MCDONALD OBSERVATORY      HOBBY-EBERLY TELESCOPE

  

TELESCOPE AND TELESCOPE STRUCTURE The HET is a tilted Arecibo-type optical telescope tailored for spectroscopy. The telescope structure is tilted 55 degrees above the horizon for a 35 degree zenith angle. The structure rotates 360 degrees on eight 36-inch air bearings with no perceived vibration. The primary mirror and structure are fixed during observations with tracking in spherical focal surface. Low-resolution spectroscopy and imaging occur directly at prime focus. Fiber-fed medium- and high-resolution spectrographs are located in the "basement" below the telescope. The HET is located on Mount Fowlkes at McDonald Observatory, Fort Davis, Texas. The site altitude is 6640 feet, or about 2 kilometers. Construction cost was $13.5 million, not including instruments or full commissioning.

Concept rendering of the HET HET Telescope Structure

Tracker installed on top hex of HET structure

TRACKER Tracker field of view is 12 degrees. The tracker implements a 6-axis coordinate system: x, y, z, theta, phi, rho. HET sees declination range from -10 deg. 20 min. to +70 deg. 40 min. This declination range covers 70% of the sky visible from McDonald Observatory. Maximum tracking times range from .75 hour at equator to 2.5 hours at north limit.

OPTICAL SYSTEM The primary mirror consists of 91 interchangeable segments forming a 11x10 meter hexagon. Segments (and thus the entire primary) have a spherical figure, figured to better than 1/15 wave. The primary's focal length is 13.08 meters, with 77.6 square meters of collecting area. Segment radii of curvature are matched to 0.5 mm via a test against a master sphere. The entrance pupil diameter is 9.2 meters on the primary. The central obstruction is 2.5 meters. Segments are Zerodur, 50 mm thick, hexagonal shape, one meter across the flats. Each segment rests on a modified Hindle mount with three tetrahedrons and a 10:1 lever. Each segment is computer controlled via three actuators (273 total actuators). Each actuator is a commercial motor micrometer. The primary mirror truss is kinematically mounted to the telescope structure. The Spherical Aberration Corrector is a four-element Gregorian design with a 490 mm maximum diameter. SAC mirrors are ULE with an invar metering structure. Overall image quality goal is 0.6 arcseconds.

Completed Primary Mirror with all 91 segments installed.   HET structure, tracker, and segmented primary mirror

Dome skeleton Hobby-Eberly telescope dome and George T. Abell Gallery

DOME AND FACILITY The dome is a spherical, 85-foot-diameter, aluminum geodesic dome, about 100 feet tall. It incorporates a downdraft ventilation system capable of 20 air exchanges per hour. There is a 2500-square-foot control and service building. The large spectrograph room is in the "basement" directly below the telescope. There is a 90-foot-tall Center of Curvature Alignment Sensor (CCAS) tower next to the dome. The CCAS tower houses a shearing interferometer for segment tilt and piston alignment. There is also a 1000-square-foot George T. Abell Gallery for public visitors.

FACILITY INSTRUMENTS LRS Low-Resolution Spectrograph MRS Medium-Resolution Spectrograph HRS High-Resolution Spectrograph  PRIME FOCUS INSTRUMENT PACKAGE (PFIP) The PFIP is the payload carried by the tracker on its hexapod mechanism. It includes the Spherical Aberration Corrector and acquisition and guide cameras. It also implements various other auxiliary functions. It mounts the LRS and the Fiber Instrument Feed for the MRS and HRS.

PLANNING AND QUEUE SCHEDULING The HET operates in a queue-scheduled, service observing mode 85% of the time. PIs uses software tools to plan HET observations and submit proposals. PIs submits detailed observing scripts after time has been granted. Observing scripts are incorporated into a relational database used to schedule the HET. Plans are ranked according to TAC- and PI-assigned priorities. Resident Astronomers do observations (identify targets, do exposures, verify data quality). RAs can also revise the queue in real time as conditions change.