You searched for: “aperture”
Word Entries at Get Words: “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.
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)