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Nearly every telescope seems to have some parts of the sky that are
Trees, mountains, and even other telescopes obscure some areas above the
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.