Determination of the optical turbulence parameters from the adaptive optics telemetry: critical analysis and on-sky validation (original) (raw)
Related papers
2020
Advanced Adaptive Optics (AO) instruments on ground-based telescopes require accurate knowledge of the strength and velocity of atmospheric turbulence. Measuring these parameters as a function of altitude assists point spread function reconstruction, AO temporal control techniques, smart scheduling of science cases and is required by wide-field AO systems to optimise the reconstruction of an observed wavefront. The variability of the atmosphere makes it important to have a measure of the turbulence profile in real-time. This measurement can be performed by iteratively fitting an analytically generated covariance matrix to the cross-covariance of Shack-Hartmann Wavefront Sensor (SHWFS) centroids. In this study we explore the benefits of reducing the number of cross-covariance data points and fitting to a covariance map Region of Interest (ROI). Both of these methods are based on the SLOpe Detection And Ranging (SLODAR) technique. A technique for using the covariance map ROI to measure and compensate for SHWFS misalignments is also introduced. We compare the accuracy of covariance matrix and map ROI optical turbulence profiling using simulated data from CANARY, an AO demonstrator on the 4.2 m William Herschel Telescope (WHT), La Palma. It is shown that the covariance map ROI optimises the accuracy of turbulence profiling. In addition, we show that the covariance map ROI reduces the fitting time for an Extremely Large Telescope-scale (ELT-scale) system by a factor of 72. SLODAR spatio-temporal analysis can be used to visualise the wind velocity profile. However, the limited altitude-resolution of current AO systems makes it difficult to disentangle the movement of independent layers. We address this issue and introduce a novel technique that uses SLODAR data analysis for automated wind velocity profiling. Simulated data from CANARY is used to demonstrate the proficiency of the technique. We apply our turbulence and wind velocity profiling techniques on-sky using data from both CANARY and the Adaptive Optics Facility (AOF). The AOF is on the 8.2 m Yepun telescope at the Very Large Telescope (VLT), Paranal. On-sky turbulence and wind velocity profiles from CANARY are compared to contemporaneous profiles from Stereo-SCIDAR, a dedicated high-resolution atmospheric profiler. Wind velocity profiles from CANARY and the AOF are compared to the European Centre for Medium-range Weather Forecasts (ECMWF). We also present AOF time sequences that show detailed examples of turbulence and wind velocity profiles at the VLT. The software packages that we developed to collect all of the presented results are open-source. They can be configured to any tomographic AO system.
Atmospheric Turbulence Characterization with the Keck Adaptive Optics Systems. I. Open-Loop Data
Applied Optics, 2003
We present a detailed investigation of different methods of the characterization of atmospheric turbulence with the adaptive optics systems of the W. M. Keck Observatory. The main problems of such a characterization are the separation of instrumental and atmospheric effects and the accurate calibration of the devices involved. Therefore we mostly describe the practical issues of the analysis. We show that two methods, the analysis of differential image motion structure functions and the Zernike decomposition of the wave-front phase, produce values of the atmospheric coherence length r 0 that are in excellent agreement with results from long-exposure images. The main error source is the calibration of the wave-front sensor. Values determined for the outer scale ᏸ 0 are consistent between the methods and with typical ᏸ 0 values found at other sites, that is, of the order of tens of meters.
Turbulence profiling methods applied to ESO's adaptive optics facility
Adaptive Optics Systems IV, 2014
The Adaptive Optics Facility (AOF) project envisages transforming one of the VLT units into an adaptive telescope and providing its ESO (European Southern Observatory) second generation instruments with turbulence corrected wavefronts. For MUSE and HAWK-I this correction will be achieved through the GALACSI and GRAAL AO modules working in conjunction with a 1170 actuators Deformable Secondary Mirror (DSM) and the new Laser Guide Star Facility (4LGSF). Multiple wavefront sensors will enable GLAO and LTAO capabilities, whose performance can greatly benefit from a knowledge about the stratification of the turbulence in the atmosphere. This work, totally based on end-to-end simulations, describes the validation tests conducted on a C 2 n profiler adapted for the AOF specifications. Because an absolute profile calibration is strongly dependent on a reliable knowledge of turbulence parameters r 0 and L 0 , the tests presented here refer only to normalized output profiles. Uncertainties in the input parameters inherent to the code are tested as well as the profiler response to different turbulence distributions. It adopts a correction for the unseen turbulence, critical for the GRAAL mode, and highlights the effects of masking out parts of the corrected wavefront on the results. Simulations of data with typical turbulence profiles from Paranal were input to the profiler, showing that it is possible to identify reliably the input features for all the AOF modes.
Performance of the Keck Observatory Adaptive-Optics System
Applied Optics, 2004
The adaptive-optics ͑AO͒ system at the W. M. Keck Observatory is characterized. We calculate the error budget of the Keck AO system operating in natural guide star mode with a near-infrared imaging camera. The measurement noise and bandwidth errors are obtained by modeling the control loops and recording residual centroids. Results of sky performance tests are presented: The AO system is shown to deliver images with average Strehl ratios of as much as 0.37 at 1.58 m when a bright guide star is used and of 0.19 for a magnitude 12 star. The images are consistent with the predicted wave-front error based on our error budget estimates. When this research was performed, M. A. van Dam and B. A. Macintosh were with the Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, California 94550. M. A. van Dam ͑mvandam@keck.hawaii.edu͒ and D. Le Mignant are now with W. M. Keck Observatory, 65-1120 Mamalahoa Highway, Kamuela, Hawaii 96743.
First Successful Adaptive Optics PSF Reconstruction at W. M. Keck Observatory
2012
We present the last results of our PSF reconstruction (PSF-R) project for the Keck-II and Gemini-North AO systems in natural guide star mode. Our initial tests have shown that the most critical aspects of PSF-R are the determination of the system static aberrations and the optical turbulence parameters, and we have set up a specific observation campaign on the two systems to explore this. We demonstrate that deformable mirror based seeing monitor works well, and 10% accuracy is easily obtained. Phase diversity has been demonstrated to work on sky sources. Besides, residual phase stationarity is an important assumption in PSF-R, and we demonstrate here that it is basically true. As a result of these tests and verifications, we have been able for the first time to obtain a very good PSF reconstruction for the Keck-II system, in bright natural guide star mode.
Advanced Maui Optical and Space Surveillance Technologies Conference, 2007
Atmospheric turbulence degrades the resolution of images of space objects beyond that predicted by diffraction alone. Adaptive optics telescopes have been widely used for compensating these effects, but as users seek to extend the envelopes of operation of adaptive optics telescopes to more demanding conditions, such as daylight operation and operation at low elevation angles, the level of compensation provided will degrade. We have been investigating the use of advanced wave front reconstructors and post detection image reconstruction to overcome the effects of turbulence on imaging systems in these more demanding scenarios. In this paper we show results comparing the optical performance of the exponential reconstructor, the least squares reconstructor, and the stochastic parallel gradient descent algorithm in a closed loop adaptive optics system using a conventional continuous facesheet deformable mirror and a Hartmann sensor. The performance of these reconstructors has been evaluated under a range of source visual magnitudes, and zenith angles up to 67 degrees. We have also simulated satellite images, and applied speckle imaging, multiframe blind deconvolution algorithms, and deconvolution algorithms that presume the average point spread function is known to compute object estimates.
Applied Optics, 2009
Compensation of extended (deep) turbulence effects is one of the most challenging problems in adaptive optics (AO). In the AO approach described, the deep turbulence wave propagation regime was achieved by imaging stars at low elevation angles when image quality improvement with conventional AO was poor. These experiments were conducted at the U.S. Air Force Maui Optical and Supercomputing Site (AMOS) by using the 3:63 m telescope located on Haleakala, Maui. To enhance compensation performance we used a cascaded AO system composed of a conventional AO system based on a Shack-Hartmann wavefront sensor and a deformable mirror with 941 actuators, and an AO system based on stochastic parallel gradient descent optimization with four deformable mirrors (75 control channels). This first-time field demonstration of a cascaded AO system achieved considerably improved performance of wavefront phase aberration compensation. Image quality was improved in a repeatable way in the presence of stressing atmospheric conditions obtained by using stars at elevation angles as low as 15°.
Characterization of adaptive optics at Keck Observatory: part II
Advancements in Adaptive Optics, 2004
This paper is a continuation of the characterization of adaptive optics (AO) at Keck Observatory (SPIE 5169-01). The bandwidth and measurement noise error terms are often the important sources of wave-front error. Here, we show how the magnitude of these two terms is estimated. First, the Bayesian wave-front reconstructor employed at Keck Observatory is presented and shown to perform better than a conventional SVD reconstructor. A novel technique used to estimate the size of the spot on a Shack-Hartmann wave-front sensor quad cell detector is introduced, along with experimental results using this technique. The spot size is an essential component of the dynamic model of the AO system, which is presented and used to find the bandwidth and noise error terms.