Detector

1. A device that is responsive to the presence or absence of a stimulus.
2. In an AM radio receiver, a circuit or device that recovers the signal of interest from the modulated wave.
3. In FM reception, a circuit called a discriminator[?] is used to convert frequency variations to amplitude variations.
4. In an optical communications receiver, a device that converts the received optical signal to another form.

Note: Currently, this conversion is from optical to electrical power; however, optical-to-optical techniques are under development.

Source: from Federal Standard 1037C

In astronomy, the detecting devices generaly used is charge-coupled devices (CCD), although, before the 1990s, photographic plates were the most common.