Laboratory verification of Fast & Furious phase diversity: Towards controlling the low wind effect in the SPHERE instrument
Laboratory verification of Fast & Furious phase diversity: Towards controlling the low wind effect in the SPHERE instrument
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Fast的实验室验证
DOI：
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发表时间：
2018
影响因子：
6.5
通讯作者：
J. Beuzit
中科院分区：
文献类型：
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作者：
M. Wilby;C. Keller;J. Sauvage;K. Dohlen;T. Fusco;D. Mouillet;J. Beuzit
上下文。低风效应是指当球面风速降至3ms−1以下时，球体仪器一致观测到的一组准静态波前像差。低风效应在恒星点扩展函数中产生明亮的低阶散斑，这严重限制了球体在其他最佳观测条件下的对比度性能。
目标。在本文中，我们提出了一种可行的软件解决方案--Fast&Furious(F&F)相位分集算法，它将利用球面差动倾斜传感器(DTTS)的图像序列，通过参考斜率偏移量对AO系统的Shack-Hartmann波前传感器进行校正。
方法：研究方法。我们在MITHIC高对比度测试台上，在模拟受LWE影响的DTTS图像的条件下，评估了F&F的闭环性能。将这些结果与各种收敛测试的预测模拟进行了对比，以评估F&F在SPHERE中的天空实施的预期性能。
结果。对于所有合适的实验室测试用例，该算法被发现能够在五次迭代内将受LWE影响的图像返回到90%以上的Strehl比率。这些结果具有很强的预测仿真代表性，并证明了该算法对图像低信噪比(S/N)、小图像视场和幅度误差等多种因素的稳定性。仿真中还发现，闭环系统的稳定性可以保持在低至5的S/N图像上，同时仍能改善整体波前质量，即使在微弱目标上也能可靠地工作。
结论。Fast&Furious算法是一种非常有前景的LWE实时补偿解决方案，它可以与科学观测同时运行，并且可以在球面上实现，而不需要额外的硬件。F&F的坚固性和相对较大的有效动态范围也使其适用于一般的波前优化应用，包括分段ELT级望远镜的共相位。
Context. The low wind effect (LWE) refers to a characteristic set of quasi-static wavefront aberrations seen consistently by the SPHERE instrument when dome-level wind speeds drop below 3 ms−1. The LWE produces bright low-order speckles in the stellar point-spread function (PSF), which severely limit the contrast performance of SPHERE under otherwise optimal observing conditions.
Aims. In this paper we propose the Fast & Furious (F&F) phase diversity algorithm as a viable software-only solution for real-time LWE compensation, which would utilise image sequences from the SPHERE differential tip-tilt sensor (DTTS) and apply corrections via reference slope offsets on the AO system’s Shack-Hartmann wavefront sensor.
Methods. We evaluated the closed-loop performance of F&F on the MITHIC high-contrast test-bench, under conditions emulating LWE-affected DTTS images. These results were contrasted with predictive simulations for a variety of convergence tests, in order to assess the expected performance of an on-sky implementation of F&F in SPHERE.
Results. The algorithm was found to be capable of returning LWE-affected images to Strehl ratios of greater than 90% within five iterations, for all appropriate laboratory test cases. These results are highly representative of predictive simulations, and demonstrate stability of the algorithm against a wide range of factors including low image signal-to-noise ratio (S/N), small image field of view, and amplitude errors. It was also found in simulation that closed-loop stability can be preserved down to image S/N as low as five while still improving overall wavefront quality, allowing for reliable operation even on faint targets.
Conclusions. The Fast & Furious algorithm is an extremely promising solution for real-time compensation of the LWE, which can operate simultaneously with science observations and may be implemented in SPHERE without requiring additional hardware. The robustness and relatively large effective dynamic range of F&F also make it suitable for general wavefront optimisation applications, including the co-phasing of segmented ELT-class telescopes.