James R. Fowler

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.

Ten Year Review of Queue Scheduling of the Hobby‐Eberly Telescope

ABSTRACT This paper presents a summary of the first 10 years of operating the Hobby‐Eberly Telescope (HET) in queue mode. The scheduling can be quite complex but has worked effectively for obtaining the most science possible with this uniquely designed telescope. The queue must handle dozens of separate scientific programs, the involvement of a number of institutions with individual Telescope Allocation Committees, as well as engineering and instrument commissioning. We have continuously revised our queue operations as we have learned from experience. The flexibility of the queue and the simultaneous availability of three instruments, along with a staff trained for all aspects of telescope and instrumentation operation, have allowed optimum use to be made of variable weather conditions and have proven to be especially effective at accommodating targets of opportunity and engineering tasks. In this paper, we review the methodology of the HET queue, along with its strengths and weaknesses.

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.

The Hobby-Eberly Telescope: performance upgrades, status, and plans

The Hobby-Eberly Telescope (HET) is a fixed-elevation, 9.2-m telescope with a spherical primary mirror and a tracker at prime focus to follow astronomical objects. The telescope was constructed for

Development of the segment alignment maintenance system (SAMS) for the Hobby-Eberly Telescope

13.9M over the period 1994-1997. A series of extensive engineering upgrades and corrective actions have been completed recently, resulting in significantly improved delivered image quality and increased operational efficiency. The telescopes Spherical Aberration Corrector (SAC) optics were recoated with a highly reflective and durable broadband coating at Lawrence Livermore National Laboratory. The software mount model that maintains optical alignment of the SAC with the 11-m primary mirror array was recalibrated and improved. The acquisition and guiding optics for both the High Resolution Spectrograph (HRS) and the Low Resolution Spectrograph (LRS) were reworked and improved, allowing for better focus and SAC alignment monitoring and control. Recoating of the primary mirror segment array was begun. Telescope images of 0.82 arcseconds have been recorded for sustained periods in preliminary testing following the engineering upgrade, an improvement of 50% over previous best performance. Additional engineering upgrades are scheduled to consolidate these performance gains and to continue improving delivered image quality, throughput, and telescope operational efficiency. The HET is now capable of the science performance for which it was designed.

Primary Mirror Figure Maintenance of the Hobby-Eberly Telescope using the Segment Alignment Maintenance System

A sensing and control system for maintaining optical alignment of ninety-one 1-meter mirror segments forming the Hobby-Eberly Telescope (HET) primary mirror array is now under development. The Segment Alignment Maintenance System (SAMS) is designed to sense relative shear motion between each segment edge pair and calculated individual segment tip, tilt, and piston position errors. Error information is sent to the HET primary mirror control system, which corrects the physical position of each segment as often as once per minute. Development of SAMS is required to meet optical images quality specifications for the telescope. Segment misalignment over time is though to be due to thermal inhomogeneity within the steel mirror support truss. Challenging problems of sensor resolution, dynamic range, mechanical mounting, calibration, stability, robust algorithm development, and system integration must be overcome to achieve a successful operational solution.

Commissioning experience with the 9.2-m Hobby-Eberly Telescope

The Segment Alignment Maintenance System (SAMS) was installed on McDonald Observatorys Hobby-Eberly Telescope (HET) in August 2001. The SAMS became fully operational in October 2001. The SAMS uses a system of 480 inductive edge sensors to correct misalignments of the HETs 91 primary mirror segments when the segments are perturbed from their aligned reference positions. A special observer estimates and corrects for the global radius of curvature (GRoC) mode, a mode unobservable by the edge sensors. The SAMS edge sensor system and GRoC estimator are able to maintain HETs primary figure for longer durations than previously had been observed. This paper gives a functional description of the SAMS control system and presents performance verification data.

Hobby-Eberly Telescope: commissioning experience and observing plans

The HET is unique among 9-meter class telescopes in featuring an Arecibo-like design with a focal surface tracker. The focal surface tracker causes image quality and pointing/tracking performance to interact in a complex way that has no precedent in astronomical telescope system design and that has presented unusual demands upon commissioning. The fixed-elevation, segmented primary-mirror array offers some simplifications over traditional telescope design in principle, but has presented challenges in practice. The sky access characteristics of the HET also place unique demands on observational planning and discipline. The HET is distinguished by uniquely low construction and operating costs which affected commissioning. In this contribution, we describe those aspects of our commissioning experience that may impact how similar telescopes are designed, especially those with larger aperture, and review the challenges and lessons learned from commissioning a 9-meter class telescope with a small technical team.

Imaging Performance of the Hobby-Eberly Telescope *

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.

Control of the Hobby-Eberly Telescope primary mirror array with the segment alignment maintenance system

The HET is a modified Arecibo-style telescope with a segmented spherical primary and a four-mirror spherical aberration corrector (SAC). Objects are tracked by driving the SAC along the focal sphere of the primary. In the original design of the telescope the alignment of the SAC was to be maintained passively. In practice, this could not be done to specifications, leading to degraded imaging quality. We have developed a metrology system to actively control the alignment of the SAC. An autocollimator maintains the optical axis of the SAC normal to the primary mirror beneath it. An absolute distance measuring interferometer (DMI) monitors the SAC/primary mirror distance, maintaining focus. Both systems work at a wavelength of 1.5 microns, well above the operating wavelength of current or planned science instruments and therefore do not interfere with observations. The performance of the system is measured via Hartmann testing. Several upgrades are implemented in the primary mirror control system, including calibration of individual edge sensors, new control system software, and a new method of setting and controlling the overall radius of curvature of the primary array. New techniques were developed to efficiently piston the segments onto the proper sphere radius.