With some confidence the PSF estimation can now be applied to solar AO. Figure 26 plots Strehl ratio versus the Fried parameter r0. Both Strehl ratio and r0 were estimated from the AO telemetry data as part of the PSF estimation algorithm. A Strehl of S = 0.9 or greater achieved for excellent seeing conditions of r0 = 20 cm. A Strehl of S = 0.3 is achieved for an r0 of about 4 cm.
The solid line follows the Strehl ratio expected from the wavefront error variances that include the most significant AO error sources: fitting error, aliasing error, bandwidth error, and wavefront sensor noise error. The agreement between the model expectation and the actual performance is remarkably good, indicating that the error budget of the AO76 system is in general well understood and that close to theoretical performance is achieved.
The second solid line traces a branch where apparently non-optimal AO performance was achieved. This branch can be modeled by adding a constant noise term. The source of this additional noise term is not yet well understood. Possible candidates are misalignment due to pupil wobble, which the DST is prone to, or wavefront sensor noise due to strong anisoplanatism (Wöger and Rimmele, 2009).
Reconstruction of solar imagery using estimated long exposure PSFs will be discussed in Section 7. It should be mentioned that in spite of the encouraging agreement shown in Figure 25 the contrast of reconstructed solar images appears to be consistently too low when compared to model predictions. Entering this ongoing scientific debate would be beyond the scope of this article. Stray-light from uncorrected high order modes (Scharmer et al., 2010) or other stray-light sources is one possible explanation for this discrepancy.
Living Rev. Solar Phys. 8, (2011), 2
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