SLODAR: measuring optical turbulence altitude with a Shack–Hartmann wavefront sensor
01 Nov 2002-Monthly Notices of the Royal Astronomical Society (Oxford University Press)-Vol. 337, Iss: 1, pp 103-108
TL;DR: In this article, the vertical distribution of atmospheric optical turbulence above large telescopes is determined from time-averaged spatial cross-correlations of local wavefront slopes for Shack-Hartmann observations of binary stars.
Abstract: This paper discusses the use of Shack–Hartmann wavefront sensors to determine the vertical distribution of atmospheric optical turbulence above large telescopes. It is demonstrated that the turbulence altitude profile can be recovered reliably from time-averaged spatial cross-correlations of the local wavefront slopes for Shack–Hartmann observations of binary stars. The method, which is referred to as SLODAR, is analogous to the well known SCIDAR scintillation profiling technique, and a calibration against contemporaneous SCIDAR observations is shown. Hardware requirements are simplified relative to the scintillation method, and the number of suitable target objects is larger. The implementation of a Shack–Hartmann based turbulence monitor for use at the William Herschel Telescope is described. The system will be used to optimize adaptive optical observations at the telescope and to characterize anisoplanatic variations of the corrected point spread function.
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TL;DR: In this article, the authors review the development of adaptive optics as an effective tool that allows using controllable optical elements to eliminate irregular distortions that occur as light propagates in an inhomogeneous medium.
Abstract: In connection with the wide use of optoelectronic systems, we review the development of adaptive optics as an effective tool that allows using controllable optical elements to eliminate irregular distortions that occur as light propagates in an inhomogeneous medium. The subject matter of this rapidly developing field of science and technology is described. Of the ideas under development in recent years, many have been around for quite a long time, but it is only now, with the development of an up-to-date optoelectronic element base, that they have started being widely incorporated into science and engineering practice. We discuss the development of adaptive optics from mere ideas to their application in astronomy, high-power laser physics, and medicine. The current state of adaptive optics in stellar and solar astronomy is reviewed, and some results of its use in distortion correction systems of high-power laser systems and facilities are presented.
204 citations
TL;DR: In this paper, the authors determine the theoretical response of a SLODAR system based on a Shack-Hartmann WFS to a thin turbulent layer at a given altitude, and also as a function of the spatial power spectral index of the optical phase aberrations.
Abstract: Slope Detection and Ranging (SLODAR) is a technique for the measurement of the vertical profile of atmospheric optical turbulence strength. Its main applications are astronomical site characterization and real-time optimization of imaging with adaptive optical correction. The turbulence profile is recovered from the cross-covariance of the slope of the optical phase aberration for a double star source, measured at the telescope with a wavefront sensor (WFS). Here, we determine the theoretical response of a SLODAR system based on a Shack–Hartmann WFS to a thin turbulent layer at a given altitude, and also as a function of the spatial power spectral index of the optical phase aberrations. Recovery of the turbulence profile via fitting of these theoretical response functions is explored. The limiting resolution in altitude of the instrument and the statistical uncertainty of the measured profiles are discussed. We examine the measurement of the total integrated turbulence strength (the seeing) from the WFS data and, by subtraction, the fractional contribution from all turbulence above the maximum altitude for direct sensing of the instrument. We take into account the effects of noise in the measurement of wavefront slopes from centroids and the form of the spatial structure function of the atmospheric optical aberrations.
139 citations
Cites methods from "SLODAR: measuring optical turbulenc..."
...Slope Detection and Ranging (SLODAR) (Wilson 2002) is an optical triangulation method for the measurement of the atmospheric optical turbulence profile C2n(h)....
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TL;DR: In this article, a multi-object adaptive optics (MOAO) system was successfully demonstrated on-sky for the first time at the 4.2m William Herschel Telescope, Canary Islands, Spain, at the end of September 2010.
Abstract: Context. A new challenging adaptive optics (AO) system, called multi-object adaptive optics (MOAO), has been successfully demonstrated on-sky for the first time at the 4.2 m William Herschel Telescope, Canary Islands, Spain, at the end of September 2010.Aims. This system, called CANARY, is aimed at demonstrating the feasibility of MOAO in preparation of a future multi-object near infra-red (IR) integral field unit spectrograph to equip extremely large telescopes for analysing the morphology and dynamics of high-z galaxies.Methods. CANARY compensates for the atmospheric turbulence with a deformable mirror driven in open-loop and controlled through a tomographic reconstruction by three widely separated off-axis natural guide star (NGS) wavefront sensors, which are in open loop too. We compared the performance of conventional closed-loop AO, MOAO, and ground-layer adaptive optics (GLAO) by analysing both IR images and simultaneous wave-front measurements.Results. In H -band, Strehl ratios of 0.20 are measured with MOAO while achieving 0.25 with closed-loop AO in fairly similar seeing conditions (r 0 ≈ 15 cm at 0.5 μ m). As expected, MOAO has performed at an intermediate level between GLAO and closed-loop AO.
123 citations
Cites methods from "SLODAR: measuring optical turbulenc..."
...We also collected external SLODAR (Wilson 2002) and DIMM (Sarazin & Roddier 1990) data in parallel, which will be processed later along with our data....
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TL;DR: In this article, the authors proposed a method to retrieve the tomographic reconstructor using the on-sky wavefront measurements from an MOAO instrument, which is also used to calibrate the registrations between the off-axis wavefront sensors and the deformable mirrors placed in the science optical paths.
Abstract: Multi-object adaptive optics (MOAO) is a solution developed to perform a correction by adaptive optics (AO) in a science large field of view. As in many wide-field AO schemes, a tomographic reconstruction of the turbulence volume is required in order to compute the MOAO corrections to be applied in the dedicated directions of the observed very faint targets. The specificity of MOAO is the open-loop control of the deformable mirrors by a number of wavefront sensors (WFSs) that are coupled to bright guide stars in different directions. MOAO calls for new procedures both for the cross registration of all the channels and for the computation of the tomographic reconstructor. We propose a new approach, called "Learn and Apply (L&A)", that allows us to retrieve the tomographic reconstructor using the on-sky wavefront measurements from an MOAO instrument. This method is also used to calibrate the registrations between the off-axis wavefront sensors and the deformable mirrors placed in the science optical paths. We propose a procedure linking the WFSs in the different directions and measuring directly on-sky the required covariance matrices needed for the reconstructor. We present the theoretical expressions of the turbulence spatial covariance of wavefront slopes allowing one to derive any turbulent covariance matrix between two wavefront sensors. Finally, we discuss the convergence issue on the measured covariance matrices, we propose the fitting of the data based on the theoretical slope covariance using a reduced number of turbulence parameters, and we present the computation of a fully modeled reconstructor.
98 citations
TL;DR: In this article, a stereo-scintillation detection and ranging (SCIDAR) system was proposed to map the optical refractive index fluctuations, generated by the atmospheric turbulence, along the line of sight to the astronomical target.
Abstract: The next generation of adaptive optics systems will require tomographic reconstruction techniques to map the optical refractive index fluctuations, generated by the atmospheric turbulence, along the line of sight to the astronomical target. These systems can be enhanced with data from an external atmospheric profiler. This is important for Extremely Large Telescope scale tomography. Here we propose a new instrument which utilizes the generalized Scintillation Detection And Ranging (SCIDAR) technique to allow high sensitivity vertical profiles of the atmospheric optical turbulence and wind velocity profile above astronomical observatories. The new approach, which we refer to as ‘stereo-SCIDAR’, uses a stereoscopic system with the scintillation pattern from each star of a double-star target incident on a separate detector. Separating the pupil images for each star has several advantages including increased magnitude difference tolerance for the target stars; negating the need for re-calibration due to the normalization errors usually associated with SCIDAR; an increase of at least a factor of 2 in the signal-to-noise ratio of the cross-covariance function and hence the profile for equal magnitude target stars and up to a factor of 16 improvement for targets of 3 mag difference and easier real-time reconstruction of the wind-velocity profile. Theoretical response functions are calculated for the instrument, and the performance is investigated using a Monte Carlo simulation. The technique is demonstrated using data recorded at the 2.5-m Nordic Optical Telescope and the 1.0-m Jacobus Kapteyn Telescope, both on La Palma.
80 citations
Cites methods from "SLODAR: measuring optical turbulenc..."
...The most widely exploited are MASS (Multi Aperture Scintillation System, Tokovinin & Kornilov 2007), SCIDAR (SCIntillation Detection And Ranging, Vernin & Roddier 1973) and SLODAR (SLOpe Detection And Ranging, Wilson 2002)....
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References
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TL;DR: In this article, the strength and veloc-ity of atmospheric optical turbulence using a generalised SCIDAR technique is outlined and demonstrated. This method allows the full turbulent prole to be characterised from the telescope pupil up to any desired altitude.
Abstract: The measurement of the strength and veloc- ity of atmospheric optical turbulence using a generalised SCIDAR technique is outlined and demonstrated. This method allows the full turbulent prole to be characterised from the telescope pupil up to any desired altitude. A number of example proles from various astronomical ob- serving sites are presented.
107 citations
TL;DR: In this paper, the performance of an adaptive optics (AO) system on a 100m diameter ground-based telescope working in the visible range of the spectrum is computed using an analytical approach.
Abstract: ABSTRA C T The performance of an adaptive optics (AO) system on a 100-m diameter ground-based telescope working in the visible range of the spectrum is computed using an analytical approach. The target Strehl ratio of 60 per cent is achieved at 0.5mm with a limiting magnitude of the AO guide source near R magnitude , 10; at the cost of an extremely low sky coverage. To alleviate this problem, the concept of tomographic wavefront sensing in a wider field of view using either natural guide stars (NGS) or laser guide stars (LGS) is investigated. These methods use three or four reference sources and up to three deformable mirrors, which increase up to 8-fold the corrected field size (up to 60 arcsec at 0.5mm). Operation with multiple NGS is limited to the infrared (in the J band this approach yields a sky coverage of 50 per cent with a Strehl ratio of 0.2). The option of open-loop wavefront correction in the visible using several bright NGS is discussed. The LGS approach involves the use of a faintOR , 22U NGS for low-order correction, which results in a sky coverage of 40 per cent at the Galactic poles in the visible.
71 citations
TL;DR: In this paper, a method for simultaneous measurement of the vertical distribution of the optical turbulence strength and wind velocity in the Earth's atmosphere, based on an analysis of spatio-temporal correlations of stellar scintillation images obtained with generalized scidar, is presented.
Abstract: We present a method for simultaneous measurement of the vertical distribution of the optical turbulence strength and wind velocity in the Earth's atmosphere, based on an analysis of spatio-temporal correlations of stellar scintillation images obtained with generalized scidar A statistical comparison of obtained with this method and instrumented balloons supports the use of this method The algorithm used allows for the identification of dome seeing, which can be subtracted from , to obtain a turbulence profile free of dome contribution Examples of simultaneous and monitorings are presented
68 citations
TL;DR: In this article, the point spread function (PSF) of an adaptive optics system evolves in the Field Of View (FOV) and this variation strongly limits the conventional deconvolution methods for the processing of wide FOV images.
Abstract: The point spread function (PSF) of an adaptive optics system evolves in the Field Of View (FOV). This variation strongly limits the conventional deconvolution methods for the processing of wide FOV images. A theo- retical expression of this PSF variation is derived. This ex- pression is both validated on simulations and experimental data. It is then applied to the a posteriori processing of stellar elds. Using the available prior information about the object (point-like sources), this technique allows the restoration of the star parameters (positions and intensi- ties) with a precision much better than the conventional methods, in a FOV much larger than the isoplanatic eld.
64 citations