Off-axis variable focus and aberration control mirrors

Elliptical-boundary deformable mirrors have been developed for focus control of an optical beam incident at forty-five degrees with respect to the surface normal. The mirrors are silicon nitride membranes 1.4x1 mm in size, designed to accommodate a 1 mm diameter beam. Two electrostatic actuation zones provide control over spherical aberration. Focal lengths ranging from infinity to 36 mm have been achieved, and the mirror surface figure has been characterized to quantify aberration. Residual aberrations have been observed to be less than lambda/5 (peak to peak) measured at lambda = 660 nm.