The Fringe Sensor Unit (FSU) is the central element of the Phase Referenced Imaging and Micro-arcsecond Astrometry (PRIMA) dual-feed facility and provides fringe sensing for all observation modes, comprising off-axis fringe tracking, phase referenced imaging, and high-accuracy narrow-angle astrometry. It is installed at the Very Large Telescope Interferometer (VLTI) and successfully servoed the fringe tracking loop during the initial commissioning phase. Unique among interferometric beam combiners, the FSU uses spatial phase modulation in bulk optics to retrieve real-time estimates of fringe phase after spatial filtering. A R=20 spectrometer across the K-band makes the retrieval of the group delay signal possible. The FSU was integrated and aligned at the VLTI in summer 2008. It yields phase and group delay measurements at sampling rates up to 2 kHz, which are used to drive the fringe tracking control loop. During the first commissioning runs, the FSU was used to track the fringes of stars with K-band magnitudes as faint as m_K=9.0, using two VLTI Auxiliary Telescopes (AT) and baselines of up to 96 m. Fringe tracking using two Very Large Telescope (VLT) Unit Telescopes (UT) was demonstrated. During initial commissioning and combining stellar light with two ATs, the FSU showed its ability to improve the VLTI sensitivity in K-band by more than one magnitude towards fainter objects, which is of fundamental importance to achieve the scientific objectives of PRIMA.
The Giant Magellan Telescope high contrast adaptive optics phasing testbed (p-HCAT): lab tests of segment/petal phasing with a pyramid wavefront sensor and a holographic dispersed fringe sensor (HDFS) in turbulence
Characterization of sensitivity and responses of a 2-element prototype wavefront sensor for millimeter-wave adaptive optics attached to the Nobeyama 45 m telescope
我们报告的结果的原型波前传感器的性能表征毫米自适应光学（MAO）安装在Nobeyama 45米射电望远镜。微弧氧化是实现未来大口径亚毫米波望远镜的关键部件，如大型亚毫米波望远镜（LST）或阿塔卡玛大口径亚毫米波望远镜（AtLAST）。然而，微弧氧化的困难在于，在孔径上以~10 μ m的精度实时感测波前变形。我们的波前传感器工作在20 GHz的测量主镜表面的某个位置到放置20 GHz相干接收器的焦点之间的无线电路径长度。与2元件的原型，我们采样的两个位置上的主镜表面（在半径为5米和16米）在10 Hz的采样率。然后，通过对两个光路进行微分，得到两个位置之间的多余光程长度（EPL）。EPL的功率谱密度示出三个分量：低频漂移（1/f^n）、振荡和白噪声。比较了不同风况下的EPL测量结果，发现前两种现象可能是由望远镜结构的风荷载引起的。白噪声的功率对应于EPL测量中8 μ m的1西格玛统计误差。8 um r.m.s.对于LST和AtLAST所需的镜面精度（~20-40 μ m r.m.s.）而言是重要的，这表明，我们的技术也是有用的，为未来的大口径亚毫米望远镜。
