The HET is one of the world's largest optical telescopes, with an effective aperture of 9.2 meters
and a 78 square meter, hexagonal mirror array made from 91 segments. Its design is revolutionary.
It sits at a fixed elevation angle of 55º, and rotates in azimuth to access 81% of the sky visible
from McDonald Observatory (discounting the "high airmass" zone right next to the horizon which
telescopes normally avoid). The HET was built for approximately 15-20% of the cost of other 9 meter
class telescopes. The tilted Arecibo design, and the cost savings realized throughout the mechanical
systems of the HET from the fixed axis concept, broke the standard cost paradigm for large aperture
The Primary Mirror
Each of the 91 identical hexagonal segments of the primary mirror array measures 1 meter side to side
and weighs about 250 lbs. Alignment across the array is accomplished by triple actuators incorporated
into each segment support and adjustable to about 1/3 a wavelength of light (218 nm). The segments
are spherically shaped, like a contact lens, and figured to a 26 meter radius of curvature, accurate
to within 0.5mm. A sensor located at the center of the segment radius of curvature, 26 meters from
the mirror, sits on top of the tower next to the HET, the Center of Curvature
Alignment Sensor Tower (CCAS or "see-cass"). At dusk, the telescope is rotated to the CCAS Tower
for precise alignment of segments across the array, which is electronically maintained all night.
Because of the precision required of large optical mirrors for quality image resolution, large
telescopes of traditional design must compensate for the major distortion effects of gravity as the
mirrors are moved through all tilt angles while aiming at different targets.
Because the HET is fixed in its vertical axis, its primary mirror is stationary with respect to
gravity. This completely eliminates the problem of variable distortion. It also dramatically
simplifies the telescope's supporting framework. The trade-off in the HET, however, was that a
new solution was required for tracking.
Stars move, of course, across the sky, because the earth rotates, and telescopes must track them.
This is usually accomplished by attaching a drive to the rotational direction of a telescope
(azimuth) and another to its vertical direction of movement (altitude). Together the drives
coordinate, the whole telescope moves to compensate for the rotation of earth, and the telescope
viewpoint remains precisely positioned on the celestial sky.