26 research outputs found
The minimum of the time-delay wavefront error in Adaptive Optics
An analytical expression is given for the minimum of the time-delay induced
wavefront error (also known as the servo-lag error) in Adaptive Optics systems
under temporal prediction filtering. The analysis is based on the von
K\'arm\'an model for the spectral density of refractive index fluctuations and
the hypothesis of frozen flow. An optimal, temporal predictor can achieve up to
a factor 1.77 more reduction of the wavefront phase variance compared to the
zero-order prediction strategy, which is commonly used in Adaptive Optics
systems. Alternatively, an optimal predictor can allow for a 1.41 times longer
time-delay to arrive at the same residual phase variance. Generally, the
performance of the optimal, temporal predictor depends on the very product of
time-delay, wind speed and the reciprocal of turbulence outer scale.Comment: 10 pages, 5 figure
Impact of time-variant turbulence behavior on prediction for adaptive optics systems
For high contrast imaging systems, the time delay is one of the major
limiting factors for the performance of the extreme adaptive optics (AO)
sub-system and, in turn, the final contrast. The time delay is due to the
finite time needed to measure the incoming disturbance and then apply the
correction. By predicting the behavior of the atmospheric disturbance over the
time delay we can in principle achieve a better AO performance. Atmospheric
turbulence parameters which determine the wavefront phase fluctuations have
time-varying behavior. We present a stochastic model for wind speed and model
time-variant atmospheric turbulence effects using varying wind speed. We test a
low-order, data-driven predictor, the linear minimum mean square error
predictor, for a near-infrared AO system under varying conditions. Our results
show varying wind can have a significant impact on the performance of wavefront
prediction, preventing it from reaching optimal performance. The impact depends
on the strength of the wind fluctuations with the greatest loss in expected
performance being for high wind speeds.Comment: 10 pages, 8 figures; Accepted to JOSA A March 201
Extremely fast focal-plane wavefront sensing for extreme adaptive optics
We present a promising approach to the extremely fast sensing and correction
of small wavefront errors in adaptive optics systems. As our algorithm's
computational complexity is roughly proportional to the number of actuators, it
is particularly suitable to systems with 10,000 to 100,000 actuators. Our
approach is based on sequential phase diversity and simple relations between
the point-spread function and the wavefront error in the case of small
aberrations. The particular choice of phase diversity, introduced by the
deformable mirror itself, minimizes the wavefront error as well as the
computational complexity. The method is well suited for high-contrast
astronomical imaging of point sources such as the direct detection and
characterization of exoplanets around stars, and it works even in the presence
of a coronagraph that suppresses the diffraction pattern. The accompanying
paper in these proceedings by Korkiakoski et al. describes the performance of
the algorithm using numerical simulations and laboratory tests.Comment: SPIE Paper 8447-7
Focal-plane wavefront sensing with high-order adaptive optics systems
We investigate methods to calibrate the non-common path aberrations at an
adaptive optics system having a wavefront-correcting device working at an
extremely high resolution (larger than 150x150). We use focal-plane images
collected successively, the corresponding phase-diversity information and
numerically efficient algorithms to calculate the required wavefront updates.
The wavefront correction is applied iteratively until the algorithms converge.
Different approaches are studied. In addition of the standard Gerchberg-Saxton
algorithm, we test the extension of the Fast & Furious algorithm that uses
three images and creates an estimate of the pupil amplitudes. We also test
recently proposed phase-retrieval methods based on convex optimisation. The
results indicate that in the framework we consider, the calibration task is
easiest with algorithms similar to the Fast & Furious.Comment: 11 pages, 7 figures, published in SPIE proceeding
Calibrating a high-resolution wavefront corrector with a static focal-plane camera
We present a method to calibrate a high-resolution wavefront-correcting
device with a single, static camera, located in the focal plane; no moving of
any component is needed. The method is based on a localized diversity and
differential optical transfer functions (dOTF) to compute both the phase and
amplitude in the pupil plane located upstream of the last imaging optics. An
experiment with a spatial light modulator shows that the calibration is
sufficient to robustly operate a focal-plane wavefront sensing algorithm
controlling a wavefront corrector with ~40 000 degrees of freedom. We estimate
that the locations of identical wavefront corrector elements are determined
with a spatial resolution of 0.3% compared to the pupil diameter.Comment: 12 pages, 12 figures, accepted for publication in Applied Optic
Robustness of prediction for extreme adaptive optics systems under various observing conditions: An analysis using VLT/SPHERE adaptive optics data
For high-contrast imaging (HCI) systems, such as VLT/SPHERE, the performance
of the system at small angular separations is contaminated by the wind-driven
halo in the science image. This halo is a result of the servo-lag error in the
adaptive optics (AO) system due to the finite time between measuring the
wavefront phase and applying the phase correction. One approach to mitigating
the servo-lag error is predictive control. We aim to estimate and understand
the potential on-sky performance that linear data-driven prediction would
provide for VLT/SPHERE under various turbulence conditions. We used a linear
minimum mean square error predictor and applied it to 27 different AO telemetry
data sets from VLT/SPHERE taken over many nights under various turbulence
conditions. We evaluated the performance of the predictor using residual
wavefront phase variance as a performance metric. We show that prediction
always results in a reduction in the temporal wavefront phase variance compared
to the current VLT/SPHERE AO performance. We find an average improvement factor
of 5.1 in phase variance for prediction compared to the VLT/SPHERE residuals.
When comparing to an idealised VLT/SPHERE, we find an improvement factor of
2.0. Under our 27 different cases, we find the predictor results in a smaller
spread of the residual temporal phase variance. Finally, we show there is no
benefit to including spatial information in the predictor in contrast to what
might have been expected from the frozen flow hypothesis. A purely temporal
predictor is best suited for AO on VLT/SPHERE
Optimal and Robust Feedback Controller Estimation for a Vibrating Plate using Subspace Model Identification
This paper presents a method to estimate the H2 optimal and a robust feedback controller by means of Subspace Model Identification using the internal model control (IMC) approach. Using IMC an equivalent feed forward control problem is obtained, which is solved by the Causal Wiener filter for the H2 optimal controller. The robust variant, called the Cautious Wiener filter, optimizes the average performance w.r.t. probabilistic model errors. The identification of the Causal and Cautious Wiener filters are control-relevant. The method is illustrated by experiments on a 4-inputs 4-outputs vibrating plate with additional mass variation
Pre-correction Adaptive Optics performance of a 10 km Laser Link
For the next generation of very high throughput communication satellites, free-space optical (FSO) communication
between ground stations and geostationary telecommunication satellites is likely to replace conventional RF links. To
mitigate atmospheric turbulence, TNO and DLR propose Adaptive Optics (AO) to apply uplink pre-correction. In order to
demonstrate the feasibility of AO pre-correction an FSO link has been tested over a 10 km range. This paper shows that
AO pre-correction is most advantageous for low point ahead angles (PAAs), as expected. In addition, an optimum AO precorrection performance is found at 16 AO modes for the experimental conditions. For the specific test site, tip-tilt precorrection accounted for 4.5 dB improvement in the link budget. Higher order AO modes accounted for another 1.5 dB
improvement in the link budget. From these results it is concluded that AO pre-correction can effectively improve highthroughput optical feeder links
Recent progress in the upgrade of the TCV EC-system with two 1MW/2s dual-frequency (84/126GHz) gyrotrons
The upgrade of the EC-system of the TCV tokamak has entered in its realization phase and is part of a broader upgrade of TCV. The MW-class dual-frequency gyrotrons (84 or 126GHz/2s/1MW) are presently being manufactured by Thales Electron Devices with the first gyrotron foreseen to be delivered at SPC by the end of 2017. In parallel to the gyrotron development, for extending the level of operational flexibility of the TCV EC-system the integration of the dual-frequency gyrotrons adds a significant complexity in the evacuated 63.5mm-diameter HE11 transmission line system connected to the various TCV low-field side and top launchers. As discussed in [1], an important part of the present TCV-upgrade consists in inserting a modular closed divertor chamber. This will have an impact on the X3 top-launcher which will have to be reduced in size. For using the new compact launcher we are considering employing a Fast Directional Switch (FADIS), combining the two 1MW/126GHz/2s rf-beams into a single 2MW rf-beam