|
||||||||||||||
|
W.M. Keck Observatory
ITT Supplies Optical Quality Mirrors to the W.M. Keck Observatory 
Specifying superb optical quality for the world's most powerful astronomical telescopes is easy. Creating mirrors with precise shapes and curvatures is more difficult.
So difficult in fact, that when project officials building the W.M. Keck Observatory realized that traditional mirror polishing techniques could not achieve the specified surface figure, they turned to ITT. Twin Telescopes The twin Keck telescopes atop Hawaii's 13,800 ft. Mauna Kea volcano consist of a 10m-diameter visible light telescope (Keck I), and a 10 m diameter infrared telescope (Keck II). Each telescope's primary mirror consists of 36 hexagonal glass segments measuring 1.8m across the corners. The segments are arranged in a mosaic pattern, giving the mirror the appearance of a honeycomb. Each segment is designed so that 80 percent of the starlight reflected from a segment surface is concentrated within a circle measuring one-quarter arc second in diameter at the focus of the telescope. This kind of clarity and resolution is equivalent to being able to distiguish each headlight on a car 500 miles away. To manufacture the mirror segments to this level of accuracy, each segment underwent a bend-and-polish technique in which the blanks were elastically bent so that their surfaces could be polished to one of six aspheric profiles. In addition, each segment was to be mounted on the telescope in such a way as to warp them in a prescribed fashion, thereby directing the captured starlight with absolute precision. However, midway through production, project officials discovered that the process of cutting the circular blank segments into their hexagonal shape had introduced an unpredicted amount of warping that traditional polishing techniques and the mounting harnesses could not correct. The result was excessive residual surface figure error. To correct the final surfaces of the segments to within their original stringent design tolerances, project officials at California Association for Research in Astronomy (CARA) sent the mirror segments to ITT, which specializes in an optical fabrication technique called ion figuring. Segment Face-Lifting Ion figuring uses a computer-controlled beam of ionized argon atoms to remove glass material molecule-by-molecule from an optical surface. The process gives ITT's optics fabricators an unprecedented degree of control when figuring the final surface of an optic, whether for a telescope, a large camera system, or a microlithography stepper mirror. At ITT's Optics fabrication facility, opticians ion-figured several of the Keck I mirror segments. The process removed mount-induced error and residual figure error in the part, and eliminated edge-of-aperture ripple caused by the initial fabrication techniques. Each segment was figured in one iteration taking approximately a week, a major time savings over traditional polishing techniques. The ion-figuring process ensured a surface accuracy on each segment of 0.14 waves rms (0.09 microns rms). When mechanically warped on their mounts, the segments showed final surface errors of only 0.025 waves rms (0.015 microns rms), well within original specifications. If expanded to the diameter of the Earth, the surface imperfections would be only three feet high. The results so impressed Keck project officials they decided to have nearly all of the 84 Keck I and II segments (72 segments to install, plus 12 spares) undergo the figuring technique. Later, as Keck II mirror segments arrived from ITT's ion figuring facility, they were installed in the Keck I primary mirror, replacing those segments that had not already been ion-figured. The result for the Keck Observatory is clear, deep views of the sky from the Earth's finest telescopes. The telescopes have 17 times the light-gathering area of the Hubble Space Telescope, and the ability to gather scientific data from the deepest regions of the Universe with unprecedented power and precision. |
|||||||||||||
 |
|
|