At what photon energy does the index of refraction start to increase again ?
The most recent generation of gamma - ray telescopes ( 2000s ) have a resolution of the order of 6 arc minutes in the GeV range ( seeing the Crab Nebula as a single " pixel " ) , compared to 0.5 arc seconds seen in the low energy X-ray ( 1 keV ) range by the Chandra X-ray Observatory ( 1999 ) , and about 1.5 arc minutes in the high energy X-ray ( 100 keV ) range seen by High - Energy Focusing Telescope ( 2005 ) .