All telescopes perform basically the same function. They gather as much light as possible from a faint astronomical object, and focus the light onto a detector. Historically, the gathering of light was done by a lens or concave mirror, and the detector was a photographic plate, or simply the astronomer's eye. to study the invisible universe, the mirrors and the detectors in a particular telescope need to be highly specialized to detect the part of the electromagnetic spectrum they are designed to study. Four such telescopes follow.