The Hobby-Eberly telescope (HET) is a 9 meter telescope built by an international collaboration between the University of Texas at Austin, The Pennsylvania State University, and Stanford University in the United States and Ludwig-Maximilians-Universitaet Muenchen, and Goerg-August-Universitaet Goettingen in Germany. The HET is named in honor of its principal benefactors, Lt. Governor William P. Hobby of Texas and Robert E. Eberly of Pennsylvania. The HET employs a unique design that marks a fundamental departure from the usual paradigm for building large optical telescopes. The engineering approach adopted makes the HET a quality instrument which maximizes the scientific research capability and minimizes cost. The HET was built at the University of Texas McDonald Observatory near Ft. Davis Texas for a cost of $13.5 million; not including instruments. First light was achieved December 10, 1996, and the telescope was dedicated October 8, 1997. First tests of the Prime Focus Instrument Platform (PFIP) were carried out in August 1998. Full science operations will begin following a commissioning period. General Design Features Central to the HET design approach is specialization: the HET is tailored for spectroscopy, and in particular, fiber-coupled spectroscopy. By limiting observational flexibility, extremely cost-effective technical solutions are possible and these have been implemented in the HET. Specifications include a spherical main mirror whose optical axis is tipped 35° from the zenith. The mirror and telescope are mounted on a frame which turns 360° in azimuth. During an observation the telescope is fixed in azimuth and objects are tracked by moving a spherical aberration corrector to follow the reflected light. The angle of the primary mirror with respect to gravity is constant resulting in large cost savings in the mirror and mirror support systems. HET reflecting surfaces utilize enhanced silver coatings to maximize the amount of light that is delivered to the focal plane.
Technical Overview
The HET design marks a fundamental departure from the usual paradigm for building large optical telescopes. Central to the HET approach is specialization: the HET is tailored for spectroscopy, and in particular, fiber-coupled spectroscopy. By accepting some limitations on observational flexibility, extremely cost-effective technical solutions are possible and these have been implemented in the HET's design.
Primary Mirror Array
Maximum Diameter
11 meters
Area
77.6 sq. meters
Focal Length
13.08 meters
Filling Factor
~96.6%
Tracker Optical Package
Maximum Diameter
490 nm
Type
Four Element Gregorian
Effective F. Ratio (output)
4.7
Focal Plane FoV
3.5 arc minutes diameter
Final Image Scale
5 arc-sec./mm
Entrance Pupil Diameter (on primary)
9.21 meters
Telescope Parameters
Telescope Tilt Angle (Z)Azimuth
35°
Angular Range (FoV tracker)
12°
Declination Range
-10°20' to 71°40'
Site Altitude
~2 km (6640 ft.)
Primary Mirror: The primary mirror is composed of 91 segments, each hexagonal in shape and 1 meter across the flats. They form a large spherical surface having a radius of curvature of 26.164 meters. Each segment is positioned under computer control to yield a hexagonal primary mirror ll meters in diameter with an area of 77.6 square meters.
Tracking: The HET is a tilted optical Arecibo type telescope, which means that tracking of astronomical targets is achieved through moving a tracker optical package, which contains spherical aberration correction optics and instrument feeds, rather than the whole telescope. Tracking time across the 12° focal surface ranges from 0.75 hr. at the equator to 2.5 hr. at the north declination limit.
