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Currently, in most adaptive optical systems, the control loop between the wavefront sensor and the deformable mirror involves intense mathematical calculations, both during calibration and operation of the system. Although thorough research has been done to optimise the control loop, some issues like error propagation and system bandwidth will always be ultimately limited by the coupling between the mirror and the wavefront sensor. Closed-loop by direct feedback from the wavefront sensor to the deformable mirror was proposed by F. Roddier in his well-quoted curvature wavefront sensing paper. However, due to the natural properties of the defocused-image, this direct feed-back method is limited to bimorph mirror applications only. Recently, M.A.A Neil et al proposed a new modal wavefront sensor (MWFS), which can detect several Zernike modes by a simple intensity subtraction operation. One drawback of this method is that it can only handle a limited number of modes. However, in this paper, we refine this method to detect the orthogonal modes of a deformable mirror instead of Zernike modes in a to-be corrected wavefront. Since the number of actuators of a deformable mirror limits the number of mirror modes, the drawback is minimised in this application. Considering the mirror modes can be directly transformed to the deformable mirror control command set by a proper gain coefficient, it is reasonable to construct a direct-feed back adaptive optical system with the modal wavefront sensing. We will report our first stage investigation on direct feedback adaptive optical system which is to understand the response of MWFS to mirror modes.
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