It is envisaged that future large space telescopes will be lightweight and employ active optics to maintain optical quality throughout the mission lifetime. We have proposed a 4 m, two-mirror space telescope with an active optics system based on reimaging the telescope primary mirror onto a small active mirror (110 mm optical pupil). Using Zemax, we demonstrate the feasibility of using this mirror to correct low-order Zernike aberrations and show that the aberration is well corrected across the 2.5 arcmin field of the telescope, operating at 0.55 μm. We describe the modeling carried out to develop the active mirror design. Using end-to-end modeling, a 25-actuator mirror with polar actuator geometry, and a ratio of mechanical to optical pupil diameter of 2 has been chosen. A single-actuator prototype has been manufactured and used to test stroke, linearity, and hysteresis. Finally, we describe the design of a laboratory breadboard that will image phase screens onto an exact replica of the space active mirror and show the results of measuring the phase screen accuracy.

Development of a prototype active optics system for future space telescopes.

We discuss the implementation and performance of an adaptive optics (AO) system that uses two cascaded deformable phase plates (DPPs), which are transparent optofluidic phase modulators, mimicking the common woofer/tweeter-type astronomical AO systems. One of the DPPs has 25 electrodes forming a keystone pattern best suited for the correction of low-order and radially symmetric modes; the second device has 37 hexagonally packed electrodes better suited for high-order correction. We also present simulation results and experimental validation for a new open-loop control strategy enabling simultaneous control of both DPPs, which ensures optimum correction for both large-amplitude low-order, and complex combinations of low- and high-order aberrations. The resulting system can reproduce Zernike modes up to the sixth radial order with stroke and fidelity up to twice better than what is attainable with either of the DPPs individually. The performance of the new AO configuration is also verified in a custom-developed fluorescence microscope with sensorless aberration correction.

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Cascading optofluidic phase modulators for performance enhancement in refractive adaptive optics

Shape Control of a Reflector Based on Generalized Zernike Functions

For a large adaptive optics system such as a large segmented mirror telescope SMT, it is often difficult, although not impossible, to directly apply common multi-input multi-output MIMO controller design methods due to the computational burden imposed by the large dimension of the system model. In this article, a practical controller design method is proposed which significantly reduces the system dimension for a system where the dimension required to represent the dynamics of the plant is much smaller than the dimension of the full plant model. The proposed method decouples the dynamic and static parts of the plant model by a modal decomposition technique to separately design a controller for each part. Two controllers are then combined using the so-called sensitivity decoupling method so that the resulting feedback loop becomes the superposition of the two individual feedback loops of the dynamic and static parts. A MIMO controller was designed by the proposed method using the H∞ loop-shaping technique for an SMT model to be compared with other controllers proposed in the literature. Frequency-domain analysis and time-domain simulation results show the superior performance of the proposed controller.

/pdf/active-control-of-adaptive-optics-system-in-a-large-iga1twyxys.pdf

Active control of adaptive optics system in a large segmented mirror telescope

Maintaining geometrical high precision for a graphite fiber reinforced composite (GFRC) reflector is a challenging task. Although great efforts have been placed to improve the fabrication precision, geometry adaptive control for a reflector is becoming more and more necessary. This paper studied geometry adaptive control for a GFRC reflector with piezoelectric ceramic transducer (PZT) actuators assembled on the ribs. In order to model the piezoelectric effect in finite element analysis (FEA), a thermal analogy was used in which the temperature was applied to simulate the actuation voltage, and the piezoelectric constant was mimicked by a Coefficient of Thermal Expansion (CTE). PZT actuator’s equivalent model was validated by an experiment. The deformations of a triangular GFRC specimen with three PZT actuators were also measured experimentally and compared with that of simulation. This study developed a multidisciplinary analytical model, which includes the composite structure, thermal, thermal deformation and control system, to perform an optimization analysis and design for the adaptive GFRC reflector by considering the free vibration, gravity deformation and geometry controllability.

Geometry adaptive control of a composite reflector using PZT actuator

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 alone.

/pdf/control-of-a-deformable-mirror-subject-to-structural-2u0ccgjz3d.pdf

Control of a deformable mirror subject to structural disturbance

High resolution persistent imaging in space necessitates a large aperture space telescope. Achieving a large aperture in space is very challenging because of the size, weight, surface accuracy, manufacturing schedule, and cost constraints of the telescope primary mirror. In this paper, four viable cost-effective approaches for large aperture space imaging are discussed. The first approach is the use of segmented mirrors. The second approach is the use of a deformable mirror for reduced surface requirements. The third approach is to use sparse aperture system. The fouth approach is to assemble in orbit. Illustrative examples and related research at the Naval Postgraduate School are also presented for these approaches.

Cost-effective Large Apertures for Future Imaging Satellites

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.

Correction of active space telescope mirror using woofer-tweeter adaptive optics

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.

https://www.spiedigitallibrary.org/journals/Journal-of-Astronomical-Telescopes-Instruments-and-Systems/volume-2/issue-2/029001/Correction-of-an-active-space-telescope-mirror-using-a-deformable/10.1117/1.JATIS.2.2.029001.pdf

Correction of an active space telescope mirror using a deformable mirror in a woofer-tweeter configuration

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.

Matthew R. Allen

Papers

Control of a deformable mirror subject to structural disturbance

Cost-effective Large Apertures for Future Imaging Satellites

Correction of active space telescope mirror using woofer-tweeter adaptive optics

Correction of an active space telescope mirror using a deformable mirror in a woofer-tweeter configuration

Correction of an active space telescope mirror using a gradient approach and an additional deformable mirror