Experimental verification of compliant mirror wavefront correction using a single actuator

The concavity of an initially flat wavefront typically increases after each reflection of the ten-reflection beam transport system at the Navy Precision Optical Interferometer (NPOI). Ideally, the exiting wavefront contour from the beam transport system preserves the original contour that enters. The beam transport system is common to and separate from the front-end, which includes primary light collectors such as siderostats or telescopes, and the back-end which includes major subsystems such as the optical delay lines, beam combiners and detectors. The beam transport system should have minimal influence on the interferometer. However, manufacturing tolerances and mount-induced deformations of each mirror collude to alter each reflected wavefront. All beam transport mirrors at the NPOI are slightly concave and each reflection adds to the concavity in the resultant wavefront. To improve the flatness of the resultant wavefront, we counter-deform a single mirror in the ten-reflection transport system. Previous analytical work using finite element analysis demonstrated the feasibility of this approach. In the present work, we have undertaken the task of verifying this approach experimentally. We set up a nine-reflection system of NPOI transport mirrors and measured the resultant beam wavefront contour. We applied a single actuator to the backside of one of the mirrors in the system and measured the contour of the exiting wavefront. Additionally, we compared the reduced concavity of the exiting wavefront to our finite element method results from the previous work, and excellent agreement was observed. In this paper, we describe our wavefront improvement approach, experimental method and results, and recommendations.