HET Goes “Back to Nature”

Fort Davis, Texas -- McDonald Observatory staff are cutting garage-door-sized holes all around the enclosure of the 9.2-meter Hobby-Eberly Telescope (HET), and fitting them with giant steel “venetian blinds” called louvers. This should allow the HET, one of the world’s largest telescopes, to operate at the same temperature as the outside air.

“You can’t beat Mother Nature,” said HET Chief Engineer John Booth. “This should go a long way toward allowing the HET to produce sharp astronomical images.

“The problem is heat. As the temperature drops outside at night, the walls of the dome keep the air inside warmer than the air outside,” Booth said. “When the warm air and cool air mix, at the location of the dome opening, they bend the light rays coming into the telescope. This mixing creates what we call bad ‘seeing’ -- it blurs the images of stars and galaxies we want to study.”

So after consultations and tests, McDonald Observatory decided to ‘open up the dome,’ to allow wind to blow through the building. This is being done by cutting 15-foot by 17-foot windows in the bottom half of the building, called the ring wall. The first cut was made on January 8. After each opening is cut, a pair of louvers – which looks like a huge venetian blind with four blades – is lifted by a crane attached to the rotating portion of the dome. Each pair of louvers weights 5,000 pounds – about the same as a large sport utility vehicle. The first louver was lifted into place on February 1. In all, 26 louvers (12 pairs and two singles) will be installed at the HET by May.

This is the first stage in the $500,000 HET dome ventilation project. Smaller heat sources, including a few instruments on the telescope itself, will also be shielded or moved out of the dome. Later, smaller louvers will be installed in the upper, geodesic part of the dome.

The Hobby-Eberly Telescope is a joint project of The University of Texas
at Austin, The Pennsylvania State University (Penn State), Stanford
University, Ludwig-Maximilians-Universität München, and
Georg-August-Unversität Göttingen.