Lowell Observatory, UArizona to Complete DCT Mirror

Lowell Observatory and the University of Arizona have finalized a $3 million, three-year contract to complete the Discovery Channel Telescope primary mirror.

The 4.3-meter-diameter (14 ft), approximately 6700-pound mirror is the center of Lowell Observatory’s new Discovery Channel Telescope (DCT), currently under construction at Happy Jack, Ariz., 40 miles southeast of Flagstaff.

The completed Discovery Channel Telescope mirror blank with Lowell Observatory staff Byron Smith (left) and Bob Millis. (Photo: Corning Inc.)

The $40 million DCT, a joint project of Lowell Observatory and Discovery Communications Inc., is located within the Mogollon Rim Ranger District of the Coconino National Forest. Unique to the DCT’s optical configuration is the ability to switch from an ultrawide-field (prime focus) imaging and surveying mode to a significantly longer focal length mode optimized for spectroscopy, infrared imaging and other applications.

When fully operational in 2010, the telescope will be the fifth largest in the continental US and will allow Lowell astronomers to more effectively enter new research areas and conduct existing programs. The DCT and its resulting research will also be the focus of TV programs about astronomy, science and technology airing on Discovery networks.

University of Arizona (UA) optical scientists will polish and figure the mirror in an exacting, delicate process expected to take about three years. If the mirror were the size of the United States, all the imperfections would be polished down to less than one inch high.

The DCT mirror was cast and fused by Corning Inc. in Canton, N.Y., to Lowell Observatory specifications. It is made of Corning’s ultralow-expansion (ULE) glass, which exhibits no dimensional changes over extreme temperature variations, and is only 100 mm (four inches) thick. "These are very important features," said Lowell's DCT project manager, Byron Smith. "The thinness of the mirror helps it cool rapidly at night, reducing heat waves that would blur the images." Both characteristics help to ensure the sharpest possible images from the telescope.

Artist's illustration of the completed Discovery Channel Telescope (Image: Lowell Observatory)

The DCT mirror will be delivered to UA’s optical sciences facility at the end of August. Over the next six months or so, the engineering team will bond a minimum of 120 pucks to the mirror’s convex backside and make a support structure that holds the mirror just as it will be held in the telescope. The system will ensure the mirror doesn’t flex under the force of grinding and polishing.

Next, grinding the mirror to get it closer to the ideal shape will take about five months, said Martin J. Valente, director of OSC's optical fabrication and engineering facility and UA's principal investigator on the project. Polishing the mirror and “figuring” it -- which is the final stage of polishing that will make the mirror accurate to within a fraction of a wavelength of light, or a few millionths of an inch -- will probably take another 15 to 18 months, he said.

Optical sciences students, working under the supervision of professor Jim Burge, co-investigator on the project, will be primarily involved in optical testing. Burge’s group will design lenses that will be used with several systems that will independently test the DCT mirror. Test systems will include a laser tracker and infrared and visible wavelength interferometers.

Lowell and Corning announced completion of the primary mirror blank in October 2005. Building a road to the DCT site near Happy Jack began in November 2004 and construction of the telescope enclosure and an auxiliary support building commenced in mid-September 2005. Thanks, in part, to an exceptionally mild winter, progress on the buildings and other site infrastructure has been rapid.

For more information, visit: www.lowell.edu/DCT