Atmospheric turbulence and the resolution limits of large ground-based telescopes

Abstract

Conventional theory of imaging through the atmosphere is based on two main assumptions: (1) atmospheric turbulence is assumed to follow a Kolmogorov spectrum and (2) the outer scale, Lo, is assumed to be much larger than any telescope. There are numerous reports in the literature, however, of image properties that are not consistent with this theory—for example, cores in star images and lack of expected image motion. In almost every case these reports are consistent with a smaller value of Lo. There is also evidence of smaller Lo from other, more direct sources such as balloonborne temperature probes and long-baseline interferometry. If Lo is smaller than previously thought, as is suggested here, many long-held ideas about imaging with ground-based telescopes will have to be modified. A much more favorable picture emerges, especially at near-infrared wavelengths. At these wavelengths, resolution in the range 0.03–0.1 arcsec should be routinely attainable with 4–10-m telescopes, even though seeing at visible wavelengths is only 1 arcsec. To attain such high levels of resolution, telescopes must be built to diffraction-limited standards rather than to the currently accepted standards, which fall well short of this limit. Recent images obtained at 2.2 μm with the 4-m Kitt Peak telescope show that very high resolution (0.1 arcsec) is attainable. The images also show that telescope aberrations prevent even higher resolution (0.05 arcsec). A further benefit of a smaller Lo is that the isoplanatic angle of the atmosphere at near-infrared wavelengths is likely to be much larger than previously thought. Thus much wider angular regions are available from which to select suitably bright stars for guiding and tracking. A small Lo also means that ground-based infrared laser beams may be focused to diffraction-limited accuracy on targets in space without necessarily having to use wave-front compensation.