At the top of Mt. Graham in the Arizona desert, the Large Binocular Telescope Observatory explores galaxies billions of light years away and discovers new exo-planets rotating foreign stars. Astronomers can rotate the 650-metric ton structure,theworld’slargestnon-segmented mirror telescope, pinpointing its dual 8.4-meter diameter mirrors to locate these infinitesimally small bodies against the vastness of space.
But to find what’s out there 1,000 light years away, LBTO engineers have to make sure they know how their telescope is positioned at 1/1000th of an inch. The telescope can be rotated to a precision of a few arcsecond . To be sure of this position, though, engineers need a measuring instrument versatile and portable enough to be positioned in the observatory’s steel structure surrounding the mirrors. They found their solution in the Omnitrac 2 (OT2) ADM laser tracker from API. As the world’s first completely wireless laser tracker, the Omnitrac 2 could be mounted in the observatory without concern for power supply.
Efficiency and Stability
The Large Binocular Telescope, whose offices are based on the campus of the University of Arizona in Tucson is a powerful astronomical tool. Its two mirrors can act together to provide the resolution of a single 11.8-meter mirror, which is impossible to create with today’s technology. Its angular resolution would match that of a hypothetical 22.8-meter wide mirror.
The LBTO staff first became familiar with API technology in 2007, when then-telescope scientist Andrew Rakich acquired a Tracker 3. As the temperature atop Mt. Graham fluctuates 30 to 40 degrees over the course of the year, Rakich was concerned that the steel structure would expand or contract ever so slightly due to thermal gradients on either side of the telescope.
Rakich positioned the T3 above the two mirrors and attached spherically mounted retroreflectors to the side of the primary mirrors. To further compensate for temperature and weather- related effects, the observatory uses activators to change the shape of the mirrors.
After optical engineer/scientist Lee Dettman joined the LBTO in 2013, he was interested in replacing the T3 with two laser trackers that could be mounted in the steel structure, dedicated to measuring thermal gradation.
The Omnitrac 2’s competitive price attracted him. His accuracy needs did not require an interferometer. In fact, it was beneficial to not have the interferometer in the tracker, as it reduced the heat that it would generate. To help the trackers compensate for the temperature right at the measurement spot, API provided extra- long cables for the weather stations.
API introduced the Omnitrac 2 in 2013 as a rugged and portable system that would take laser metrology into new applications and environments. In addition to its battery power and ability to transmit data over WiFi, the Omnitrac 2 can be mounted at a 90 degree angle, or even upside down, allowing it to be positioned in the most convenient spot for measurement. And at only 24 pounds, it is easy to carry and affix into position.
Dettman could have used one Omnitrac 2, but two trackers are more efficient, allowing him to run both tracker operations on each side simultaneously and in parallel. He analyzes the data through a single, customized version of Spatial Analyzer.
“It’s certainly great for positioning objects and it fits the tolerances we need,” Dettman said. “Our needs and the OT2 are pretty well matched.”
The trackers really show off their portability and versatility, as they are mounted up in the steel structure. Up there, they are constantly turned on and ready to run a measurement. The Omnitrac 2 can be permanently on without any ill effect because it has no laser tube.
The Tracker 3 has not been retired, either. It has been put to use for telescope maintenance and other special measurement projects.
Before laser trackers, Dettman was forced to use tedious and error-fraught measurement procedures, including aligning mirrors in collimators to see how well images reflect back on the focal plane.
“With a laser tracker, you’re directly measuring the things you want to measure,” he said.
A novel concept for an astronomer, no doubt.
The goal of the LBT project is to construct a binocular telescope consisting of two 8.4-meter mirrors on a common mount. This telescope will be equivalent in light-gathering power to a single 11.8 meter instrument. Because of its binocular arrangement, the telescope will have a resolving power (ultimate image sharpness) corresponding to a 22.8-meter telescope. The feasibility study for the project was completed in early 1989. In 1992, the original partners (Arizona, Italy and Research Corporation) decided to proceed to the construction phase even though the funds available were sufficient only to complete a “reduced first light” telescope with only one primary mirror in place. With the addition of LBTB and Ohio State University to the consortium in 1997, the project began to construct the full binocular telescope. The telescope was completed in Italy and shipped to Arizona in the summer of 2002.
The Large Binocular Telescope (LBT) is a collaboration between the Italian astronomical community (represented by the Instituto Nazionale di Astrofisica (INAF)), The University of Arizona, Arizona State University, Northern Arizona University, the LBT Beteiligungsgesellschaft in Germany (Max-Planck-Institutfür Astronomie in Heidelberg, Landessternwarte in Heidelberg, Leibniz Institute for Astrophysics inPotsdam, Max-Planck-Institut für Extraterrestrische Physik in Munich, and Max-Planck-Institut für Radioastronomie in Bonn), The Ohio State University, Research Corporation in Tucson, and the University of Notre Dame.