The term First Light refers to a milestone in the construction and deployment of a (research) telescope: the first time an image is viewed or recorded through it, demonstrating the optics of the telescope is in working order. This may happen some time before scientific observations are first carried out, e.g., before final tweaks to the optics, or instruments are installed, or the facilities readied for science work. There may be some variation in the use of the term. The term is generally used for telescopes even if they are operating out of the visible range, for an analogous milestone when necessary, but occasionally an analogous phrase is used. It may also be used for detectors other than telescopes, such as Neutrino detectors.
Discovery Channel Telescope (DCT)
European Extremely Large Telescope (E-ELT)
Gemini Planet Imager (GPI)
Large Millimeter Telescope (LMT)
Telescopio Nazionale Galileo (TNG)