Randall L. Ricklefs

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.

ICESat Geolocation Validation Using Airborne Photography

NASAs ICESat satellite launched in January of 2003, carrying the Geoscience Laser Altimeter. During the initial phase of this mission, many validation procedures were implemented to verify the accuracy associated with a variety of altimetry-derived data products. Of specific interest was the need to validate the geodetic position of the ICESat footprints, which is a convolution of laser-pointing determination, satellite position, and ranging measurements. This paper describes the methodology and implementation of one effort using aerial photography to image the laser spots on the surface during a satellite overflight. The spot locations are determined based on the relative positions of accurately placed geodetic infrared-emitting markers within the overflight area and apparent in the aerial photograph. One specific overflight opportunity captured six successive ICESat footprints with the airborne camera system. The mean geolocation predictions of those spots using the ground fiducial placement in the image provide a data product validation to better than 3.1-m rms on the surface with an estimated accuracy of ±3.6 m when compared to the ICESat solution. These results are within the ICESat mission requirement of 4.5 m on the surface (1.5-arcsecond pointing knowledge) for geodetic position determination.

Commissioning experience with the 9.2-m Hobby-Eberly Telescope

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.

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.

High Precision Lunar Tracking For Laser Ranging

Laser ranging to lunar surface retro-reflectors has provided a valuable source of data for the investigation of the lunar orbit, the earths orientation in space, general relativity, and many other aspects of solar system dynamics.1,2,3 Due to the moons relatively large distance, and the requirement to keep the beam from the relatively low-power laser very narrow, acquiring this data type requires very accurate telescope pointing and tracking capabilities. The nominal requirement is to keep the telescope on target within approximately 1 arc-sec of its predicted location for at least several minutes. An expansion of this requirement and the methods we have used to accomplish this high precision pointing are presented. Difficulties encountered and a few future goals of automating the McDonald station are also discussed.