Spotlight Summary by Remy Avila
First on-sky results of the CO-SLIDAR Cn2 profiler
Optical turbulence in the atmosphere---turbulent fluctuations of the refractive index of air---provokes a drastic degradation of the angular resolution in astronomical observations in the visible and near infrared regimes. The wavefronts reaching the telescopes are spatially distorted and vary in time scales of milliseconds. To recover, at least partially, the lost angular resolution, opticians and astronomers have developed adaptive optics techniques that consist of rapidly measuring the wavefront and correcting its form using, typically, deformable mirrors, before the light coming from the astronomical object reaches the detector. Next generation adaptive optics systems are being designed with the goal of enlarging the corrected field of view. The design and optimal operation of those systems require detailed knowledge of the vertical distribution of the optical turbulence strength (determined by the refractive index structure constant, Cn2). The precise measurement of Cn2 profiles is the subject of this paper by Voyez et al. where the authors combine in a single instrument two complementary techniques.
Instruments for optical turbulence profiling have been developed using either scintillation (like in the Scidar) or wavefront slopes (like in the Slodar) that result from the passage of stellar light through turbulent layers. The local slopes of the wavefront produced by a layer at any altitude are conserved down to the wavefront sensor. Thus, turbulence profilers based on slopes are sensitive to turbulence all along the optical path and their sensitivity does not depend on the altitude of the layers. On the other hand, scintillation due to the phase fluctuations produced at a given altitude can be detected only after the wave has travelled a certain distance, because the scintillation variance is proportional to h5/6, where h is the distance separating the turbulent layer and the plane where the detector is focused on. Then what is the interest in using scintillation as the signal for turbulence profiling? Both methods rely on the spatial autocorrelation of the signals they detect (either slopes or scintillation) coming from a double star. The altitude resolution goes like the star separation multiplied by the minimum distinguishable separation of two autocorrelation peaks. The autocorrelation of slopes is much wider than that of scintillation. Consequently the altitude resolution with scintillation is much better than with slopes, if the spatial sampling used to compute the corresponding autocorrelations respects the Nyquist criterion for each of the two signals under consideration. This characteristic was exploited in the instrument Slodar-Lolas (Slope Detection and Ranging – Low Layer Scidar) that combined the two techniques using the same telescope and camera.
The approach followed by Voyez et al. goes further toward the integration of the two types of signals. The authors developed a method, called CO-SLIDAR, that uses a Shack-Hartmann wavefront sensor whose plane is made conjugate to the telescope pupil. The slopes and the scintillation within each subaperture are registered. Each subaperture covers a square area of 5-cm in side on the pupil plane. In the visible, this size is larger than the scintillation autocorrelation width for turbulent layers up to 8 km approximately, which produces a spatial filtering, but is adequate for slope measurements. Considering the tests shown by the authors, this filtering effect is most certainly well corrected for during the data reduction. The numerical simulations presented in this paper show that scintillation-only signal is suitable for the turbulence profile measurement from 1 km up to 15 km. Turbulence at ground level cannot be measured with scintillation since the propagation distance h is null (or too short). At that level, only slopes give the optical turbulence strength. To retrieve the profiles from the autocorrelation measurements, two different inversion methods are investigated: maximum likelihood and maximum a posteriori solutions. They give similar results. The authors also present on-sky real measurements. In this case they cannot compare their results with the “true” profiles since there is no way such profiles can be determined. Nevertheless, they compare their measurement with profiles obtained with the NCEP/NCAR Reanalysis, which is a forecast product. This approach has been used by other authors. The comparison results are satisfactory.
Other methods for turbulence profiling exist. The great advantage of the CO-SLIDAR is that it can be implemented on Shack-Hartmann wavefront sensors used in an adaptive optics system. There is no need of an additional instrument to do turbulence profiling. The only requirement is to point the telescope to a suitable double star and run the appropriate acquisition software. This is a step forward towards a dynamically configurable single- or multi-conjugate adaptive optics system that would adapt to the turbulence profile at the moment of observation.
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Instruments for optical turbulence profiling have been developed using either scintillation (like in the Scidar) or wavefront slopes (like in the Slodar) that result from the passage of stellar light through turbulent layers. The local slopes of the wavefront produced by a layer at any altitude are conserved down to the wavefront sensor. Thus, turbulence profilers based on slopes are sensitive to turbulence all along the optical path and their sensitivity does not depend on the altitude of the layers. On the other hand, scintillation due to the phase fluctuations produced at a given altitude can be detected only after the wave has travelled a certain distance, because the scintillation variance is proportional to h5/6, where h is the distance separating the turbulent layer and the plane where the detector is focused on. Then what is the interest in using scintillation as the signal for turbulence profiling? Both methods rely on the spatial autocorrelation of the signals they detect (either slopes or scintillation) coming from a double star. The altitude resolution goes like the star separation multiplied by the minimum distinguishable separation of two autocorrelation peaks. The autocorrelation of slopes is much wider than that of scintillation. Consequently the altitude resolution with scintillation is much better than with slopes, if the spatial sampling used to compute the corresponding autocorrelations respects the Nyquist criterion for each of the two signals under consideration. This characteristic was exploited in the instrument Slodar-Lolas (Slope Detection and Ranging – Low Layer Scidar) that combined the two techniques using the same telescope and camera.
The approach followed by Voyez et al. goes further toward the integration of the two types of signals. The authors developed a method, called CO-SLIDAR, that uses a Shack-Hartmann wavefront sensor whose plane is made conjugate to the telescope pupil. The slopes and the scintillation within each subaperture are registered. Each subaperture covers a square area of 5-cm in side on the pupil plane. In the visible, this size is larger than the scintillation autocorrelation width for turbulent layers up to 8 km approximately, which produces a spatial filtering, but is adequate for slope measurements. Considering the tests shown by the authors, this filtering effect is most certainly well corrected for during the data reduction. The numerical simulations presented in this paper show that scintillation-only signal is suitable for the turbulence profile measurement from 1 km up to 15 km. Turbulence at ground level cannot be measured with scintillation since the propagation distance h is null (or too short). At that level, only slopes give the optical turbulence strength. To retrieve the profiles from the autocorrelation measurements, two different inversion methods are investigated: maximum likelihood and maximum a posteriori solutions. They give similar results. The authors also present on-sky real measurements. In this case they cannot compare their results with the “true” profiles since there is no way such profiles can be determined. Nevertheless, they compare their measurement with profiles obtained with the NCEP/NCAR Reanalysis, which is a forecast product. This approach has been used by other authors. The comparison results are satisfactory.
Other methods for turbulence profiling exist. The great advantage of the CO-SLIDAR is that it can be implemented on Shack-Hartmann wavefront sensors used in an adaptive optics system. There is no need of an additional instrument to do turbulence profiling. The only requirement is to point the telescope to a suitable double star and run the appropriate acquisition software. This is a step forward towards a dynamically configurable single- or multi-conjugate adaptive optics system that would adapt to the turbulence profile at the moment of observation.
Article Reference
First on-sky results of the CO-SLIDAR Cn2 profiler
- Opt. Express 22(9) 10948-10967 (2014)
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