Grind and Polish!
Production of the first primary mirror segment for the Giant Magellan Telescope has reached another milestone: the front surface "generation" has been completed, and the "grinding and polishing phase" has begun.
Tucson, AZ. The latest photos from the University of Arizona’s Steward
Observatory Mirror Lab show the 8.4 meter mirror segment undergoing
“loose abrasive grinding” on the Large Optics Generator, which is a
computer controlled mill.
| GMT1 8.4 meter mirror segment undergoing loose abrasive grinding. Photo by SOML. |
The device you see above the mirror’s surface is called a “stressed lap.” This is a polishing tool that, under computer control, changes its shape and pressure as it rotates and moves across the surface of the mirror. The stressed lap polisher is one of the key technologies that make it possible to shape and polish the GMT mirrors.
| The Stressed Lap polishing tool changes its shape and pressure as it moves across the surface of the mirror. Photo by SOML. |
Remember that the GMT primary mirrors will be mounted “off-axis.” (That is, instead of lying flat, like in more conventional telescopes, the mirrors are mounted at an angle, and they are mounted off to one side of the center line.) Therefore the primary mirrors must have a highly curved asymmetric shape. Also, the mirrors must match each other to high precision. It is essential that all the primary mirror segments accurately focus on the secondary mirror assembly.
| Underside of Stressed Lap reveals deformable polishing plate. Photo by SOML. |
When polishing is complete, the surface accuracy will be on the order of ±20 nanometers. (A nanometer is one billionth of a meter.) That means if the mirror were the size of the continental United States, the largest “hill” on the mirror surface would be about ½ inch high!
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