اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدAccurate measurement of Cn2 profile with Shack-Hartmann data
108
0
0.0
(
0
)
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
The precise reconstruction of the turbulent volume is a key point in the development of new-generation Adaptive Optics systems. We propose a new Cn2 profilometry method named CO-SLIDAR (COupled Slope and scIntillation Detection And Ranging), that uses correlations of slopes and scintillation indexes recorded on a Shack-Hartmann from two separated stars. CO-SLIDAR leads to an accurate Cn2 retrieval for both low and high altitude layers. Here, we present an end-to-end simulation of the Cn2 profile measurement. Two Shack-Hartmann geometries are considered. The detection noises are taken into account and a method to subtract the bias is proposed. Results are compared to Cn2 profiles obtained from correlations of slopes only or correlations of scintillation indexes only.
قيم البحث
اقرأ أيضاً
Adaptive optics (AO) systems using tomographic estimation of three-dimensional structure of atmospheric turbulence requires vertical atmospheric turbulence profile, which describes turbulence strength as a function of altitude as a prior information.
We propose a novel method to reconstruct the profile by applying Multi Aperture Scintillation Sensor (MASS) method to scintillation data obtained by a Shack-Hartmann wavefront sensor (SH-WFS). Compared to the traditional MASS, which uses atmospheric scintillation within 4 concentric annular apertures, the new method utilizes scintillation in several hundreds of spatial patterns, which are created by combinations of SH-WFS subapertures. Accuracy of the turbulence profile reconstruction is evaluated with Bayesian inference, and it is confirmed that turbulence profile with more than 10 layers can be reconstructed thanks to the large number of constraints. We demonstrate the new method with a SH-WFS attached to the 50 cm telescope at Tohoku university and confirm that general characteristics of atmospheric turbulence profile is reproduced.
We investigate the improvements in Shack-Hartmann wavefront sensor image processing that can be realised using total variation minimisation techniques to remove noise from these images. We perform Monte-Carlo simulation to demonstrate that at certain
signal-to-noise levels, sensitivity improvements of up to one astronomical magnitude can be realised. We also present on-sky measurements taken with the CANARY adaptive optics system that demonstrate an improvement in performance when this technique is employed, and show that this algorithm can be implemented in a real-time control system. We conclude that total variation minimisation can lead to improvements in sensitivity of up to one astronomical magnitude when used with adaptive optics systems.
In typical adaptive optics applications, the atmospheric residual turbulence affects the wavefront sensor response decreasing its sensitivity. On the other hand, wavefront sensors are generally calibrated in diffraction limited condition, and, so, th
e interaction matrix sensitivity differs from the closed loop one. The ratio between the two sensitivities, that we will call the sensitivity loss factor, has to be estimated to retrieve a well-calibrated measurement. The spots size measurement could give a good estimation, but it is limited to systems with spots well sampled and uniform across the pupil. We present an algorithm to estimate sensitivity loss factor from closed loop data, based on the known parameters of the closed loop transfer functions. Here we preferred for simplicity the Shack-Hartmann WFS, but the algorithm we propose can be extended to other WFSs.
In recent years, detectors with sub-electron readout noise have been used very effectively in astronomical adaptive optics systems. Here, we compare readout noise models for the two key faint flux level detector technologies that are commonly used: E
MCCD and scientific CMOS (sCMOS) detectors. We find that in almost all situations, EMCCD technology is advantageous, and that the commonly used simplified model for EMCCD readout is appropriate. We also find that the commonly used simple models for sCMOS readout noise are optimistic, and recommend that a proper treatment of the sCMOS rms readout noise probability distribution should be considered during instrument performance modelling and development.
Real-time seeing and outer scale estimation at the location of the focus of a telescope is fundamental for the adaptive optics systems dimensioning and performance prediction, as well as for the operational aspects of instruments. This study attempts
to take advantage of multiwavelength long exposure images to instantaneously and simultaneously derive the turbulence outer scale and seeing from the full-width at half-maximum (FWHM) of seeing-limited images taken at the focus of a telescope. These atmospheric parameters are commonly measured in most observatories by different methods located away from the telescope platform, and thus differing from the effective estimates at the focus of a telescope, mainly because of differences in pointing orientation, height above the ground, or local seeing bias (dome contribution). Long exposure images can either directly be provided by any multiwavelength scientific imager or spectrograph, or alternatively from a modified active optics Shack-Hartmann sensor (AOSH). From measuring simultaneously the AOSH sensor spot point spread function FWHMs at different wavelengths, one can estimate the instantaneous outer scale in addition to seeing. Although AOSH sensors are specified to measure not spot sizes but slopes, real-time r0 and L0 measurements from spot FWHMs can be obtained at the critical location where they are needed with major advantages over scientific instrument images: insensitivity to the telescope field stabilization, and being continuously available. Assuming an alternative optical design allowing simultaneous multiwavelength images, AOSH sensor gathers all the advantages for real-time seeing and outer scale monitoring. With the substantial interest in the design of extremely large telescopes, such a system could have a considerable importance.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
