reach even higher resolutions, for the first time down to tens of microarcseconds at visible wavelengths.