While the National Science Foundation’s Daniel K. Inouye Solar Telescope will stare at the Sun to detect magnetic fields, it first has to focus on objects far away – other celestial objects visible only in the night sky.

The Integration, Test and Commissioning (ITC) team with the DKI Solar Telescope (DKIST), led by Predrag Sekulic, ITC Optical Group Manager, recently began initial pointing tests and focusing experiments with the recently installed primary and secondary mirrors. These experiments included night-time observations of stars, planets and the Moon.

“The observatory ultimately will operate only during daytime hours to observe magnetic fields on the Sun,” says Mark Warner, project manager for DKIST. “But during the integration and commissioning process we’re not ready for the full Sun. So, the construction team uses a variety of night-sky objects to align and calibrate the telescope including stars, planets, and the Moon.”

DKIST’s first target at primary focus, located at a point between the primary (M1) and secondary mirror (M2), was the bright, double star, Castor in the constellation Gemini. The tests performed by the ITC Team show the Telescope Mount Assembly and primary mirror are successfully pointing within required specifications.

“The images demonstrate that the telescope optics have now been aligned and calibrated to where it has reached seeing limited imaging performance,” says Thomas Rimmele, DKIST Principal Investigator and Associate Director of the National Solar Observatory.

The DKI Solar Telescope is an all‐reflecting, 4‐meter, off‐axis Gregorian telescope being constructed at Haleakalā Observatories in Maui, Hawaii. Ten large-diameter reflecting mirrors and six large refractive dichroic distribution optics gather, condition and deliver light to a suite of science instruments located in a large, rotating Coudé instrument platform beneath the telescope.

Unlike their night-time counterparts, solar telescopes use a Gregorian design, in combination with heat stops, to help eliminate or reduce the high heat created by focusing on the Sun itself. The primary mirror in this type of reflecting telescope creates a primary focus before the secondary mirror. It is at this location that DKIST’s high-tech heat stop is placed. The majority of the Sun’s light and heat is rejected by the heat stop. The heat stop reduces the 2,000 arcsecond disk of the Sun to just 300 arcseconds and allows for a manageable heat load on the optical elements that follow.

The telescope also includes a sophisticated wavefront control system that helps manipulate the high-order adaptive optics system along with the active, deformable, optics at the primary and secondary mirrors and other critical optical elements. This combination provides diffraction-limited images to the focal-plane instruments at the Coudé science instruments. A diffraction-limited system behaves at the theoretical limit of its ability rather than limited by other factors, such as imperfections in the optics.

The choice of Castor was based on convenience as it was one of the brightest stars in the sky at the time of the test. Jupiter and the Moon were used to visually check corrections applied to M1 and M2.

“The images are the best focus available given setup and the seeing conditions that night. Over the coming months, the ITC team will ‘follow the light,’” says Warner. “They will systematically continue to carefully install and test the remainder of the optics after M3 all the way to the instruments in the Coudé lab. The images from DKIST will only get better and sharper as the team installs more optical components.”

The remaining optics are expected to be installed later this year, with the first images of the Sun expected by fall. The Sun’s surface, atmosphere and magnetic fields will be captured at resolutions that have never before been seen. It will be Sun as we’ve never seen it!

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