Question

Why are telescopes called detectors?

Answer 1

Because you use telescopes to detect planets and contellations and other things.

Answer 2

telescope is an instrument used to gather light radiated from distant objects. A secondary mirror (like the eyepiece lens of a refractor) is often used to channel the light rays to a detector.

Our eyes are detectors, but in astronomical studies an electronic device is often used instead.

so it is called detectors .