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Direct detection of exoplanets around nearby stars requires advanced AdaptiveOptics (AO) systems. High order systems are needed to reach high Strehl Ratio(SR) in near infrared and optical wavelengths on future Giant Segmented MirrorTelescopes (GSMTs). Direct detection of faint exoplanets with the ESO ELT willrequire some tens of thousand of correction modes. Resolution and sensitivityof the wavefront sensor (WFS) are key requirements for this science case. Wepresent a new class of WFSs, the Bi-Orthogonal Foucault-knife-edge Sensors (orBi-O-edge), that is directly inspired by the Foucault knife edge test (Foucault1859). The idea consists of using a beam-splitter producing two foci, each ofwhich is sensed by an edge with orthogonal direction to the other. We describetwo implementation concepts: The Bi-O-edge sensor can be realised with a sharpedge and a tip-tilt modulation device (sharp Bi-O-edge) or with a smoothgradual transmission over a grey edge (grey Bi-O-edge). A comparison betweenthe Bi-O-edge concepts and the 4-sided classical Pyramid Wavefront Sensor (PWS)gives some important insights into the nature of the measurements.Our analysisshows that the sensitivity gain of the Bi-O edge with respect to the PWSdepends on the system configuration. The gain is a function of the number ofcontrol modes and the modulation angle. We found that for the sharp Bi-O-edge,the gain in reduction of propagated photon noise variance approaches atheoretical factor of 2 for a large number of control modes and smallmodulation angle, meaning that the sharp Bi-O-edge only needs half of thephotons of the PWS to reach similar measurement accuracy.

The Bi-O-edge wavefront sensor: How Foucault-knife-edge variants can boost eXtreme Adaptive Optics