First light (astronomy)

First light image from the Kepler spacecraft

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 during 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. Usually, a well-known and spectacular astronomical 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.

For example, the 200-inch 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.

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.

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 to an end cosmological dark ages.