Telescope obstruction data

Nearly every telescope seems to have some parts of the sky that are inaccessible. Trees, mountains, and even other telescopes obscure some areas above the horizon. Furthermore, many telescopes have peculiar mountings or other mechanical limitations on where they may look. In a surprising number of cases, the inaccessible regions extend into the parts of the sky one would normally use to measure extinction stars. Such restrictions should be placed in a separate ``obstruction'' table for each telescope.