Keck Telescope Construction Started

CONSTRUCTION STARTED ON WORLD'S LARGEST TELESCOPE

The W.M. Keck Observatory in Mauna Kea, Hawaii is home of the Keck Telescope designed by Stefan Medwadowski, Ph.D. with the Algor FEA system. Photo provided by the California Association for Research in Astronomy (CARA).

The Keck telescope is the largest telescope in the world - its 400-inch, computer controlled primary mirror features four times the light-gathering capacity of the 200-inch telescope at Mt. Palomar. The Keck's unique structure was designed by Stefan Medwadowski, Ph.D., a structural engineer in San Francisco, California. With the help of Algor's Finite Element Analysis (FEA) system, Medwadowski was able to keep the weight of the telescope's steel structure to a minimal 270 tons while preserving the necessary strength to support the 14-ton segmented mirror and to aim it precisely at any point in the sky.

Many interesting features were incorporated into the design of the Keck Telescope. For example, the mirror itself is segmented and consists of 36 hexagonal parts. A segmented mirror possesses several advantages over a monolithic one. These include much lower weight (the Keck mirror is only 7.5 cm thick), ease of transportation, and ease of removal and replacement required for re-aluminizing without loss of observing time.

Construction Underway

Assembly of the Keck Telescope structure is now complete, and workers at the California Association for Research in Astronomy (CARA) are now balancing the telescope, powering the bearings and drive motors, and installing the numerous cables that will carry the electronic signals for controlling the telescope's motion.

TIW Systems, Inc., Sunnyvale, California, was responsible for the telescope's construction and installation. The company is also responsible for fabrication of the systems that move the telescope to aim at and track celestial objects.

A team of TIW employees and Hawaiian steelworkers began assembling the telescope shortly after its arrival on the Big Island in July 1989. No serious problems have occurred, but the work was delayed in part by a record-breaking snowstorm that buried the Mauna Kea summit in February 1990.

This computer model of the Keck Telescope, developed and analyzed for stress and vibration with Algor FEA software, has 4525 nodes and 7529 Plate/Shell elements.

CARA is installing and testing the drive systems and has developed the computer controls for operating them. CARA employees have also tested both hydrostatic bearings, which float the telescope on a thin film of high-pressure oil, enabling relatively low-power motors to move the massive structure. The telescope should soon be fully movable.

Presently, technicians from the Lawrence Berkeley Laboratory (LBL) are installing precision components called "sub-cells" that will receive the mirror segments and their support systems. The sub-cells allow the mirrors to be precisely positioned during installation. Dummy weights representing the mirrors are attached for balancing the telescope.

One-third of the segments for the Keck Telescope's mirror array have now been polished. When the 12 polished segments are mounted in the telescope they will provide an aperture equal to a 228-inch, or 5.7-meter, mirror. That is bigger than the five-meter Hale Telescope at Palomar Observatory and nearly the size of the Soviet Union's six-meter telescope. Polishing of the remaining mirror segments continues at both Itek Optical Systems in Massachusetts and at Tinsley Laboratories in California.

The W.M. Keck Telescope. Photo provided by CARA.

Itek has finished polishing and cutting six of the "Type 2" segments. Six different types, or shapes, of segments make up the full 36-segment mirror array. A seventh, spare Type 2 segment has been polished but not yet cut into a hexagon.

The first five mirror segments for the Keck Telescope reached the observatory at Mauna Kea the week of February 5, 1990. The segments were shipped from Boston to Honolulu by plane, from Honolulu to Hilo by barge, and from Hilo to Mauna Kea summit by truck.

A technical support team from Lawrence Berkeley Laboratory has taken charge of the mirror segments and will uncrate them, reattach them to their whiffletree and radial supports, aluminize them in a vacuum chamber at the observatory, and install them in the telescope.