Ground-layer wave front reconstruction from multiple natural guide stars

Observational tests of open-loop ground-layer wave front recovery have been made using a constellation of four natural guide stars at the 1.55 m Kuiper telescope in Arizona. Such tests explore the effectiveness of wide-field seeing improvement by correction of low-lying atmospheric turbulence with ground-layer adaptive optics (GLAO). The wave fronts from the four stars were measured simultaneously on a Shack-Hartmann wave front sensor (WFS). The WFS placed a 5 × 5 array of square subapertures across the pupil of the telescope, allowing for wave front reconstruction up to the fifth radial Zernike order. We find that the wave front aberration in each star can be roughly halved by subtracting the average of the wave fronts from the other three stars. Wave front correction on this basis leads to a reduction in width of the seeing-limited stellar image by up to a factor of 3, with image sharpening effective from the visible to near-infrared wavelengths over a field of at least 2'. We conclude that GLAO correction will be a valuable tool that can increase resolution and spectrographic throughput across a broad range of seeing-limited observations.