The ıQuad sensor: a new Fourier-based wave front sensor derived from the 4 quadrants coronagraph
Résumé
Some coronagraph masks can be turned into wave front sensing masks thanks to minor modification. For instance, one only has to divide by two the depth of the central well to convert the Roddier & Roddier coronograph into the Zernike wave front sensor (WFS). Physically, the opposition of phase in coronagrapy becomes a quadrature phase in wave front sensing. Here, we replicate this idea to the Four Quadrant Phase Mask (FQPM) coronagraph by introducing a sensor that we call the ıQuad WFS, generated by a mask which has the same geometrical structure as the FQPM but with a modified differential piston. An optical and mathematical description of this new WFS is firstly provided showing its great elegance and the central role played by the Hilbert transform in its understanding. We then compare its performance criteria with two classical wave front sensors. We finally show the ıQuad sensor has major similarities to the Pyramid sensor making it a wonderful theoretical object to improve our understanding of this sensor.
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