Wavefront Error Compensation Capabilities Of Multi-Actuator Deformable Mirrors

The concept of a multi-actuator deformable mirror as a high-pass spatial frequency filter which operates upon the spatial frequency spectrum of the incident wavefront error allows us to apply the well-known techniques of linear systems theory to the wavefront error compensation process. The deformable mirror filter function, given by the ratio of the spatial frequency spectrum of the residual wavefront error after compensation to the spatial frequency spectrum of the uncorrected wavefront error, can be considered to be a transfer function characterizing the wavefront error compensation capabilities of the mirror. This concept has been implemented by constructing a realistic transfer function from measured actuator influence function data and incorporating the effects of several different types of wavefront sensors and control algorithms. For the special case of random wavefront errors with Gaussian statistics this transfer function can be used to predict the variance of the residual wavefront error as a function of the autocovariance length of the uncorrected wavefront error for any desired actuator density. The resulting design curves provide a simple method of determining the actuator density and stroke requirements of a deformable mirror capable of achieving a specified degree of wavefront error compensation. The actuator influence function is also explored as a variable in the design of deformable mirrors. The simple analytic model developed here is a valuable tool in the design and evaluation of wavefront error compensation systems.