Currently under construction on the Pacific Island of Maui, the 41.5 m tall Daniel K. Inouye Solar Telescope (DKIST) will be the world’s largest solar telescope. Once operational, the DKIST will be able to provide the sharpest views ever taken of the solar surface, which will allow scientists to learn even more about the Sun and solar-terrestrial interactions. The DKIST will allow astronomers to resolve the extremely small, violently active, magnetic fields that control the temperature of the corona and the solar wind that produce flares and x-ray emissions, and help to improve prediction of the way these “space weather” phenomena influence the earth.

The telescope will be built atop the Haleakala volcano on the Pacific island of Maui, which was chosen from a list of 72 possible global locations after 2 years of monitoring daytime seeing conditions. Haleakala has the darkest clearest skies, and its tropical location and elevation mean that the telescope sits above the turbulent inversion layer so there is little turbulence to blur its view or moisture to block the infrared spectrum.

At the heart of the telescope is a huge 13ft (4m) primary mirror which, when combined with adaptive optics technology that reduces the amount of blurring from Earth’s atmosphere, produces images 33 times sharper than those of common telescopes. The resolution of the DKIST is comparable with space telescopes, but at a much lower cost and with the benefit of greater accessibility. Unlike a space telescope, it will be relatively easy to upgrade the technology of the DKIST throughout its lifetime.

A solar telescope-specific problem is the heat generated by the tightly-focused sunlight. Unlike most large ground-based telescopes, which are used at nighttime to capture a small number of photons from distant astronomical bodies, the DKIST will spend its working life pointed directly at the sun, absorbing large quantities of focused light and heat energy.

A Heat Stop is an integral part of the design of solar telescopes, and represents one of its larger engineering challenges. It performs the role of what is called a “field stop” in a conventional telescope, limiting the field of vision to the area with minimal distortion. Located at the prime focus, the Heat Stop prevents unwanted solar disc light from heating and scattering on subsequent optics. In a solar telescope such as the DKIST, in addition to blocking light, the Heat Stop must also dissipate huge amounts of thermal energy.

For the upcoming DKIST, the heat load is 2.5 MW/m2, reducing the heat load on subsequent optics from an enormous 12 kW to a minuscule 300 W (a reduction factor of 40). Designed by Thermacore, the Heat Stop Assembly is actively cooled by an internal system of porous metal heat exchangers that dissipate approximately 1,700 W at peak operating load (see side box for a discussion of porous metal heat exchangers).

The Heat Stop must not only be able to survive this heat load (without cooling, the Heat Stop reflector would last only about 30 seconds before catastrophic failure), but also must remain cool enough not to induce any additional turbulence inside the telescope’s dome.