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Mirror Coating Smooths the Way for HET Upgrade

The HETDEX project has passed an important milestone with the successful coating of the three largest mirrors in the wide-field corrector, a system that will focus light from the Hobby-Eberly Telescope’s primary mirror and direct it to the scientific instruments.

An aerial view of the Hobby-Eberly Telescope shows the current tracker, which is attached to the hexagonal frame, above the telescope’s main mirror. [Martin Harris/McDonald Observatory]

An aerial view of the Hobby-Eberly Telescope shows the current tracker, which is attached to the hexagonal frame, above the telescope’s main mirror. [Martin Harris/McDonald Observatory]

“The coating met all our specifications and it’s very durable,” says HETDEX principal investigator Gary Hill, the chief astronomer for McDonald Observatory. “There was risk associated with that because we had to transport the mirrors and handle them, and if one of them had broken, the project would have been derailed for several years.”

The University of Arizona Optical Sciences Center is building the corrector, which consists of four mirrors. Two of them are one meter in diameter, a third is 0.9 meters, and the fourth is just 0.23 meters (40, 36, and 9 inches, respectively). The three largest mirrors have received a reflective coating to improve their optical performance, and were returned to Arizona in April. The final mirror will be coated in May, allowing Arizona engineers to complete work on the corrector assembly.

“We can’t safely take the old telescope down until we’re sure that the corrector will make images as it’s supposed to,” says Hill. “So there are a series of optical tests once they align the mirrors to a few microns precision to demonstrate that it will meet its specs when we put it on-sky. Once we have that, we can start dismantling the current HET.”

Engineers will remove the current tracker assembly, which holds an image corrector and other equipment, and replace it with a new tracker that is being completed at The University of Texas at Austin Center for Electromechanics. The wide-field corrector, which will allow HET to view a much larger portion of the sky, will be carried by the tracker.

“All of the [tracker’s] software is done for the motion control system and we’re about to check it against all its specs in terms of how it tracks and moves, so that’s coming along,” says Hill. “Then we’ll start packing the tracker up to send it out [to McDonald] as well. We’ve retired a huge amount of critical risk at this point with the corrector and the tracker.”

Current plans call for the telescope to be shut down this summer to allow for the installation of the new tracker, corrector, and other equipment. The upgraded telescope will begin making scientific observations early next year. Over the following months, more of the VIRUS spectrographs will be installed, eventually bringing the total to 150 instruments.

The spectrographs will gather the light from roughly one million distant galaxies and break the light into its individual wavelengths. By analyzing that light HETDEX scientists will learn about the effects of dark energy on the expansion of the early universe.