Membrane based Deformable Mirror: Intrinsic aberrations and alignment issues

TL;DR

This paper investigates the intrinsic aberrations, especially astigmatism, introduced by a membrane deformable mirror at various incidence angles, crucial for optimizing adaptive optics systems.

Contribution

It provides a first-order estimation of aberrations caused by a curved deformable mirror at different angles, supported by experimental measurements and theoretical simulations.

Findings

Astigmatism is the dominant aberration induced.

Aberrations depend on the incidence angle and surface deformation.

Feasibility of using such mirrors in adaptive optics is discussed.

Abstract

A Deformable Mirror (DM) is an important component of an Adaptive Optics system. It is known that an on-axis spherical/parabolic optical component, placed at an angle to the incident beam introduces defocus as well as astigmatism in the image plane. Although the former can be compensated by changing the focal plane position, the latter cannot be removed by mere optical re-alignment. Since the DM is to be used to compensate a turbulence-induced curvature term in addition to other aberrations, it is necessary to determine the aberrations induced by such (curved DM surface) an optical element when placed at an angle (other than 0 degree) of incidence in the optical path. To this effect, we estimate to a first order, the aberrations introduced by a DM as a function of the incidence angle and deformation of the DM surface. We record images using a simple setup in which the incident beam is reflected by a 37 channel Micro-machined Membrane Deformable Mirror for various angles of incidence. It is observed that astigmatism is a dominant aberration which was determined by measuring the difference between the tangential and sagital focal planes. We justify our results on the basis of theoretical simulations and discuss the feasibility of using such a system for adaptive optics considering a trade-off between wavefront correction and astigmatism due to deformation.