TELESCOPES

Tile by tile, engineers are installing and aligning the 91 individual mirrors that form the 11-m segmented primary mirror of the Hobby-Eberly Telescope (HET; McDonald Observatory, TX). When completed in mid-1997, the mirror will be the largest yet made. The blue trusswork (bottom) provides a spherical mounting surface for the 1-m-diameter hexagonal mirror segments (shown here covered with protective foam), which are each supported on a nine-point Hindle mount that includes three actuators. The m

TELESCOPES

Hobby-Eberly Telescope mirror comes into focus

Tile by tile, engineers are installing and aligning the 91 individual mirrors that form the 11-m segmented primary mirror of the Hobby-Eberly Telescope (HET; McDonald Observatory, TX). When completed in mid-1997, the mirror will be the largest yet made. The blue trusswork (bottom) provides a spherical mounting surface for the 1-m-diameter hexagonal mirror segments (shown here covered with protective foam), which are each supported on a nine-point Hindle mount that includes three actuators. The mounting surface is adjusted in piston (radial runout) to a contiguous spherical surface to within 25 µm; three actuators on each of the individual mirror mounts maintain fine alignment of the mirror surfaces during observation and thermal cycling.

The silver strutwork carries a suspended secondary package (not shown) that includes a guide/star-tracking assembly and correcting optics to address spherical aberration. Mounted at the focus of the primary, the secondary translates above the primary mirror, following the object under observation. This design, optically analogous to the radio telescope at Arecibo, Chile, significantly reduced the cost of the mirror support system. For gross alignment, the entire telescope assembly and dome can be rotated to reach any angular position.

As of this writing, 14 of the mirror segments were installed; the $13.5 million telescope has already registered first light (see Laser Focus World, Feb. 1997, p. 13). Intended primarily for spectroscopic studies, the HET is a collaborative effort involving the University of Texas (Austin, TX), Pennsylvania State University (University Park, PA), Stanford University (Stanford, CA), Georg-August-Universität (Göttingen, Germany), and Ludwig-Maximilians-Universität (Munich, Germany).

Kristin Lewotsky

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