Measurement of atmospheric turbulence with a Shack-Hartmann wavefront sensor at the new MMT's prime focus

In order to prepare for the adaptive secondary program at the new 6.5 meter MMT, we have begun a campaign to measure atmospheric turbulence with a Shack-Hartmann wavefront sensor (WFS) camera. Our first efforts, prior to second aluminization of the primary, consisted of taking turbulence data with our WFS camera with special coma-correcting optics at the MMT prime focus, without a Cassegrain secondary mirror. Our first measurements consisted of 1000 frames of approximately 10 millisecond duration, taken approximately 3 seconds apart. We measure a 5/3 power law structure function, suggesting Kolmogorov turbulence, with an r₀ = 15 cm, but with possible hints of an outer scale and tracking jitter in the structure function. At the end of our data acquisition, we deliberately put 2 μm of astigmatism into the primary mirror with its actuators, and in our analysis, we recover 1.7±0.3 μm of astigmatism. A brief analysis of the low-order modal amplitudes with the 3 second frame delay shows that there are significant self-correlations of the low-order modes even on this long time-scale.