The Collaborative International Dictionary of English v.0.48:

coelostat \coe"lo*stat\ n.
   an optical instrument used to follow the path of a celestial
   body and reflect its light into a telescope; it has a movable
   and a fixed mirror.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

coelostat
    n 1: optical device used to follow the path of a celestial body
         and reflect its light into a telescope; has a movable and a
         fixed mirror