You searched for:
“aperture”
aperture
The clear diameter of the light-gathering surface of a telescope.
For a refractor telescope, it is the diameter of the objective lens, and for a reflecting telescope, it is the diameter of the primary mirror.
This entry is located in the following unit:
Astronomy and related astronomical terms
(page 2)
A hole or an opening. (2)
This entry is located in the following unit:
Word a Day Revisited Index of Cartoons Illustrating the Meanings of Words
(page 12)