Measurement of focus and off-axis anisoplanatism using a sodium resonance beacon and binary stars

We have obtained the first measurements with a sodium laser beacon of focus anisoplanatism over large aperture, at the Multiple Mirror Telescope (MMT). Focus anisoplanatism sets the fundamental limit to wavefront correction of large telescopes. An artificial star, of visual magnitude 12.5, was created by a 0.5 W beam from a continuous-wave dye laser tuned to the sodium D2 resonance line, projected from a telescope centered and coaxial with the main array of six 1.8 m mirrors. Atmospheric measurements were made by placing the laser beacon exactly coaxial with a natural star. A high-speed CCD camera in a plane midway between the foci of the star and beacon recorded the individual images from each of the six telescope mirrors, deliberately separated by adjusting the telescope alignment. From the instantaneous image centroids, six low-order Zernike coefficients for both the star and beacon wavefronts were computed and compared. The measured strength of focus anisoplanatism is found to be in good agreement with atmospheric models. In complementary studies, the atmospheric turbulence at the high altitude and on the large scale responsible for the measured focus anisoplanatism was explored by observations of binary stars of different separations. We confirm the predictions of Kolmogorov theory, and derive an effective height for the turbulence of 5050 m above the telescope. These results confirm that the sodium laser guide star planned for use with the 6.5 m telescope conversion of the MMT in 1996 will allow diffraction limited infrared observations in the H and K bands.