By Lori Stiles

New approaches and technologies embodied in the LBT give astronomers features they have long dreamed about at prices they can afford.

• Innovative Mirror Lab technology has produced the world's largest monolithic glass mirrors. The mirrors are stiff and lightweight "honeycomb" structures of borosilicate glass formed in a ceramic fiber mold spun in a giant rotating furnace. They are polished with a computer-controlled stressed-lap disk that adjusts its shape, speed and pressure to a nearly perfect mirror shape.
• The deeply dished mirrors focus light closer to their surfaces than other mirrors do. Short focal-length mirrors permit a very compact telescope building, which is a big factor in keeping costs down.
• Positioning the giant mirrors side-by-side on a single mount is another major advance in telescope design. Side-by-side mounting gives the telescope its ability to produce images 10 times sharper at optical and infrared wavelengths than Hubble Space Telescope images.
• Unlike any other telescope ever made, the LBT has secondary and tertiary mirrors mounted on swing arms. The telescope's Italian designers planned this feature to give astronomers special advantages. One is, if weather conditions change, astronomers can switch out instruments and optics in minutes for a different observing program, rather than shut down for the night.
• Like other giant telescopes, the LBT is equipped with "adaptive optics," a system that corrects for atmospheric turbulence that distorts starlight. But the LBT will use a new method to take the twinkle out of stars, a method successfully tested last year at the 6.5-meter (21-foot) UA/Smithsonian Institution MMT Observatory telescope on Mount Hopkins, Ariz.
  Researchers from Italy's Osservatorio Astrofisico de Arcetri in Florence and Steward Observatory developed the new adaptive optics technique. LBT secondary mirrors will change shape in milliseconds, virtually in real time, to compensate for atmospheric blurring. The mirrors, which are less than 2 millimeters thick (less than eight-hundredths of an inch) and float in a magnetic field, are gripped by hundreds of computer-controlled electro-magnetic devices. These "actuators" tweak the mirrors into place, nanometer by nanometer, so they focus light as steadily as if Earth had no atmosphere.
• The LBT is an "alt/azimuth" telescope, that is, it can point in any direction in the sky just like a battleship gun. What's new in mechanical design is that the structure that supports all the optics and instruments is mounted on two huge (14-meter diameter) C-rings that swivel across the azimuth, or horizon, and tilt up and down. Steward Observatory engineer Warren Davison suggested mounting the optical support structure on giant C-rings so its weight is borne directly by the pier, the base on which the LBT rotates. The design gives the LBT great stiffness, a feature that astronomers need to find and track stars, especially in wind.
• LBT's primary mirrors will be aluminized, or coated with aluminum, while they are in the telescope. The conventional technique is to remove mirrors from the telescope and aluminize them in a separate facility. Ohio State University helped develop the LBT aluminizing technique, which will fit a huge bell jar-like chamber to the LBT mirror cell and pump it to a vacuum. Researchers successfully tested the new technique on the 6.5-meter MMT Observatory telescope.