Daniel K. Inouye Solar Telescope, Solar telescope at Haleakalā Observatory, Maui, Hawaii, US

The Daniel K. Inouye Solar Telescope is a research facility on Mount Haleakalā on Maui that focuses on observing the sun with advanced optical systems. The primary mirror measures about 4 meters across and works with wavelengths ranging from visible light to near-infrared.

The contract to build the telescope was awarded in 2010, with a planned completion in 2017. Physical construction on Haleakalā began in January 2013, and the telescope housing was finished that September. The 4.24-meter primary mirror arrived on the night of 1–2 August 2017. First science observations began on 23 February 2022, and the telescope was named after Daniel K. Inouye, US Senator for Hawaii.

The name honours Daniel K. Inouye, a US Senator for Hawaii and the first Japanese American to serve in the Senate. Since 2018 there have been calls to rename the telescope over allegations about him. The facility is run by the National Solar Observatory with funding from the National Science Foundation, and 22 institutions were collaborating as of 2014.

The telescope stands at about 3,000 metres on Haleakalā, where daytime weather is clear but conditions shift fast and the air is thin. The summit road climbs through several climate zones, so allow time to acclimatise and carry warm layers. The building itself is a working research facility and is not open to the public.

The primary mirror is a 4.24-meter disc cast from Zerodur by Schott and polished at the University of Arizona's Richard F. Caris Mirror Laboratory; its outer 12 cm is masked, leaving a 4-meter off-axis aperture. The 75 mm thick mirror is an off-axis section of a 12-meter f/0.67 concave parabola, a shape that avoids a central obstruction and keeps scattered light low.