Adaptive-optics corrections available for the whole sky

Adaptive-optics systems can in principle allow a telescope to achieve perfo rmance at its theoretical maximum (Limited only by diffraction), by correct ing in real time for the distortion of starlight by atmospheric turbulence( 1). For such a system installed on an 8-m-class telescope(2,3), the spatial resolution and sensitivity could be up to 100 times better than convention al imaging(4,5). Adaptive-optics corrections have hitherto been achieved on ly for regions of the sky within a few arcseconds of a bright reference sou rce. But it has been proposed theoretically that by using multiple guide st ars, the tomography of atmospheric turbulence could be probed and used to e xtend adaptive-optics corrections to the whole sky(6,7). Here we report the experimental verification of such tomographic(8) corrections, using three off-axis reference stars similar to 15 arcsec from the central star. We use d the observations of the off-axis stars to calculate the deformations of t he wavefront of the central star, and then compare them with the real measu red values. This tomographic approach is found to reduce variations in the wavefront by similar to 92%. Our result demonstrates that a serious barrier to achieving diffraction-limited seeing over the whole sky has been remove d.