Active Optical Control of Quasi-Static Aberrations for ATST

The Advanced Technology Solar Telescope (ATST) requires active control of quasi-static telescope aberrations in order to achieve the image quality set by its science requirements. Four active mirrors will be used to compensate for optical misalignments induced by changing gravitational forces and thermal gradients. These misalignments manifest themselves primarily as low-order wavefront aberrations that will be measured by a Shack-Hartmann wavefront sensor. When operating in closed-loop with the wavefront sensor, the active optics control algorithm uses a linear least-squares reconstructor incorporating force constraints to limit force applied to the primary mirror while also incorporating a neutral-point constraint on the secondary mirror to limit pointing errors. The resulting system compensates for astigmatism and defocus with rigid-body motion of the secondary mirror and higher-order aberrations with primary mirror bending modes. We demonstrate this reconstruction method and present simulation results that apply the active optics correction to aberrations generated by finite-element modeling of thermal and gravitational effects over a typical day of ATST operation. Quasi-static wavefront errors are corrected to within limits set by wavefront sensor noise in all cases with very little force applied to the primary mirror surface and minimal pointing correction needed.