It takes only a glance to see that the Hobby-Eberly Telescope is different from McDonald Observatory’s other telescopes. Its mirror is supported by an open framework, not a closed tube, for example. And instead of a glass doughnut, the mirror itself looks like a polished tile floor, composed of 91 hexagonal segments.
Not surprisingly, a telescope that looks so different also operates differently from the others. It points at the sky differently, it needs some extra steps to bring its targets into focus, and astronomers line up to collect data differently.
One other obvious difference is the techniques used to keep the giant mirror cool. If the mirror is warmer than the cool nighttime air, it creates ripples in the air above it, like the heat waves that shimmer above a desert highway. These ripples blur the telescope’s view.
Several techniques are used to eliminate this problem. The dome is coated with a silver tape that reflects more sunlight than its original white paint. Giant louvers in the bottom of the dome allow more outside air to flow over the telescope. And industrial-strength air conditioning keeps the dome cool during the day.
When the dome is opened at night, HET’s 91 mirror segments must be aligned before observations can begin. Small motors move each one-meter mirror segment tiny amounts to create the proper curvature for the overall mirror. Without this step, tiny misalignments between the mirrors would ruin the mirror’s focus. As the mirror moves and adjusts to changing temperatures during the night, it is periodically realigned to maintain its sharp focus.
HET moves differently from the other McDonald telescopes.
The other telescopes can both rotate in a full circle and tilt from near the horizon to the overhead point. That allows them to view any point in the sky, and to track a target for hours as Earth rotates on its axis and the stars wheel above them.
HET rotates through a full 360 degrees, as the other telescopes do, but it cannot tilt up and down. Instead, it always aims at the same altitude above the horizon. This design greatly reduced the telescope’s cost, but limits its ability to track astronomical targets.
To compensate, telescope designers added a tracking assembly at the top of the telescope, above the mirror. This assembly moves back and forth across the primary mirror, allowing HET to track targets for up to an hour at a time.
The tracker also contains lenses that correct for HET’s intentionally flawed vision. The overall mirror has a spherical shape, like the surface of a ball. This design held down costs because it means all 91 mirror segments are the identical shape, so each one was cast from the same mold.
However, a spherical mirror “smears” the light from distant objects, producing a fuzzy blob instead of a crisp point of light. This is the effect that astronomers saw in the early images from Hubble Space Telescope; a manufacturing defect created a more spherical mirror than planned.
To compensate for its blurred vision, known as spherical aberration, HET uses a similar technique to that devised for Hubble: corrective lenses. These lenses bring the light into sharp focus.
HET is designed for one astronomical technique, known as spectroscopy, which splits light into its individual wavelengths, or colors. This technique reveals details about a target object’s temperature, composition, motion through space, and much more.
Some spectrographs are attached to the telescope itself, while others are located in special climate- and light-controlled rooms in the dome’s basement. Light is fed into these instruments through bundles of fiber-optic cables.
One other major difference between HET and the other McDonald telescopes is the way in which astronomers use it.
For the other telescopes, astronomers travel to McDonald for several days at a time. During that period, the astronomer typically has complete control of the telescope. The astronomer also operates the telescope, staying up all night to point the telescope at the proper targets and collect data.
HET, however, follows a different system. Because of the special skills needed to align and operate the telescope, it is controlled by a telescope operator who lives and works full-time at the Observatory.
The operator points the telescope at a list of targets provided by astronomers. So during most nights, HET looks at multiple targets for several astronomers. Data are archived and sent to each astronomer the next day. The astronomers sometimes journey to McDonald for their observations, but often they stay at home and wait for the data to be delivered to their computers for analysis.
Over the coming years, upgrades in conjunction with the Hobby-Eberly Telescope Dark Energy Experiment will improve the telescope’s performance even more. The upgrades, which will include a completely new tracker assembly, will provide a wider field of view, more stable pointing accuracy, and other performance improvements. They will make HET a more valuable resource for all the astronomers who use it.