The Collaborative International Dictionary of English v.0.48:

Heliometer \He`li*om"e*ter\, n. [Helio- + -meter: cf. F.
   h['e]liom[`e]tre.] (Astron.)
   An instrument devised originally for measuring the diameter
   of the sun; now employed for delicate measurements of the
   distance and relative direction of two stars too far apart to
   be easily measured in the field of view of an ordinary
   telescope. Heliometric

WordNet (r) 3.0 (2006):

heliometer
    n 1: an instrument used to measure the angular separation of two
         stars that are too far apart to be included in the field of
         view of an ordinary telescope