first light

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 is 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.

The term engineering first light is sometimes used, which underscores the point that the telescope is not yet ready for research. Other qualifiers might be used to indicate some particular milestone in the telescope's deployment that first produces some type or class of observation data.

The term first-light instrument is sometimes used: this may refer to some basic instrument ready for the initial tests, but sometimes appears to refer to the instruments provided around the time that science observation begins.

The term first light is also used to indicate light from the very first stars in the universe. Such stars are a current observation goal.

Another astronomy-related use of first light is to indicate the time of day when light first appears, i.e., the beginning of dawn.