Eyes to the universe

Eric M. Jukelevics Arizona Daily Wildcat Dean Ketelsen, UA senior research specialist, sweeps abrasive material onto a slowly turning mirror Thursday while an automatic polisher smooths its surface. The mirror, cast at UA's Steward Observatory Mirror Laboratory, is the largest in the world.

One of the projects - creating the world's largest mirror.

The mirror, one of a pair that will be installed in the Large Binocular Telescope on Mount Graham, is an 8.4-meter, 16-ton slab of glass.

The second mirror will be cast early in the year 2000, said Buddy Martin, polishing scientist for the lab.

The two mirrors will work in tandem to have a light-collecting power "which is beyond anything else that currently exists," said James Slagle, assistant director for the telescope project.

Just as the pupil of an eye dilates to make objects in a dark room more visible, a larger mirror can pick out fainter objects such as distant galaxies.

While the large mirrors are effective, they take nearly two years to complete, Martin said.

Building the mirror's ceramic mold takes nine months. Then the glass is cast at 2000 degrees Fahrenheit inside the lab's spinning oven to form a concave mirror. Martin said the molten glass then takes three months to cool.

While there are two other labs in the world that cast large mirrors, the UA lab is unique in that it also polishes them, he said.

"It is the only place in the world making large lightweight mirrors," Martin said. "It is the only place in the world making mirrors from start to finish."

The UA scientists are busy working on secondary mirrors for the Multiple Mirror Telescope Observatory on Mount Hopkins.

A 6.5-meter mirror was already constructed and installed in March.

But a telescope isn't complete until it has a smaller, secondary mirror, which takes light gathered by the larger primary and focuses it onto an eyepiece or detection equipment.

Three secondary mirrors are being constructed for the telescope, and will be used interchangeably.

Besides a standard 1-meter secondary mirror, two others are being designed in the lab.

A 1.7-meter, "wide-field" mirror will be able to take images of the sky the size of two full moons. The mirror features a robotic detection system that will be able to measure the chemical composition and distance of 300 celestial objects simultaneously.

The third measures 0.7-meters and features an adaptive optics system - a new technology that compensates for atmospheric disturbance by minutely changes the shape of the mirror.

Even on the clearest night, the atmosphere is hazy and turbulent, making stars twinkle and obscuring the view of dimmer objects. A murky view limits the abilities of ground-based telescopes, leading scientists to launch expensive telescopes into space.

With adaptive optics, a laser is beamed high into the atmosphere and creates a spot. As the spot is warped by atmospheric disturbances, scientists can adjust the mirror accordingly.

A set of more than 300 "actuators" - electromagnetic coils that push and pull at the glass to make microscopic changes in the shape of the mirror - negates any disturbance, said Craig Foltz, director of the Multiple Mirror Telescope Observatory.

The atmosphere changes quickly, so the actuators make hundreds of corrections every second. Astronomers then can get almost as clear a view into space as if there were no atmosphere at all, he said.

But adaptive optics will not make orbital telescopes obsolete, Martin said.

The atmosphere blocks out ultraviolet rays and some infrared, so orbital telescopes will still be necessary to observe those types of radiation.

"There are certain things you can only do from space, but only a handful of people get observation time," he said.

While the Mirror Lab scientists are trying to perfect the ground-based telescopes, they are also working on the "next generation" of space-based telescopes.

Set to be launched in 2007, the NASA-sponsored project is dubbed "The Next Generation Space Telescope." It will feature an ultrathin, lightweight mirror that can be launched into orbit around the sun.

The 2-meter prototype is fresh out of the oven and is only 2 millimeters thick.

A set of actuators will adjust the shape of the mirror to keep it from warping in the frigid temperatures of outer space, said James Burge, assistant professor of optical science and astronomy and mirror technician for the project.

The mirror will have to withstand temperatures as low as -400 degrees Fahrenheit, he said.

Martin said he expects next year to be as busy as this one. Since the creation of the 8.4-meter mirror for the Large Binocular Telescope, Martin predicted the lab may get orders for more.

"Some people think there might be an explosion of 8-meter-class telescopes in the '90s like there was for 4-meter-class telescopes in the '80s," he said. "Once there were a few out there, everyone wanted to get their hands on one."

For more history on the Mirror Lab, which started as one astronomer's back-yard experiment, visit: http://medusa.as. arizona.edu/mlab/mlhist.html.