4 ANISOPLANATISM 29Figure 8: Focal and angular anisoplanatism <strong>PSF</strong>s (0.73 × 0.73 arc second field of view) in the analytical von Karman modelwith L 0 =36m,at(0, 0), (15, 15) and (30, 30) arc seconds.outer scale in the analytical von Karman calculations varied between L 0 = {1000, 100, 36, 18} meters, while thenumerical simulation only produced results at L 0 = {36, 18} meters (since the phase screens were FFT-based with alength of 72 meters).An overview of Strehl ratios obtained in the various cases are given in Table 1, showing exancellent agreementbetween the analytical von Karman model and numerical simulations. The von Karman model also reproduces theKolmogorov result at very large outer scales. The effect of the finite outer scale on the structure function and the OTFcan be plotted in the case of NGS anisoplanatism (since it is stationary), as shown in Figs. 6 and 6. Figures 7 and9 plot the relative <strong>PSF</strong> error, i.e. the normalized <strong>PSF</strong> subtraction computed as (numerical − analytical)/numerical,<strong>for</strong> a line segment of the <strong>PSF</strong>s (the positive x-axis). While in some parts the reported numbers may seem large (0.5implies a 50% error), the relative error in the <strong>PSF</strong> halo should be weighted by the fact that the energy levels aretwo or three orders of magnitude lower than in the central region of the <strong>PSF</strong>. The encircled energy curves in Fig. 7and 10 also show that even when the relative error in the wings of the <strong>PSF</strong> is large, this still amounts to a negligibleerror in an absolute sense. As long as the overall energy distribution is correct, a small error in the <strong>PSF</strong> halo hasno impact on the encircled energy. Even in the worst case considered (dashed gray curves in Fig. 9 and 9), theanalytical von Karman model is still very accurate and the error practically negligible.NGS anisoplanatism 0 ′′ 21.2 ′′ 42.4 ′′analytical KolmogorovL 0 = ∞ 1.000 0.343 0.101analytical von KarmanL 0 = 1000 1.000 0.343 0.101L 0 = 100 1.000 0.350 0.106L 0 = 36 1.000 0.380 0.127L 0 = 18 1.000 0.438 0.183numerical von KarmanL 0 = 36 1.000 0.383 0.127L 0 = 18 1.000 0.442 0.183LGS anisoplanatism 0 ′′ 21.2 ′′ 42.4 ′′analytical von KarmanL 0 = 36 0.782 0.387 0.143L 0 = 18 0.803 0.440 0.192numerical von KarmanL 0 = 36 0.791 0.385 0.137L 0 = 18 0.812 0.445 0.192Table 1: NGS and LGS anisoplanatism Strehl ratios, comparing analytical predictions to numerical simulations, <strong>for</strong> differentouter scales and different off-axis angles θ.
4 ANISOPLANATISM 30Figure 9: Left: Sample LGS anisoplanatism <strong>PSF</strong> profile (cut along the positive x-axis), comparing the numerical simulation(solid black line) and the analytical von Karman model. Right: Relative <strong>PSF</strong> error of the analytical von Karman model <strong>for</strong>LGS anisoplanatism, measured against the numerical simulation <strong>for</strong> the θ =0 ′′ and θ =21.2 ′′ cases (cut of <strong>PSF</strong>s along thepositive x-axis).4.4 ConclusionsStructure functions <strong>for</strong> angular and focal anisoplanatism were derived analytically including a finite outer scale ina von Karman type turbulence model. The analytical result is in excellent agreement with numerical Monte Carlosimulations, and offers an analytical method to produce anisoplanatism kernels <strong>for</strong> <strong>PSF</strong> <strong>reconstruction</strong> problemsand <strong>PSF</strong> modeling in general. The model may replace Kolmogorov-based models where the impact of a finite outerscale becomes non-negligible. In the current investigation it is found that <strong>for</strong> 10-meter-class telescopes, an outerscale smaller than ∼30 meters may have a non-negligible impact on the anisoplanatism estimation, and under theseconditions the von Karman model should be preferred.Not taken into account in the current analysis is the spatial filtering of the wave-front per<strong>for</strong>med by the <strong>AO</strong>system. This effect can be accounted <strong>for</strong> by setting the weighting functions w i equal to the de<strong>for</strong>mable mirror(DM) influence functions in Eq. (110). This renders the structure function non-stationary, but in the case of NGSanisoplanatism the expression can still be put on a <strong>for</strong>m that allows <strong>for</strong> a relatively efficient computation of theOTF. For completeness, this result is given in Appendix A, but the method is unlikely to be employed <strong>for</strong> <strong>PSF</strong><strong>reconstruction</strong> since: 1) in the case of focal anisoplanatism, the expression becomes computationally expensive andimpractical <strong>for</strong> <strong>PSF</strong> <strong>reconstruction</strong>, so the approximation in Eq. (127) must be invoked in any case, and 2) theeffect is small in high-order <strong>AO</strong> systems that have d/r 0 < 1, where d is the DM actuator spacing. As shown in Fig.10, the effect of spatial filtering is to reduce the amount of scattered light from anisoplanatism in the <strong>PSF</strong> halo,which should be modeled by the fitting error instead. If the fitting error is small, however, omitting spatial filteringin the anisoplanatism model has a small effect on the the <strong>PSF</strong> structure, on the Strehl ratio, and on the resultingphotometry estimates produced with this model. For the current WMKO <strong>AO</strong> system (d =0.56 m), in this modelthe LGS Strehl ratio at L 0 = 36 m at the three evaluations points without spatial filtering is [0.792, 0.384, 0.134],and with spatial filtering they are [0.829, 0.416, 0.0.147]. Given that the added complexity of trying to model thiseffect realistically makes it impractical <strong>for</strong> LGS <strong>PSF</strong> <strong>reconstruction</strong>, one possible compromise might be to use theunfiltered models derived in Sect. 4.1 and Sect. 4.2, and implement a heuristic adjustment to the fitting error inorder to account <strong>for</strong> the fact that some high-spatial-frequency wavefront error is added by the anisoplanatism term.This route is being further investigated within the <strong>PSF</strong> <strong>reconstruction</strong> project that motivated the current research.
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