An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

1) Someone or something that observes.
2) Idealized person or piece of equipment, often hypothetical, that measures relevant properties of a physical system.

Agent noun, from → observe + → -or.

A place located either at the top of the tube of a large telescope from where one observes or at the back of the tube where instruments are attached to the → Cassegrain focus.

→ observer; → cage.