Zernike based wavefront alignment in a deployable optic control test bed

Abstract

An automated optical alignment method is necessary for the efficient operation of deployable optic-equipped earth observation micro-satellites. Implementation of a linear output feedback controller for the alignment of an active optic is investigated on the Control Test Bed (CTB). The output feedback controller calculates the required positional adjustments of the optic using a multi-variable output measurement. The CTB is a refractive optical system by which the wavefront of an incoming collimated beam is modified by control inputs that adjust the position of a lens. The position of this lens can be translated in the x, y, and z axes (despace and defocus) and rotated about the x and y axes (tip and tilt). In the CTB, a Shack-Hartmann wavefront sensor (SHWFS) measures the output as Zernike coefficients. This setup is intended to emulate the necessary adjustments in a simple deployable optic. We describe the process of developing the output feedback controller that can align the CTB as well as discuss the implications of the scheme to a space-based deployable optic telescope. The aberrations induced by misalignment of the active lens were characterized experimentally by disturbing the system along each degree of freedom about the optically aligned point. The experimental results derive a state observer that can sufficiently estimate the misalignment of the lens from a set of relevant Zernike coefficients. Iterative correction of the estimated misalignment in a closed-loop allowed consistent convergence of the CTB to a nearly flat wavefront.