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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS
dust and gas emission from very young galaxies, early in the history of the universe. Surveys tracing emission from carbon monoxide molecules will lead to three-dimensional maps of the large-scale distribution of spiral galaxies out to cosmologically exciting distances. Images with high spatial and spectral resolution will unveil the kinematics of optically obscured galactic nuclei and quasars on spatial scales smaller than 300 light-years. Images of young stars taken with existing millimeter interferometers have already demonstrated the existence of enough material in orbit around younger versions of the sun to make 10 to 100 Jupiters. The MMA will measure the mass, temperature, and composition of such protoplanetary disks with greatly improved sensitivity and resolution.
The recommended program in adaptive optics will give existing and future large telescopes the ability to remove atmospheric distortions, thereby increasing the resolution and sensitivity of astronomical measurements. “Adaptive optics” is different from “active optics.” The latter refers to techniques, like those planned for the Keck 10-m optical telescope or the Green Bank 100-m radio telescope, to correct for minute- or hour-long drifts in mirror shape due to gravity, wind, and temperature drifts. “Adaptive optics,” however, attempts to compensate for the rapid, hundredth-of-a-second effects of atmospheric turbulence. Of the two, adaptive optics is the more challenging, but also the more rewarding scientifically.
Turbulence in the atmosphere scrambles light waves in patches larger than a characteristic size, r0, about 8 in. at visible wavelengths. Consequently, light reaching a telescope of diameter larger than this is so badly disordered that diffraction-limited imaging is normally impossible. European astronomers, as well as U.S. scientists, have developed techniques to monitor the wave-front errors in each r0-sized patch of a telescope mirror, and to correct them with reference to a nearby standard star by warping the mirror appropriately. Corrections must be made within the “coherence time” of the atmosphere, τ0, roughly every hundredth of a second at visible wavelengths. Complete phase corrections require a reference star brighter than a visual magnitude of about 9 located within an isoplanatic angle θ0 fof the object of interest. This angle is only a few arcseconds at visible wavelengths. Partial corrections can still improve angular resolution and might utilize stars as faint as 15 magnitude separated by larger distances.
The size parameter, r0, the coherence time, τ0, and the allowable distance between object and reference star all increase with wavelength. Thus adaptive optics will probably be applied first in the infrared. The number of r0 patches across a telescope is much smaller in the infrared than in the visible, so that the number of correcting actuators is tens instead of hundreds. Corrections are