Dark Energy Spectroscopic Instrument

The Dark Energy Spectroscopic Instrument (DESI) is a multi-object spectrograph measuring redshifts of luminous red galaxies, quasars, emission line galaxies, and Lyman-alpha QSOs, in order to create a 3D cosmological map for dark energy research. It images targets simultaneously using 5000 fibers positioned with a robotic fiber positioner. It resides on the Mayall 4m Telescope at Kitt Peak National Observatory (KPNO) and began its planned five-year survey in April 2021 after some delay due to the pandemic. An early release of data was announced in June 2023 and DESI DR1 data was released in March 2025, along with several papers based upon the as-yet-unreleased DESI DR2 (which are said to support the notion that dark energy is not merely a cosmological constant).

The DESI Legacy Imaging Surveys are not DESI observations, but are three preliminary surveys to identify promising targets for DESI. A final data release was announced in January 2021.