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Hobby-Eberly Telescope

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The HET is one of the world's largest optical telescopes, with an effective aperture of 10 meters and a 78 square meter, hexagonal mirror array made from 91 segments. Its design is revolutionary. It sits at a fixed elevation angle of 55º, and rotates in azimuth to access 81% of the sky visible from McDonald Observatory (discounting the "high airmass" zone right next to the horizon which telescopes normally avoid). The HET was built for approximately 15-20% of the cost of other 9 meter class telescopes. The tilted Arecibo design, and the cost savings realized throughout the mechanical systems of the HET from the fixed axis concept, broke the standard cost paradigm for large aperture telescopes.

The Primary Mirror

Each of the 91 identical hexagonal segments of the primary mirror array measures 1 meter side to side and weighs about 250 lbs. Alignment across the array is accomplished by triple actuators incorporated into each segment support and adjustable to about 1/3 a wavelength of light (218 nm). The segments are spherically shaped, like a contact lens, and figured to a 26 meter radius of curvature, accurate to within 0.5mm. A sensor located at the center of the segment radius of curvature, 26 meters from the mirror, sits on top of the tower next to the HET, the Center of Curvature segment support Alignment Sensor Tower (CCAS or "see-cass"). At dusk, the telescope is rotated to the CCAS Tower for precise alignment of segments across the array, which is electronically maintained all night.

Because of the precision required of large optical mirrors for quality image resolution, large telescopes of traditional design must compensate for the major distortion effects of gravity as the mirrors are moved through all tilt angles while aiming at different targets. Because the HET is fixed in its vertical axis, its primary mirror is stationary with respect to gravity. This completely eliminates the problem of variable distortion. It also dramatically simplifies the telescope's supporting framework. The trade-off in the HET, however, was that a new solution was required for tracking.

Tracking

Stars move, of course, across the sky, because the earth rotates, and telescopes must track them. This is usually accomplished by attaching a drive to the rotational direction of a telescope (azimuth) and another to its vertical direction of movement (altitude). Together the drives coordinate, the whole telescope moves to compensate for the rotation of earth, and the telescope viewpoint remains precisely positioned on the celestial sky.

 

tracker and ccas towerThe system developed for tracking on the HET has no precedent in a large optical telescope. Objects are tracked by a moving instrument package located 13 meters above the mirror at prime focus. Essentially, on the HET, it is the eyepiece that does the tracking. As a star moves overhead, its light bounces off of the large stationary mirror and the tracker package moves to catch it, always precisely at the exact location of focus.

Accomplishing this is extremely complex. It combines focusing and tracking in a single system. If the primary mirror were flat, movement of the tracker within a single plane parallel to the mirror would be enough for it to accomplish its job. The mirror, however is spherical, so the focal surface is also spherical. The tracker must move within a six axis coordinate system (hexapod) to achieve focus as it tracks. The process requires the simultaneous motion of 10 motors to track a star precisely.

Other instruments also ride on the tracker, which weighs in total about 8 tons. These include cameras, correcting optics, fiber feeds to spectrographic instruments located in the basement below the HET, and the Low Resolution Spectrograph.

Spectroscopy

In 1814, Joseph von Fraunhofer first correctly identified the curious dark bands that are found in spectra when light is passed through a narrow opening and then a prism. The dark areas correspond to a reduction or absence of energy at particular wavelengths, and studying them, a wealth of information can be inferred. This is, in essence, Spectroscopy. It is no exaggeration to say that three-quarters or more of astronomical knowledge would be unknown if the optical spectroscope had never been invented. (Optical Astronomical Spectroscopy, C.R. Kitchin, 1995, p. xi).

spectra-vega vs. sun

The HET is designed for Spectroscopy. It is equipped with three spectrographs, of low, medium and high resolution. The Low Resolution Spectrograph (LRS) is located at prime focus on the tracker. The MRS and the HRS are located beneath the telescope in a climate controlled basement, and are fed by fiber optic cable. The configuration of the spectroscopes enables rapid switching between instruments which makes them well integrated with the observing program of the telescope.

Queue Scheduled Observing

steel balls The HET operates a queue scheduled observing program. Under this system, operation of the telescope, and the acquisition of targets on the nightly observing list is the primary responsibility of the Resident Astronomer, who has flexibility to optimize the observing plan. This mode of operations is particularly well suited to the HET because it makes full use of the entire night for the maximum scientific output. Optimized scheduling overcomes any drawback from the telescope accessing specific areas of the sky only in specific windows of time. The observing program functions superbly for large-scale spectroscopic surveys, rapid follow-up inspections of just discovered, and often short-lived (explosive), astronomical events, and long-term monitoring by periodically re-observing the evolving spectra of some kinds of objects.

truss node Queue scheduled observing brings further significant benefits. It largely "immunizes" participating investigators from bad weather, since the Resident Astronomer has extensive observing time flexibility unlike the irreplaceable time slot awarded to the traditional visitor-observer. Secondly, the observing service provided by a resident observer saves on overall expenses and resources, and avoids equipment-use errors from lack of familiarity on the part of an infrequent visitor-observer.

Amazing Truss

Many systems of the HET are remarkable for their precision and creatively engineered design. The truss supporting the primary mirror array fits this category. The truss was made by MERO Industries, in Wuerzburg, Germany, at extremely low cost. The steel balls shown above were robotically milled into truss nodes, like the example at left, part of MERO's patented connection technique.

The structure is made from 383 nodes and 1,747 struts. Few of the parts are interchangable. The struts were manufactured to a precision of 0.0004 inches, about one-tenth the thickness of a human hair. Before shipment to Texas, the truss was test assembled. Each node in the top surface of the truss was accurate to its theoretical position to better than 0.08 inches.

mirror segments At McDonald Observatory, the pieces arrived in a single truckload and were assembled in six weeks by a MERO crew. Under the dome of the HET, and the 27,000 pound weight of the primary mirror, the truss is deflected just 2.5 millimeters.

Images and Amazing Truss content courtesy of The West Texas Time Machine: Creating the Hobby-Eberly Telescope, Little Hands of Concrete Productions, 1998.