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 will improve its capabilities and provide an even more powerful platform for HETDEX.
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.
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.
A series of upgrades will increase HET’s capabilities for HETDEX.
A new tracker assembly at the top of the telescope will incorporate new optics that greatly widen its field of view. Currently, the telescope has a field of view that is about four arcminutes wide, which is about one-seventh the diameter of the full Moon. The new assembly will increase the view to about 22 arcminutes, which is three-quarters of the Moon’s diameter. That means that during each observation, HET will see an area of the sky that is more than 30 times greater than it sees today.
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 will be larger and heavier than the existing unit, and will accommodate new electronics and other critical telescope systems. The telescope’s support structure will be strengthened to support the tracker’s extra weight.
The upgrades will not only accommodate HETDEX, they will improve the telescope’s overall science capabilities for many other research projects.