Simulating the Effects of an Extended Source on the Shack-Hartmann Wavefront Sensor through Turbulence

Perspective elongation in Shack-Hartmann wavefront sensor (SHWFS) spots is a phenomenon caused by an extended three-dimensional scattering beacon in the mesosphere (80-100km). Elongated spots cause errors in wavefront sensor measurements, which leads to poor turbulence compensation and decreased image resolution of the optical system. In order to compensate for elongated spots, a proper beacon model must be developed to simulate the error. In this paper, a documented theory for modeling an elongated sodium beacon and elongated SHWFS spots using sodium layer "slices" was tested. It was found that nine evenly-spaced slices were adequate to model the elongated beacon in the most stringent, turbulence-included case. Furthermore bench-top source was developed and tested to model SHWFS spot elongation in the lab. The source demonstrated the principle, but requires a more robust design to simulate sodium layer depth. Being the first documented experiment using an extended source on an adaptive optics (AO) system, it opens the door for more research to include: the effects of deep turbulence on AO systems and correlation based wavefront sensing with extended sources.