n.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
英汉双解大词典
heliometer/ˌhiːlɪˈɒmɪtə/
1.
N（名词）
a refracting telescope having a split objective lens that is used to determine very small angular distances between celestial bodies 太阳仪