The Naval Postgraduate School’s segmented mirror telescope (SMT) was developed using prototype silicon carbide active hybrid mirror technology to demonstrate lower cost and rapid manufacture of primary mirror segments for a space telescope. The developmental mirror segments used too few actuators limiting the ability to adequately correct the surface figure error. To address the unintended shortfall of the developmental mirrors, a deformable mirror is added to the SMT and control techniques are developed. The control techniques are similar to woofer-tweeter adaptive optics, where the SMT segment represents the woofer and the deformable mirror represents the tweeter. The optical design of an SMT woofer-tweeter system is presented, and the impacts of field angle magnification on the placement and size of the deformable mirror are analyzed. A space telescope woofer-tweeter wavefront control technique is proposed using a global influence matrix and closed-loop constrained minimization controller. The control technique simultaneously manipulates the woofer and tweeter mirrors. Simulation and experimental results demonstrate a significant improvement in wavefront error of the primary mirror and the control technique shows significant wavefront error improvement compared to sequentially controlling the woofer and tweeter mirrors.
High development cost is a challenge for space telescopes and imaging satellites. One of the primary reasons for this high cost is the development of the primary mirror, which must meet diffraction limit surface figure requirements. Recent efforts to develop lower cost, lightweight, replicable primary mirrors include development of silicon carbide actuated hybrid mirrors and carbon fiber mirrors. The silicon carbide actuated hybrid mirrors at the Naval Postgraduate School do not meet the surface quality required for an optical telescope due to high spatial frequency residual surface errors. A technique under investigation at the Naval Postgraduate School is to correct the residual surface figure error using a deformable mirror in the optical path. We present a closed loop feedback gradient controller to actively control a SMT active segment and an additional deformable mirror to reduce residual wavefront error. The simulations and experimental results show that the gradient controller reduces the residual wavefront error more than an integral controller.
Future large aperture space telescopes may use lightweight correctable active mirrors. The Naval Postgraduate School’s Segmented Mirror Telescope (SMT) test bed uses 1-meter silicon carbide (SiC) active mirror segments to form a sixsegment deployable 3-meter telescope. The active segments suffer from residual surface errors after a correction is applied. A deformable mirror is added at the SMT pupil plane to improve this residual error. The large active SMT segment represents the woofer, and a small continuous micro-electro-mechanical system (MEMS) deformable mirror represents the tweeter. A global influence matrix and closed loop constrained least squares controller command the active segment and additional deformable mirror as a single device. An interferometer measures the surface error and provides feedback to the controller. Simulation and experimental results demonstrate a significant improvement in wavefront error compared to a 2-step sequential woofer-tweeter constrained least squares control approach.
Future space based deployable telescopes will be subject to non-atmospheric disturbances. Jitter and optical
misalignment on a spacecraft can be caused by mechanical noise of the spacecraft, and settling after maneuvers. The
introduction of optical misalignment and jitter can reduce the performance of an optical system resulting in pointing
error and contributing to higher order aberrations. Adaptive optics can be used to control jitter and higher order
aberrations in an optical system. In this paper, wavefront control methods for the Naval Postgraduate School adaptive
optics testbed are developed. The focus is on removing structural noise from the flexible optical surface using discrete
time proportional integral control with second order filters. Experiments using the adaptive optics testbed successfully
demonstrate wavefront control methods, including a combined iterative feedback and gradient control technique. This
control technique results in a three time improvement in RMS wavefront error over the individual controllers correcting
from a biased mirror position. Second order discrete time notch filters are also used to remove induced low frequency
actuator and sensor noise at 2Hz. Additionally a 2 Hz structural disturbance is simulated on a Micromachined
Membrane Deformable Mirror and removed using discrete time notch filters combined with an iterative closed loop
feedback controller, showing a 36 time improvement in RMS wavefront error over the iterative closed loop feedback