6.4 Measured AO system performance

The PSF estimation method described in the previous section can be used to measure and optimize the performance of an AO system with the caveat that non-common path errors are not detected. The performance is estimated at the WFS focal plane. Since the PSF is measured indirectly by this method it is desirable to verify its results in some independent fashion. This was done by Marino (2007) and Marino and Rimmele (2010Jump To The Next Citation Point) who modified the AO76 at the DST to enable the system to lock on bright stars (e.g., Sirius), albeit with significantly reduced AO performance due to S/N issues. Figure 25View Image, from Marino and Rimmele (2010Jump To The Next Citation Point), shows several comparisons between estimated PSF and the actual PSF (the Sirius image). In spite of the low Strehl achieved with the solar AO running in night-time mode the agreement between actual and estimated PSF is excellent. Significant deviation is visible only for the images with lowest Strehl and tends to affect the residual seeing halo more than the core of the PSF.
View Image

Figure 25: Estimated PSF vs. actual PSF. The AO76 was looked on the bright star Sirius. Long exposure images of this point source directly measure the AO PSF, which can be compared to the estimated PSF provided by the AO76 telemetry (from Marino and Rimmele, 2010Jump To The Next Citation Point).

With some confidence the PSF estimation can now be applied to solar AO. Figure 26View Image 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, 2009Jump To The Next Citation Point).

View Image

Figure 26: Strehl vs. Fried parameter r0. The Strehl was estimated using the AO76 telemetry data and the long exposure PSF estimation method. The AO was locked on a small sunspot. Seeing was highly variable spanning a wide range of r0 (from Marino and Rimmele, 2010Jump To The Next Citation Point).

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 25View Image 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., 2010Jump To The Next Citation Point) or other stray-light sources is one possible explanation for this discrepancy.

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