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Going Deep

HET mirror

Hobby-Eberly Telescope’s segmented primary mirror spans 11 meters (36 feet). [Martin Harris]

World’s third-largest telescope will provide wide, deep view of distant galaxies

The HETDEX project has two critical technological requirements. One is a set of instruments that can quickly gather and analyze the light from many distant galaxies at once. The other is a giant telescope that can see far into the universe, providing good views of galaxies that existed when the universe was only a few billion years old.

The first requirement is met by VIRUS, a set of about 150 spectrographs. The second is met by the Hobby-Eberly Telescope (HET), the third-largest optical telescope in the world. A set of upgrades has improved its capabilities, providing an even more powerful platform for HETDEX.

Special Design

While other large optical telescopes perform a variety of roles, the HET is designed to observe astronomical objects spectroscopically — in other words, to split light from an object into its component wavelengths. Combined with innovative technical approaches, the approach yielded a world-class research instrument at a fraction of the cost of other larger telescopes.

Aerial View of Hobby-Eberly Telescope

An aerial view of Hobby-Eberly Telescope, with blue sky reflecting off its primary mirror. [Tim Jones]

In most telescopes, the primary mirror is a single piece of glass. The HET’s mirror, though, consists of 91 identical six-sided segments that fit together like the tiles on a floor. Each segment, which is made from a glass/ceramic material, is one meter across and two inches thick. Small computer-controlled motors attached to the back of each segment periodically adjust the positions of individual segments to maintain the proper overall mirror shape.

The combined segments yield a light-gathering surface equivalent to a single 11-meter (36-foot) telescope. Not all of this surface can be used at one time, though, because of one of HET’s original design features.

Unlike most other telescopes, HET’s mirror is always tilted at the same angle, 55 degrees above the horizon. To compensate for this, a tracking system is installed at the top of the telescope above the primary mirror. As the tracker moves, it captures light from different portions of the mirror. With this system, up to 9.2 meters of the mirror can be used for any given observation.

The telescope rotates between exposures to view different regions of the sky. The combination of this rotation and the tracker’s motion in six directions allows the HET to cover 70 percent of the sky that is visible from McDonald Observatory.

Field of view diagram

This diagram shows HET’s current (right) and upgraded field of view compared to the size of the full Moon. [Tim Jones]

Upgrading for the Future

A series of upgrades has increased HET's capabilities for HETDEX.

A new tracker assembly at the top of the telescope incorporates new optics that greatly widen HET's field of view. Before the upgrade, the telescope had a field of view that was about four arcminutes wide, which is about one-seventh the diameter of the full Moon. The new assembly has increased the view to about 22 arcminutes, which is three-quarters of the Moon's diameter. That means that during each observation, HET sees an area of the sky that is more than 30 times larger than when the telescope was built.

The wider view is critical for HETDEX because the project must survey a large swatch of sky to amass data on enough galaxies to meet its goal of measuring the effects of dark energy at earlier times in the history of the universe. Without the larger field of view, it would take decades instead of years to complete the project.
The new tracker is larger and heavier than the previous unit, and accommodates new electronics and other critical telescope systems. The telescope's support structure was strengthened to support the tracker's extra weight.

The upgrades not only accommodate HETDEX, they improved the telescope's overall science capabilities for many other research projects as well.