An astronomer looks at them and sees optical parts that need to be mated properly to any other optical instrument being employed, such as a telescope or pair of binoculars. Understanding pupils is important for skywatchers who want to know which telescope eyepieces or binoculars to buy. It is also the key to some relatively unknown aspects of visual observing.
Pupils come in two types.
The entrance pupil refers to the aperture through which light enters an optical instrument such as a telescope. The exit pupil is a small circle just behind a telescope through which all emerging light rays pass. You can see it as a little disk of light floating in the air behind the eyepiece when the instrument is pointed at a bright surface, such as a wall or a daytime sky. This disk is an image of the telescope's aperture. Its size equals the aperture divided by the magnifying power.