Wide Field Adaptive Optics Systems Research Articles

PML: a generalized monitor of atmospheric turbulence profile with high vertical resolution.

The future generation of extremely large telescopes will be certainly equipped with wide-field adaptive optics systems. All the components of such adaptive optics systems have to be precisely specified, and most of the technical specifications are related to atmospheric turbulence parameters, particularly the profile of the refractive index structure constant Cn2(h). The monitor Profiler of Moon Limb (PML) for the extraction of the Cn2(h) profile with high vertical resolution for nighttime and daytime conditions by the observation of the moon limb or sun edge has been developed and is now routinely exploited at the Calern Observatory on the French Riviera. The PML instrument uses a differential method with two small subapertures through which the moon limb or sun edge are observed, leading to a continuum of double stars that allows a scan of the whole atmosphere with high resolution in altitude. The PML is an autonomous instrument aided by a set of equipment such as an all-sky camera, a small meteorological station, and an automatic system to cover the two subapertures with solar filters to switch from night/moon to day/solar observations. In addition, the PML instrument provides in real time a large characterization of the atmospheric turbulence since it can measure other turbulence parameters, such as the total seeing and the isoplanatic angle.

Towards an automatic wind speed and direction profiler for Wide Field adaptive optics systems

Wide Field Adaptive Optics (WFAO) systems are among the most sophisticated AO systems available today on large telescopes. The knowledge of the vertical spatio-temporal distribution of the wind speed (WS) and direction (WD) are fundamental to optimize the performance of such systems. Previous studies already proved that the Gemini Multi-Conjugated AO system (GeMS) is able to retrieve measurements of the WS and WD stratification using the SLODAR technique and to store measurements in the telemetry data. In order to assess the reliability of these estimates and of the SLODAR technique applied to such a kind of complex AO systems, in this study we compared WS and WD retrieved from GeMS with those obtained with the atmospherical model Meso-Nh on a rich statistical sample of nights. It has been previously proved that, the latter technique, provided an excellent agreement with a large sample of radiosoundings both, in statistical terms and on individual flights. It can be considered, therefore, as an independent reference. The excellent agreement between GeMS measurements and the model that we find in this study, proves the robustness of the SLODAR approach. To by-pass the complex procedures necessary to achieve automatic measurements of the wind with GeMS, we propose a simple automatic method to monitor nightly WS and WD using the Meso-Nh model estimates. Such a method can be applied to whatever present or new generation facilities supported by WFAO systems. The interest of this study is, therefore, well beyond the optimization of GeMS performance.

Monte Carlo modelling of multi-object adaptive optics performance on the European Extremely Large Telescope

The performance of a wide-field adaptive optics system depends on input design parameters. Here we investigate the performance of a multi-object adaptive optics system design for the European Extremely Large Telescope, using an end-to-end Monte-Carlo adaptive optics simulation tool, DASP, with relevance for proposed instruments such as MOSAIC. We consider parameters such as the number of laser guide stars, sodium layer depth, wavefront sensor pixel scale, actuator pitch and natural guide star availability. We provide potential areas where costs savings can be made, and investigate trade-offs between performance and cost, and provide solutions that would enable such an instrument to be built with currently available technology. Our key recommendations include a trade-off for laser guide star wavefront sensor pixel scale of about 0.7 arcseconds per pixel, and a field of view of at least 7 arcseconds, that EMCCD technology should be used for natural guide star wavefront sensors even if reduced frame rate is necessary, and that sky coverage can be improved by a slight reduction in natural guide star sub-aperture count without significantly affecting tomographic performance. We find that adaptive optics correction can be maintained across a wide field of view, up to 7 arcminutes in diameter. We also recommend the use of at least 4 laser guide stars, and include ground-layer and multi-object adaptive optics performance estimates.

Experience with wavefront sensor and deformable mirror interfaces for wide-field adaptive optics systems

Recent advances in adaptive optics (AO) have led to the implementation of wide field-of-view AO systems. A number of wide-field AO systems are also planned for the forthcoming Extremely Large Telescopes. Such systems have multiple wavefront sensors of different types, and usually multiple deformable mirrors (DMs). Here, we report on our experience integrating cameras and DMs with the real-time control systems of two wide-field AO systems. These are CANARY, which has been operating on-sky since 2010, and DRAGON, which is a laboratory adaptive optics real-time demonstrator instrument. We detail the issues and difficulties that arose, along with the solutions we developed. We also provide recommendations for consideration when developing future wide-field AO systems.

Monte Carlo modelling of multiconjugate adaptive optics performance on the European Extremely Large Telescope

The performance of a wide-field adaptive optics system depends on input design parameters. Here we investigate the performance of a multi-conjugate adaptive optics system design for the European Extremely Large Telescope, using an end-to-end Monte-Carlo adaptive optics simulation tool, DASP. We consider parameters such as the number of laser guide stars, sodium layer depth, wavefront sensor pixel scale, number of deformable mirrors, mirror conjugation and actuator pitch. We provide potential areas where costs savings can be made, and investigate trade-offs between performance and cost. We conclude that a 6 laser guide star system using 3 DMs seems to be a sweet spot for performance and cost compromise.

Scintillation correction for astronomical photometry on large and extremely large telescopes with tomographic atmospheric reconstruction

We describe a new concept to correct for scintillation noise on high-precision photometry in large and extremely large telescopes using telemetry data from adaptive optics (AO) systems. Most wide-field AO systems designed for the current era of very large telescopes and the next generation of extremely large telescopes require several guide stars to probe the turbulent atmosphere in the volume above the telescope. These data can be used to tomographically reconstruct the atmospheric turbulence profile and phase aberrations of the wavefront in order to assist wide-field AO correction. If the wavefront aberrations and altitude of the atmospheric turbulent layers are known from this tomographic model, then the effect of the scintillation can be calculated numerically and used to normalize the photometric light curve. We show through detailed Monte Carlo simulation that for an 8 m telescope with a 16 × 16 AO system we can reduce the scintillation noise by an order of magnitude.

Using the ${C_{n}^{2}}$ and wind profiler method with wide-field laser-guide-stars adaptive optics to quantify the frozen-flow decay

We use the spatio-temporal cross-correlations of slopes from five Shack-Hartmann wavefront sensors to analyse the temporal evolution of the atmospheric turbulence layers at different altitudes. The focus is on the verification of the frozen-flow assumption. The data come from the Gemini South Multiconjugate Adaptive Optics System (GeMS). First, we present the C-n(2) and wind profiling technique. This method provides useful information for the operation of the adaptive optics system, such as the number of existing turbulence layers, their associated velocities, altitudes and strengths, and also a mechanism to estimate the dome-seeing contribution to the total turbulence. Next, by identifying the turbulence layers, we show that it is possible to estimate the rate of decay in time of the correlation among turbulence measurements. We reduce on-sky data obtained during the 2011, 2012 and 2013 campaigns. The first results suggest that the rate of temporal decorrelation can be expressed in terms of a single parameter that is independent of the layer altitude and turbulence strength. Finally, we show that the decay rate of the frozen-flow contribution increases linearly with the layer speed. The observed evolution of the decay rate confirms the potential interest of the predictive control for wide-field adaptive optics systems.

First results of the PML monitor of atmospheric turbulence profile with high vertical resolution

Aims. Future extremely large telescopes will certainly be equipped with wide-field adaptive optics systems. The optimization of the performances of these techniques requires a precise specification of the different components of these AO systems. Most of these technical specifications are related to the atmospheric turbulence parameters, particularly the profile of the refractive index structure constantC 2 (h). A new monitor called Profiler of Moon Limb (PML) for the extraction of theC 2 N (h) profile with high vertical resolution and its first results are presented. Methods. The PML instrument uses an optical method based on the observation of the Moon limb through two subapertures. The use of the lunar limb leads to a continuum of double stars allowing a scan of the whole atmosphere with high resolution in altitude. Results. The first prototype of the PML has been installed at Dome C in Antarctica and the first results of the PML are presented and compared to radio-sounding balloon profiles. In addition to the C 2 (h) profile obtained with high vertical resolution, PML is also able to provide other atmospheric turbulence parameters such as the outer scale profile, the total seeing, and the isoplanatic and isopistonic angles.

Linear Quadratic Gaussian Control Applied to WFAO Systems: Simulation and Experimental Results

Adaptive optics (AO) is a proven technique to correct turbulence on ground-based astronomical telescopes. The corrected Field of View (FoV) is however limited by the anisoplanatism effect. To extend the FoV, new Wide Field AO (WFAO) concepts have been recently developed. They aim at providing a wide FoV correction through the use of multi-guide-star WaveFront Sensors (WFSs) and several Deformable Mirrors (DMs). Such WFAO systems require a tomographic reconstruction of the atmospheric turbulence, leading to a multivariable problem of much higher complexity than in classic AO. They also raise new questions in terms of calibration and control. The Linear Quadratic Gaussian (LQG) control formalism is a natural way to cope with this issue. It enables both tomographic reconstruction and distinction between controlled output and measurements. In this paper, LQG control is presented and applied to WFAO systems. Performance is evaluated thanks to simulations based on theoretical studies, but also through laboratory experiments. We present the first results of the implementation of LQG control on a WFAO system, the HOMER bench, that has been developed recently at ONERA and which is devoted to WFAO laboratory research.

High-resolution lidar observations of mesospheric sodium and implications for adaptive optics

Observations of sodium density variability in the upper mesosphere/lower thermosphere, obtained using a high-resolution lidar system, show rapid fluctuations in the sodium centroid altitude. The temporal power spectrum extends above 1 Hz and is well-fit by a power law having a slope that is -1.95±0.12. These fluctuations produce focus errors in adaptive optics systems employing continuous-wave sodium laser guide stars, which can be significant for large-aperture telescopes. For a 30 m aperture diameter, the associated rms wavefront error is approximately 4 nm per meter of altitude change and increases as the square of the aperture diameter. The vertical velocity of the sodium centroid altitude is found to be ~23 ms(-1) on a 1 s time scale. If these high-frequency fluctuations arise primarily from advection of horizontal structure by the mesospheric wind, our data imply that variations in the sodium centroid altitude on the order of tens of meters occur over the horizontal scales spanned by proposed laser guide star asterisms. This leads to substantial differential focus errors (~107 nm over a 1 arc min separation with a 30 m aperture diameter) that may impact the performance of wide-field adaptive optics systems. Short-lasting and narrow sodium density enhancements, more than 1 order of magnitude above the local sodium density, occur due to advection of meteor trails. These have the ability to change the sodium centroid altitude by as much as 1 km in less than 1 s, which could result in temporary disruption of adaptive optics systems.

Real-time turbulence profiling with a pair of laser guide star Shack–Hartmann wavefront sensors for wide-field adaptive optics systems on large to extremely large telescopes

Real-time turbulence profiling is necessary to tune tomographic wavefront reconstruction algorithms for wide-field adaptive optics (AO) systems on large to extremely large telescopes, and to perform a variety of image post-processing tasks involving point-spread function reconstruction. This paper describes a computationally efficient and accurate numerical technique inspired by the slope detection and ranging (SLODAR) method to perform this task in real time from properly selected Shack-Hartmann wavefront sensor measurements accumulated over a few hundred frames from a pair of laser guide stars, thus eliminating the need for an additional instrument. The algorithm is introduced, followed by a theoretical influence function analysis illustrating its impulse response to high-resolution turbulence profiles. Finally, its performance is assessed in the context of the Thirty Meter Telescope multi-conjugate adaptive optics system via end-to-end wave optics Monte Carlo simulations.