The first light is the moment when a telescope is first used to take an astronomical image after its construction. Sometimes the term refers to the first image of a new instrument.

In amateur astronomy first light refers to the first use of the telescope for astronomical viewing. Optical tests have likely already been performed during daylight to adjust the components.

Typically, the first light image is of no scientific interest and has a very poor quality, since the various telescope elements must be adjusted for optimum efficiency. Usually, an important and spectacular celestial object is chosen.

Despite this, a first light is always a moment of great excitement, both for the people who designed and built the telescope, and for the astronomical community.