We describe and report first results from PALM-3000, the second-generation astronomical adaptive optics (AO) facility for the 5.1 m Hale telescope at Palomar Observatory. PALM-3000 has been engineered for high-contrast imaging and emission spectroscopy of brown dwarfs and large planetary mass bodies at near-infrared wavelengths around bright stars, but also supports general natural guide star use to V ≈ 17. Using its unique 66 × 66 actuator deformable mirror, PALM-3000 has thus far demonstrated residual wavefront errors of 141 nm rms under ∼1 seeing conditions. PALM-3000 can provide phase conjugation correction over a 6. 4 × 6. 4 working region at λ = 2.2 μm, or full electric field (amplitude and phase) correction over approximately one-half of this field. With optimized back-end instrumentation, PALM-3000 is designed to enable 10 −7 contrast at 1 angular separation, including post-observation speckle suppression processing. While continued optimization of the AO system is ongoing, we have already successfully commissioned five back-end instruments and begun a major exoplanet characterization survey, Project 1640.
This paper compares two control methods to predict and correct aero-optical wavefronts derived from recent flight-test data. The first is an optimal linear time-invariant controller constructed from an identified state-space model of the turbulence flow. The second control method is an adaptive controller based on a recursive least-squares lattice filter. The performance of these control schemes versus classical integrator methods is investigated in an adaptive optics experiment that reproduces the aberrations from in-flight measurements of aero-optical turbulence. Experimental results show the improvement in wavefront correction achieved by both prediction methods. Altering the flow characteristics of the disturbance wavefront during the control process illustrates the ability of the adaptive controller to track changes in the aberration statistics.
Structural, Thermal, and Optical Performance (STOP) analysis is important for understanding the dynamics and for predicting the performance of a large number of optical systems whose proper functioning is negatively influenced by thermally induced aberrations. Furthermore, STOP models are being used to design and test passive and active methods for the compensation of thermally induced aberrations. However, in many cases and scenarios, the lack of precise knowledge of system parameters and equations governing the dynamics of thermally induced aberrations can significantly deteriorate the prediction accuracy of STOP models. In such cases, STOP models and underlying parameters need to be estimated from the data. To the best of our knowledge, the problem of estimating transient state-space STOP models from the experimental data has not received significant attention. Similarly, little attention has been dedicated to the related problem of obtaining low-dimensional state-space models of thermally induced aberrations that can be used for the design of high-performance model-based control and estimation algorithms. Motivated by this, in this manuscript, we present a numerical proof of principle for estimating low-dimensional state-space models of thermally induced aberrations and for characterizing the transient dynamics. Our approach is based on the COMSOL Multiphysics simulation framework for generating the test data and on a system identification approach. We numerically test our method on a lens system with a temperature-dependent refractive index that is used in high-power laser systems. The dynamics of such a system is complex and described by the coupling of thermal, structural, and ray-tracing models. The approach proposed in this paper can be generalized to other types of optical systems.
High-order adaptive optics systems often suffer from significant computational latency, which ultimately limits the temporal error rejection bandwidth when classical controllers are employed. This Letter presents results from an on-sky, real-time implementation of an optimal controller on the PALM-3000 adaptive optics system at Palomar Observatory. The optimal controller is computed directly from open-loop wavefront measurements using a multichannel subspace system identification algorithm, and mitigates latency by explicitly predicting incident turbulence. Experimental results show a significant reduction in the residual wavefront error over the controlled spatial modes, illustrating the superior performance of the optimal control approach versus the nominal integral control architecture.
Optimizing on-sky single-mode fiber (SMF) injection is an essential part of developing precise Doppler spectrometers and new astrophotonics technologies. We installed and tested a prototype SMF injection system at the Large Binocular Telescope (LBT) in April 2016. The fiber injection unit was built as part of the de-risking process for a new instrument named iLocater that will use adaptive optics (AO) to feed a high resolution, near-infrared spectrograph. In this paper we report Y-band SMF coupling measurements for bright, M-type stars. We compare theoretical expectations for delivered Strehl ratio and SMF coupling to experimental results, and evaluate fundamental effects that limit injection efficiency. We find the pupil geometry of the telescope itself limits fiber coupling to a maximum efficiency of ρ tel ≈ 0.78. Further analysis shows the individual impact of AO correction, tip-tilt residuals, and static (non-common-path) aberrations contribute coupling coefficients of ρ Strehl ≈ 0.33, ρ tip/tilt ≈ 0.84, and ρ ncpa ≈ 0.8 respectively. Combined, these effects resulted in an average Y-band SMF efficiency of 0.18 for all observations. Finally, we investigate the impact of fiber coupling on radial velocity (RV) precision as a function of stellar apparent magnitude.
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