The 4.3-meter (14.1ft) diameter DCT mirror blank was manufactured by Corning, Inc. in 2004-2005 in a multi-step process that began with production of 13 boules of Corning's patented ultra-low-expansion glass (ULE®). The boules were shaped into hexagons and fused together to produce a single ULE disk for the mirror blank. ULE is ideally suited for large optics such as the DCT primary mirror rendering special properties such as a light weight, thin shape, and the ability to maintain a temperature close to that of the surrounding air as the temperature drops during the night.
Officials from Lowell and Corning inspect the completed mirror blank. Photo courtesy of Corning, Inc., Sept. 2005
The DCT primary mirror will also be relatively flexible, supported by an active system that maintains optimum mirror shape regardless of the orientation of the telescope. A mirror of the correct shape in temperature equilibrium is crucial to producing ultra sharp images of celestial objects.
In early 2005, the fused mirror blank went into a large furnace and was slumped over a mold to set its curvature, giving the blank its approximate final shape. The last step in the blank fabrication process involved grinding the mirror’s front surface to within a couple of mils of the final shape and loading it into a 9-ton, custom-
designed shipping crate. Weighing 6,700 lbs and just 4 inches thick, the DCT primary mirror blank was completed at the end of 2005.
With the blank completed, the next phase in fabricating the primary mirror is final figuring and polishing, a painstaking three-year process involving numerous grinding runs and high precision testing using advanced metrology equipment. Lowell Observatory conducted a competitive bid process for this critical phase of the mirror, awarding the contract to University of Arizona, College of Optical Sciences (OSC) in Tucson in summer 2006.
Discovery Channel's film crew is on hand for the delivery of the primary mirror to OSC's facility in Tucson. Photo courtesy of Discovery Communications, Aug 2006
OSC has extensive experience producing very complex optical systems, including large optics for NASA’s Next Generation Space Telescope (James Webb Space Telescope) scheduled for launch in 2013 and the European Space Agency’s Far Infrared Space Telescope (Herschel Space Observatory) launched in 2007.
The mirror blank, which had been stored temporarily in Pittsburgh until the contract was awarded, was carefully loaded onto the bed of a custom shipping truck and transported cross country to OSC's facility in Tucson.
Visit our public photo tour site to view more pictures of the transportation and delivery of the mirror blank from post-figuring storage in Pittsburgh to the University of Arizona College of Optical Sciences facility.
Following delivery of the mirror blank to OSC’s optical facility in August 2006, the first step in the figuring process involved precisely aligning and bonding 120 “pucks” to the mirror’s convex backside and making a hydraulic support structure to hold the mirror on the polishing table just as it will be held in the telescope. The support system, completed in August 2007, ensures the mirror doesn’t flex under the force of grinding and polishing in order to render a pristine optical finish.
After positioning the mirror on the polishing table, the initial grinding phase (progressive grinding through 9 Microns) began immediately and continued through July 2008. Next, the mirror underwent a series of careful metrology checks using a laser tracker as well as infrared and visible wavelength interferometers. With precise calculations in hand, optical technicians entered the last stage of figuring which involved polishing the mirror’s surface to an accuracy of a few millionths of an inch or a fraction of a wavelength of light, and then final acceptance testing. The mirror was officially completed in February 2010 with a finished diameter of 4.28 meters. It was crated and delivered to the Happy Jack site on June 8, 2010. The next step will be to coat the mirror and integrate it into the mirror cell and support structure.
Visit our public photo tour site to view more pictures of the mirror figuring and polishing process at University of Arizona – OSC.
FIRST LIGHT INSTRUMENTS