Hobby-Eberly Telescope

With its 11-meter (433-inch) mirror, the Hobby-Eberly Telescope (HET) is one of the world's largest optical telescopes. It was designed specifically for spectroscopy, the decoding of light from stars and galaxies to study their properties. This makes it ideal in searching for planets around other stars, studying distant galaxies, exploding stars, black holes and more. The telescope is especially suited to conduct large survey projects using spectroscopy.

The control room of the HET. The control room of the HET.

Dedicated in 1997, the HET is a joint project of The University of Texas at Austin, Pennsylvania State University, and two German universities: Ludwig-Maximilians-Universität München, and Georg-August-Universität Göttingen.

Astronomers submit proposals to use the HET for their research projects, and they are chosen through a competitive process. However, the projects are not scheduled for specific dates. Instead, the HET uses a queue scheduling process. The projects are chosen for completion within a four-month period. Each night, the telescope operator decides which project is best suited to be carried out, based on factors like priority ranking of the projects, weather, and Moon phase. Queue scheduling makes sure that the HET is used efficiently, and makes the telescope especially well suited to studying targets of opportunity -- those events in the heavens that arise without warning, such as exploding stars.

The telescope's unique design allowed the partner universities to build a very large modern telescope at a fraction of the cost of similarly sized instruments. Unlike most other telescopes, which tilt up and down in altitude, the HET's mirror is always tilted at 55 degrees above the horizon. However, the tracker mounted above the telescope moves in six directions, allowing the HET to study 70 percent of the sky visible from its location (6,640 feet up, atop Mount Fowlkes in the Davis Mountains of West Texas).

The segmented mirror of the Hobby-Eberly Telescope.The segmented mirror of the Hobby-Eberly Telescope.

The telescope's mirror looks like a honeycomb. It's made up of 91 hexagonal mirrors. To make good observations, the 91 segments must be aligned exactly, to form a perfect reflecting surface. The mushroom-shaped tower to the side of the HET dome contains a laser-alignment system that works to keep the segments in proper alignment. The mirror segments form a reflecting surface that is 11 by 10 meters. However, the HET is known as a 9.2-meter telescope because that's how much of the mirror is actually in use at any given time. This makes the HET, scientifically speaking, the third largest telescope in the world.

The HET is designed to use three main instruments: The Marcario Low-Resolution Spectrograph (built by an international consortium of institutions under the direction of Dr. Gary J. Hill of McDonald Observatory), a Medium-Resolution Spectrograph (built at Pennsylvania State University under the direction of HET project scientist Dr. Larry Ramsey), and a High-Resolution Spectrograph (built at The University of Texas at Austin under the direction of Dr. Robert Tull).

The distinctive features of the HET dome are designed to regulate the environmental conditions inside the dome. The louvers, which look like garage-door-sized Venetian blinds surrounding the dome's base, allow air to flow freely through the telescope structure. The chrome coating on the dome also helps regulate temperature.

Telescope Facts

Primary Mirror

Composed of 91 hexagonal mirror segments Effective Diameter: 9.2 meters Actual Diameter: 11 meters Weight: 13 tons Thickness: 52 mm Material: Schott "Zerodur"

Telescope Weight

80 tons

Dome

Diameter: 86 feet Weight: 19.9 tons

Design

Construction started: 1994 Construction completed: 1996