Frank B. Ray

Early performance and present status of the Hobby-Eberly Telescope

The Hobby-Eberly telescope (HET) is a recently completed 9- meter telescope designed to specialize in spectroscopy. It saw first light in December 1996 and during July 1997, it underwent its first end-to-end testing acquiring its first spectra of target objects. We review the basic design of the HET. In addition we summarize the performance of the telescope used with a commissioning spherical aberration correlator and spectrograph, the status of science operations and plans for the implementation of the final spherical aberration corrector and facility class instruments.

Development and performance of Hobby-Eberly Telescope 11-m segmented mirror

The Hobby Eberly Telescope features a unique eleven-meter spherical primary mirror consisting of a single steel truss populated with 91 ZerodurTM mirror segments. The 1 meter hexagonal segments are fabricated to 0.033 micron RMS spherical surfaces with matched radii to 0.5 mm. Silver coatings are applied to meet reflectance criteria for wavelengths from 0.35 to 2.5 micron. To support the primary spectroscopic uses of the telescope the mirror must provide a 0.52 arc sec FWHM point spread function. Mirror segments are co-aligned to within 0.0625 ar sec and held to 25 microns of piston envelope using a segment positioning system that consists of 273 actuators (3 per mirror), a distributed population of controllers, and custom developed software. A common path polarization shearing interferometer was developed to provide alignment sensing of the entire array from the primary mirrors center of curvature. Performance of the array is being tested with an emphasis on alignment stability. Distributed temperature measurements throughout the truss are correlated to pointing variances of the individual mirror segments over extended periods of time. Results are very encouraging and indicate that this mirror system approach will prove to be a cost-effective solution for large optical collecting apertures.

Development of a star tracker for the Hobby-Eberly Telescope

A large prime-focus robotic star tracking device has been designed and constructed and is now undergoing commissioning atop the 9.2-meter Hobby-Eberly Telescope at McDonald Observatory in West Texas. The novel, cost-effective tracker represents a major departure in the way very large astronomical telescopes are controlled in pointing, tracking, and guiding. The tracker development and design implementation included detailed structural analysis, the application of minimum constraint kinematic design to a large gantry-type motion control system, and the unique application of a large precision hexapod to solve the dynamic tilting and focus motion problems. Challenging fabrication, test, and on-telescope assembly problems were overcome. Performance data of the completed device demonstrate that the tracker design and implementation efforts were successful.

Hobby-Eberly Telescope low-resolution spectrograph: optical design

The Hobby Eberly Telescope (HET) is a revolutionary large telescope of 9.2 meter aperture, which is currently undergoing commissioning at McDonald Observatory. First light was obtained on December 11, 1996. Scientific operations are expected in 1998. The Low Resolution Spectrograph (LRS, a collaboration between the University of Texas at Austin, the Instituto de Astronomia de la Universidad Nacional Autonoma de Mexico, Stanford University, Ludwig-Maximillians-Universitat, Munich and Georg-August-Universitat, Gottingen) is a high throughput, imaging spectrograph which rides on the HET tracker at prime focus. The LRS will be the first HET facility instrument. The unique nature of the HET has led to interesting optical design solutions for the LRS, aimed at high performance and simplicity. The LRS is a grism spectrograph with a refractive collimator and a catadioptric f/1.4 camera. The beam size is 140 mm, resulting in resolving powers between (lambda) /(Delta) (lambda) approximately 600 and 3000 with a 1 arcsec wide slit. The LRS optics were designed and partially fabricated at the IAUNAM. We present a description of the LRS specifications and optical design, and describe the manufacturing process.

Hobby-Eberly Telescope: commissioning experience and observing plans

Experience in bringing into operation the 91-segment primary mirror alignment and control system, the focal plane tracker system, and other critical subsystems of the HET will be described. Particular attention is given to the tracker, which utilizes three linear and three rotational degrees of freedom to follow sidereal targets. Coarse time-dependent functions for each axis are downloaded to autonomous PMAC controllers that provide the precise motion drives to the two linear stages and the hexapod system. Experience gained in aligning the sperate mirrors and then maintaining image quality in a variable thermal environments will also be described. Because of the fixed elevation of the primary optical axis, only a limited amount of time is available for observing objects in the 12 degrees wide observing band. With a small core HET team working with McDonald Observatory staff, efficient, reliable, uncomplicated methodologies are required in all aspects of the observing operations.

Extremely large telescope: a twenty-five meter aperture for the twenty-first century

The 10-meter class Hobby-Eberly telescope (HET), now nearing completion, provides technology for optical Arecibo-type telescopes which can be extrapolated to even larger apertures. Utilizing a fixed elevation angle and a spherical segmented primary mirror provides cost effective and pragmatic solutions to mirror mounting and fabrication. Arecibo-type tracking implies a greatly reduced tracking mass and no change to the gravity vector for the primary mirror. Such a telescope can address 70 percent of the available sky and exhibit optical quality easily sufficient for effective spectroscopy and photometry. The extremely large telescope takes advantage of several key engineering approaches demonstrated by the HET project to achieve a cost comparable to similarly-sized radio rather than optical telescopes. These engineering approaches include: bolted pre-manufactured primary mirror truss, factory manufactured geodesic enclosure dome, air bearing rotation of primary mirror, tracker, and dome systems directly on concrete piers, and tracking via a hexapod system. Current estimates put the cost of the ELT at

Design and status of the Spectroscopic Survey Telescope

200 million for a 25-meter aperture utilizing a 33-meter primary mirror array. Construction of the ELT would provide the astronomy community with an optical telescope nearly an order of magnitude larger than even the largest telescopes in operation or under construction today.

Extremely large telescope: further adventures in feasibility

The Spectroscopic Survey Telescope is being constructed by a consortium of universities at McDonald Observatory in the Davis Mountains of Texas. Principal partners are the University of Texas at Austin and the Pennsylvania State University. Also participating are Stanford University and the University of Munich and University of Gottingen in Germany. We describe the specific design attributes which enable the SST to be constructed for a fraction of the cost of astronomical telescopes of comparable size. Such unique features as identical spherical mirror segments, selective figuring for constant mirror mount deformation, air bearing azimuth rotation system, and pre-fabricated architectural type domes are employed. Emphasis is on simplification of design, reduction of part count and mass, and utilization of lessons learned from other recent large telescope projects.

Surface analysis of an actively controlled telescope primary mirror under static loads.

Should the astronomical community pursue development of telescopes 10 times larger than the 8 and 10 meter individual and arrayed telescopes currently under development or recently commissioned? The question devolves into two parts: Is construction of such a telescope feasible from an engineering and cost standpoint? Does the scientific benefit justify the probable cost of such development? An Extremely Large Telescope (ELT) has previously been proposed based on the Arecibo type design employed in the recently completed Hobby Eberly Telescope. Analysis of the performance and scientific viability of the ELT shows that it can have an important role in near and IR spectroscopy for cosmology providing that stringent image and background performance requirements are met. Further development of engineering design and interaction with the manufacturing community conclusively shows that not only is such a telescope feasible, but that the entire observatory can be constructed for of order

Issues in scaling the ELT up to 100 m

DLR250 million at a site likely to provide optimal optical seeing. It remains an issue for the scientific community to judge whether such capability provides benefits commensurate with the costs.