A Better Closed Loop

On July 21 we closed the loop again (but without wind on the outside of the tower). In these calm conditions we were able to obtain 55% Strelh at i’ (765 nm) which is excellent correction in 0.8″ seeing and 33mph winds. I attach a log10 stretch image below to show the very high contrast PSFs that are obtained with the Magellan AO system. This is slightly better performance then was predicted for an R=8 mag guide star in a 33 mph wind with ro=14cm at 0.55um. Normally at Magellan the seeing should be better (and the wind lower) than what we simulated here, giving us some confidence that VisAO will be an excellent visible AO science camera in >75% of the weather seen at the Magellan telescope.(clockwise) perfect PSF, the MagAO PSF, The AO ON PSF, and the AO OFF PSF (Log Scale)

Note by Jared: due to the way turbulence is simulated in the tower using the mirror itself, there is some missing power in the modes higher than ~585 (the number of actuators on our mirror). This results in an optimistic fitting error during these tests. We can estimate how much lower Strehl at the telescope would be due to this unsimulated turbulence using some AO theory (see Noll 1976). Our most conservative estimate for this correction brings our telescope Strehl down to 37%, from the 55% measured in the tower. Compared to the correction calculated by a more empirical method for the LBT (see Esposito 2010) this correction is probably a little large (resulting in a low estimate for Strehl). Even with this conservative correction, our tower results are exceeding our performance predictions for an 8th magnitude guide star by more than 5% Strehl. This is very exciting!

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