Largest solar telescope

The largest solar telescope is the Daniel K. Inouye Solar Telescope (USA), which has a primary mirror diameter of 4 m (13 ft 1.48 in). The Daniel K. Inouye Solar Telescope is located in the Haleakalā High Altitude Observatory Site, which is located near the summit of Haleakalā, an inactive volcano on the Hawaiian Island of Maui. The telescope saw its first light on 29 November 2020, and was declared complete on 22 November 2021.

The mirror's actual diameter is 4.24 m, but this includes a 12-cm masked-off area around the edge, which does not contribute to the telescope's optical power. Even this inner diameter is still more than twice as wide as the next biggest solar telescope. The mirror was polished to a 20-Angstrom surface roughness – meaning that it has no bumps or deeps that measure more than 2 millionths of a millimetre from top to bottom.

The Daniel K. Inouye Solar Telescope is capable of imaging the Sun multiple times a second, revealing features on the solar surface three times smaller than anything seen by previous solar observatories.

More than seven miles of piping distribute coolant throughout the observatory to try and reduce the heat created by focussing sunlight into an image.