NNTT's next generation: harmonizing a quartet of large telescopes.

The National New Technology Telescope (NNTT) is the National Optical Astronomy Observatories' (NOAO's) entry in what may be the coming generation of large telescopes. The NNTT, a proposed multiple-mirror telescope, would employ four separate mirrors to act together to simulate a single mirror 15 meters across, or to act separately. At a recent meeting in Houston of the American Astronomical Society, the NNTT's planners announced significant developments in its design, and the director of the project, Jacques M. Beckers, described successful tests of a new method for making the mirrors act in concert.

The plan, as it has evolved since 1984 when NOAO decided that the NNTT should be a multiple-mirror telescope, envisions four mirrors, each of 7.5 meters diameter, hung in a common altitude-azimuth mounting. In this mounting the telescope rotates in horizontal and vertical planes. The more usual equatorial mounting, in which the telescope rotates vertically and in the plane of the celestial equator, makes it easier to follow stars across the sky. However, the telescope has to be hung at an angle to the vertical, and in the case of an arrangement as bulky as the NNTT, an equatorial mounting would impose torques and shears that the system couldn't sustain. As does the housing of the existing Multiple Mirror Telescope, the entire building housing the NNTT would rotate horizontally (telescopes usually rotate inside their buildings).

Each of the 7.5-meter mirrors would be bigger than any telescope mirror would be bigger than any telescope mirror now existing. Earlier concepts of the NNTT had generally foreseen a larger number of smaller mirrors, but recent progress in spin-casting of large mirrors, pioneered by Roger Angel of the University of Arizona in Tucson (SN: 2/16/85, p. 106), has made the larger mirrors seem practical.

Together the four mirrors would simulate a single mirror 15 meters across, for imaging purposes. For interferometic work they would be the equivalent of a 21-meter baseline. Working together, they would cast their reflections into a single image. To get them into such harmony and keep them in it, telescope operators use an artificial star, a test light source. In the existing Multiple Mirror Telescope, reflections of the artificial star by the six mirrors are brought to a common focus, and the mirrors are adjusted until the image of the test source is acceptable.

The new method of coalignment that Beckers and K.L. Shu and S. Shaklan of NAOA reported at the meetings uses "optical bridges" to link the mirrors in pairs. Each mirror would be linked to each of the other three by such a bridge. Reflections of a xenon test light from each pair of mirrors would be taken into the bridge linking them, and there combined at a central mirror to give two images. One of the images would monitor the alignment of the mirrors to see that their light was reaching a common focus. The other would monitor the phase of the reflected waves to keep the different reflections in phase with one another. The apparatus, they say, can maintain the alignment to within a tenth of a second of arc and the phase to better than half a micron, or a fraction of an optical wavelength. The design is being optimized for infrared, where wavelengths run from 1 to a few microns.

According to a NOAO prospectus, the NNTT would cost about $125 million in 1985 dollars. The only one of the new-generation telescopes actually under construction, the 10-meter Keck Telescope of Caltech and the University of California, is expected to cost something over $70 million. The University of Texas large telescope project has been slowed by difficulties with the Texas state budget. The University of Arizona project for an 8-meter spin-cast mirror is still in the planning stage.