Thomas Sandrine, Evans Julia W, Gavel Donald, Dillon Daren, Macintosh Bruce
University of California at Santa Cruz, 1156 High Street, Santa Cruz, California, USA.
Appl Opt. 2009 Jul 20;48(21):4077-89. doi: 10.1364/ao.48.004077.
High-contrast imaging techniques such as coronagraphy are expected to play an important role in the imaging of extrasolar planets. Instruments like the Gemini Planet Imager (GPI) or the Spectro-Polar-Imetric High-Contrast Exoplanet Research (SPHERE) require high-dynamic range, achieved using coronagraphs to block light coming from the parent star. An extremely good adaptive optics (AO) system is required to reduce dynamic atmospheric wavefront errors to 50-100 nm rms. Systematic wavefront errors must also be controlled at the nanometer-equivalent level to remove persistent speckle artifacts. While precision AO systems can control wavefront phase errors at this level, systematic amplitude or intensity errors can also produce speckle artifacts and are uncontrolled by traditional AO phase conjugation. On the Laboratory for Adaptive Optics (LAO) extreme AO testbed, we observed a discrepancy between the coronagraphic image profile and the profile predicted by simple simulations using the measured optical phase, which could potentially be explained by amplitude variations. Measurements showed up to 7% rms intensity changes across the microelectrical mechanical (MEM) plane of the system. We identified potential sources of amplitude variation and compared them to a Fresnel model of the system. One potential concern was the surface structure of the MEM system's (MEMS) deformable mirror, but analysis shows that it induces at most 2% rms variation. The bulk of the observed intensity variation is due to nonuniform illumination of the system by the input single-mode fiber and phase errors mixing into amplitude at the nonpupil-plane due to the Talbot effect, coupled with residual astigmatism in the pupil imager.
日冕仪等高对比度成像技术有望在系外行星成像中发挥重要作用。像双子座行星成像仪（GPI）或光谱偏振高对比度系外行星研究（SPHERE）这样的仪器需要高动态范围，这是通过日冕仪阻挡来自母恒星的光线来实现的。需要一个极其出色的自适应光学（AO）系统将动态大气波前误差降低到50 - 100纳米均方根值。还必须将系统波前误差控制在纳米等效水平，以消除持续的散斑伪像。虽然精密AO系统可以在这个水平上控制波前相位误差，但系统的幅度或强度误差也会产生散斑伪像，并且传统的AO相位共轭无法对其进行控制。在自适应光学实验室（LAO）的极端AO试验台上，我们观察到日冕仪图像轮廓与使用测量的光学相位进行简单模拟预测的轮廓之间存在差异，这可能是由幅度变化所解释的。测量结果显示，整个系统的微机电（MEM）平面上强度均方根变化高达7%。我们确定了幅度变化的潜在来源，并将它们与系统的菲涅耳模型进行了比较。一个潜在问题是MEM系统（MEMS）可变形镜的表面结构，但分析表明它最多引起2%的均方根变化。观察到的强度变化主要是由于输入单模光纤对系统的不均匀照明，以及由于塔尔博特效应在非光瞳平面上相位误差混入幅度，再加上光瞳成像仪中的残余像散。
