In astronomy, first light is the first use of a telescope (or, in general, a new instrument) to take an astronomical image after it has been constructed. This is often not the first viewing using the telescope; optical tests will probably have been performed in daylight to adjust the components. The first light image is normally of little scientific interest and is of poor quality, since the various telescope elements are yet to be adjusted for optimum efficiency. Despite this, a first light is always a moment of great excitement, both for the people who design and build the telescope and for the astronomical community, who may have anticipated the moment for many years while the telescope was under construction. A well-known and spectacular astronomical object is usually chosen as a subject.
For example, the 5.08-metre (200 in) Hale Telescope saw first light January 26, 1949, targeting NGC 2261 under the direction of American astronomer Edwin Powell Hubble. The image was published in many magazines and is available on Caltech Archives.
The Isaac Newton Telescope had two first lights: one in England in 1965 with its original mirror, and another in 1984 at La Palma island. The second first light was done with a video camera that showed the Crab Pulsar flashing.
The Large Binocular Telescope had its first light with a single primary mirror on October 12, 2005, which was a view of NGC 891. The second primary mirror was installed in January 2006 and became fully operational in January 2008.
The IRIS solar space observatory achieved first light on July 17, 2013. The PI noted, "The quality of images and spectra we are receiving from IRIS is amazing. This is just what we were hoping for ..."
In physical cosmology, first light refers to the light emitted from the first generation of hyperstars, formed less than a billion years after the big bang, which brought the cosmological Dark Ages to an end.