Atmospheric Seeing Research Articles

Optical Turbulence Profile in Marine Environment with Artificial Neural Network Model

Optical turbulence strongly affects different types of optoelectronic and adaptive optics systems. Systematic direct measurements of optical turbulence profiles [Cn2(h)] are lacking for many climates and seasons, particularly in marine environments, because it is impractical and expensive to deploy instrumentation. Here, a backpropagation neural network optimized using a genetic algorithm (GA-BP) is developed to estimate atmospheric turbulence profiles in marine environments which is validated against corresponding [Cn2(h)] profile datasets from a field campaign of balloon-borne microthermal measurements at the Haikou marine environment site. Overall, the trend and magnitude of the GA-BP model and measurements agree. The [Cn2(h)] profiles from the GA-BP model are generally superior to those obtained by BP and the physically-based (HMNSP99) models. Several statistical operators were used to quantify the GA-BP model performance on reconstructing the optical turbulence profiles in marine environments. The characterization of vertical distributions of optical turbulence profiles and the main integral parameters derived from [Cn2(h)] profiles are presented. The median Fried parameter, isoplanatic angle, and coherence time are 9.94 cm, 0.69″, and 2.85 ms, respectively, providing independent optical turbulence parameters for adaptive optics systems. The proposed approach exhibits potential for implementation in ground-based optical applications in marine environments.

Wave-optics investigation of branch-point density

We use wave-optics simulations to investigate branch-point density (i.e., the number of branch points within the pupil-phase function) in terms of the grid sampling. The goal for these wave-optics simulations is to model plane-wave propagation through homogeneous turbulence, both with and without the effects of a finite inner scale modeled using a Hill spectrum. In practice, the grid sampling provides a gauge for the amount of branch-point resolution within the wave-optics simulations, whereas the Rytov number, Fried coherence diameter, and isoplanatic angle provide parameters to setup and explore the associated deep-turbulence conditions. Via Monte Carlo averaging, the results show that without the effects of a finite inner scale, the branch-point density grows without bound with adequate grid sampling. However, the results also show that as the inner-scale size increases, this unbounded growth (1) significantly decreases as the Rytov number, Fried coherence diameter, and isoplanatic angle increase in strength and (2) saturates with adequate grid sampling. These findings imply that future developments need to include the effects of a finite inner scale to accurately model the multifaceted nature of the branch-point problem in adaptive optics.

Testbed Emulator of Satellite-to-Ground FSO Downlink Affected by Atmospheric Seeing Including Scintillations and Clouds

Free Space Optics (FSO) technology enabling next-generation near-Earth communication is prone to severe propagation losses due to atmospheric-turbulence-induced fading and Mie scattering (clouds). As an alternative to the real-time evaluation of the weather effects over optical signal, a state-of-the-art laboratory testbed for verification of slant APD-based (Avalanche Photodiode) FSO links in laboratory conditions is proposed. In particular, a hardware channel emulator representing an FSO channel by means of fiber-coupled Variable Optical Attenuator (VOA) controlled by driver board and software is utilized. While atmospheric scintillation data are generated based on Radiosonde Observation (RAOB) databases combined with a statistical design approach, cloud attenuation is introduced using Mie theory together with empirical Log-Normal modeling. The estimation of atmospheric-turbulence-induced losses within the emulated optical downlink is done with an FSO IM/DD prototype (Intensity Modulation/Direct Detection) relying on two different data throughputs using a transmitter with external and internal modulation. Moreover, the receiver under-test is a high-speed 10 Gbps APD photodetector with integrated Transimpedance Amplifier (TIA) typically installed in OGSs (Optical Ground Stations) for LEO/GEO satellite communication. The overall testbed performance is addressed by a BER tester and a digital oscilloscope, providing BER graphs and eye diagrams that prove the applied approach for testing APD-TIA in the presence of weather-based disruptions. Furthermore, the testbed benefits from the used beam camera that measures the quality of the generated FSO beam.

Crosstalk in image plane beam combination for optical interferometers

ABSTRACT Image plane beam combination in optical interferometers multiplexes the interference fringes from multiple baselines on to a single detector. The beams of starlight are arranged in a non-redundant pattern at the entrance of the combiner so that the signal from each baseline can be separated from one another in the frequency domain. If the signals from different baselines overlap in the frequency domain, this can give rise to a systematic error in the fringe measurements known as baseline crosstalk. In this paper, we quantify crosstalk arising from the combination of atmospheric seeing and beam propagation over distances of the order of hundreds of metres. We find that in idealized conditions atmospheric wavefront errors and beam propagation do not contribute to crosstalk. However, when aperture stops are included in the optical beam train we observe that wavefront errors can result in squared visibility errors arising from crosstalk as high as ΔV2 = 6.6 × 10−3 under realistic observing conditions.

Estimating the astronomical seeing above Dome A using Polar WRF based on the Tatarskii equation

A seeing model based on the Tatarskii equation, which uses wind shear and temperature gradient as inputs, has been proposed for Dome A in our previous study. It has been shown that the seeing was correctly reproduced statistically. In this paper, in order to investigate the ability of the Polar version of the Weather Research and Forecasting (PWRF) model to forecast the seeing, we have tried three different PWRF-simulated wind shear and temperature gradient combinations of two levels for the near-ground. The three combinations all give good performance with the correlation coefficients being higher than 0.75, when compared with the seeing measured by the Kunlun Differential Image Motion Monitor at Dome A during the summer of 2019. Besides this, the gradient Richardson number (Ri), which was also simulated by the PWRF model, basically behaved as expected as the Ri is generally large when the measured seeing is small. These results suggest that the PWRF model can be used to forecast the astronomical seeing above Dome A.

Astronomical seeing forecast for the Thai National Observatory

Forecasting the astronomical seeing above an observatory can assist astronomers plan their observations. In this study, the astronomical seeing above the Thai National Observatory (TNO) in Doi Inthanon, Chiang Mai, Thailand was simulated using the Weather Research and Forecasting (WRF) model. The model outputs were then compared to Polaris seeing observations and using the Differential Image Motion Monitor (DIMM). Results showed that the forecasts capture the variation of the astronomical seeing fairly well. However, bias correction is needed on the simulations due to lack of data from meteorological balloons to constrain the model.

Forecasting seeing for the Maunakea observatories with machine learning

ABSTRACT The staff at the Maunakea Weather Center (MKWC) has provided daily forecasts of optical turbulence for the summit of Maunakea for more than 20 yr. Observational measures of optical turbulence at Maunakea with which to validate official MKWC forecasts have been available since mid-2009. This paper presents a machine-learning approach to translate the MKWC experience into a forecast of the nightly average optical turbulent state of the atmosphere. Maunakea observational and forecast data were collected to build a predictive model of the total and free atmospheric seeing for the following five nights. The motivation for this work is two-fold: to provide a tool/guidance to the MKWC forecaster and allow for a dynamic calibration of the optical turbulence algorithm implemented in the MKWC Weather Research and Forecasting (WRF) model.

Application limitations of a laser guide star in adaptive optoelectronic systems caused by its jitter in the atmosphere

Application limitations caused by the jitter of a laser guide star used for the compensation of atmospheric phase distortions are considered for Earth-based adaptive optoelectronic systems for the observation of natural and artificial space objects. The spatial coherence radii (Fried parameter) for which a laser beam remains within the isoplanatic angle under the jitter effect during guide star formation are estimated using locally homogeneous and isotropic atmospheric turbulence models. Beam deflections in two mutually orthogonal directions are considered to be distributed normally, and the probability of the beam remaining within the isoplanatic angle is considered to be subject to the Rayleigh law. The angular divergence of the deflected beam is estimated for a short exposure with a duration less than the atmosphere freezing time for the far and near fields of the emitting laser aperture.

Optical turbulence and wind speed distributions above the Tibetan Plateau from balloon-borne microthermal measurements

ABSTRACT The vertical profiles of wind speed and the optical turbulence are critical to the design and operation of a new generation of highly sophisticated astronomical telescopes and adaptive optics instrumentation. We present the first study of the temporal evolution behaviours and probability distributions of wind speed [V(h) profiles, as well as the 200 hPa pressure level wind speed, V200] and optical turbulence [$C_n^2(h)$ profiles, and the most relevant integrated astronomical parameters derived from $C_n^2(h)$ profiles, i.e. the seeing ε, the isoplanatic angle θAO, the wavefront coherence time τAO, the average velocity of turbulence VAO, and the seeing layer height hAO] above the Dachaidan site of the Tibetan Plateau. The field campaigns of wind speed and optical turbulence were collected using the balloon-borne microthermal measurement system. From the whole field campaigns, the results are remarkable: The median VAO is 21.1 m s−1, the median V200 is 32.5 m s−1, the median hAO is 7566 m, the median ε is 1.04 arcsec (below 1.00 arcsec 52 per cent of the time), the median θAO is 0.74 arcsec, and the median τAO is 1.33 ms; these conditions are comparable to some of the best astronomical observatories in the world. In particular, the linear relationship of average velocity and 200 hPa level wind at this site is VAO = 0.627V200. In this study, we flag the temporal evolution and probability distribution feature of wind speed, optical turbulence profile, and the relevant integrated astronomical parameters for astronomical applications.

In situ measurements and neural network analysis of the profiles of optical turbulence over the Tibetan Plateau

ABSTRACT The vertical profile of $C_n^2$ is the main factor for accurate astronomical observation and laser communication, however, hardware-based instruments and associated data are not widely available due to logistical and financial issues. In this article, we developed an indirect method, a hybrid network structure which is a combination of the backpropagation neural network and the simulated annealing algorithm, to fit the vertical profile of $C_n^2$. Radiosonde measurements from a field campaign over the Tibetan Plateau at Dachaidan (37.7○N, 95.3○E, 3180 m ASL) were performed in 2020 August to estimate the accuracy of our model, during which a balloon-borne portable turbulence meteorological radiosonde was used to measure the atmospheric optical profiles. Besides, the integrated astronomical parameters (the coherence length r0, seeing ε0, isoplanatic angle θ0, and the wavefront coherence time τ0), derived from $C_n^2$ and wind-speed vertical profiles, are investigated for astronomical applications using the proposed model. In addition, quantitative evaluations such as the correlation coefficient, the root mean squared error, and the systematic bias are used to quantify the performance of our model. More interesting, this model is found to outperform a widely used external scale model for the prevalent atmospheric conditions and shows better correlation and reliable estimates.

Aircraft to Ground Profiling: Turbulence Measurements and Optical System Performance Modeling

A series of flight experiments have demonstrated a novel approach to measuring path-resolved optical turbulence quantities, such as , along an air-to-ground slant path. This paper describes the data acquisition experiments that involved two laser beams propagating between an orbiting airborne platform and a stationary ground terminal over an 8 km slant path. Ground-based and in-flight measurements were collected simultaneously, and profiles were computed using the difference of differential-tilt variance (DDTV) technique. This paper describes the DDTV technique that enables the path-resolved measurement of turbulence strength resulting in profiles. The resulting turbulence profiles reveal what is believed to be the aero-optical contamination from the aircraft boundary layer within the statistics closest to the aircraft. Therefore, the contamination of the aero-optical environment can be quantified with respect to the rest of the atmospheric propagation path. Lastly, this paper presents analyses that compare the measured atmospheric turbulence profiles to state-of-the-art atmospheric models. The analyses extend beyond comparisons and show the measurement-versus-modeling comparison in terms of key directed energy system propagation parameters such as Greenwood frequency, coherence diameter, Rytov number, isoplanatic angle, Tyler frequency, open-loop jitter, and open-loop Strehl ratio. The slant path turbulence is analyzed in the context of air-to-ground and ground-to-air directed energy systems.

Astronomical Seeing at Maidanak Observatory during the Year 2018

Results of a four-month campaign of astronomical seeing measurements at Maidanak astronomical observatory (MAO) are presented. A differential image motion monitor (DIMM) was used for seeing estimations during the period from August to November 2018. The observation was organized within the framework of a site testing for a new telescope which is going to be installed at the observatory. The median value of seeing for the entire period was determined as 0.70 arcseconds, which agrees well with the results of the period 1996–2003. The comparison of monthly values showed that some monthly median values differ from the seasonal trend of the previous period.

Modelling of atmospheric optical turbulence with the Weather Research and Forecasting model at the Ali observatory, Tibet

ABSTRACT In this study, we make use of a meso-scale numerical model to obtain the parameters of atmospheric optical turbulence at the Ali observatory above the Tibetan Plateau. These parameters are essential for ground-based optical observations and some adaptive optics techniques. We present the characterization of the vertical distributions of atmospheric optical turbulence Cn2 and the main integral parameters, such as coherence time τ0, isoplanatic angle θ0 and seeing ε0 derived from Cn2. Meanwhile, the measurement of Cn2 obtained with a radiosonde instrument is used to quantify the model performance on reconstructing the optical turbulence above the site. This shows a high level of agreement and provides a reliable tool for astronomical site testing. The results presented in this paper demonstrate the good behaviour of the atmospheric optical turbulence condition at Ali, particularly during the summer half of the year. We find yearly median values of seeing ε0 = 0.47 arcsec, isoplanatic angle θ0 = 4.35 arcsec and atmospheric coherence time τ0 = 5.52 ms in 2016, and we also provide independent confirmation of the potential of the Ali site in adaptive optics. Moreover, it has been proven that meso-scale models can provide reliable estimations of atmospheric optical turbulence conditions above an astronomical site on the Tibetan Plateau.

Method of Estimation of Turbulence Integral Parameters

The optical effects of turbulence are directly related to turbulence integral parameters, which are integrals of the refractive index structure constant over a whole path with different path-weighting functions (PWFs). We describe a method that utilizes measurable turbulence integral parameters, such as angle-of-arrival fluctuations and scintillation, to estimate turbulence integral parameters that cannot be measured directly. The estimates of the turbulence integral parameters are based on the linear combination of the PWFs of those measurable quantities. New measurable quantities and their PWFs under different propagation conditions were studied. Some interesting and meaningful results have been obtained. This method shows the prospect of characterizing anisoplanatism in adaptive optics and allows for the estimation of some optical turbulence parameters under non-ideal conditions, such as an isoplanatic angle in a finite distance.

Method for Measuring the Second-Order Moment of Atmospheric Turbulence

The turbulence moment of order m (μm) is defined as the refractive index structure constant Cn2 integrated over the whole path z with path-weighting function zm. Optical effects of atmospheric turbulence are directly related to turbulence moments. To evaluate the optical effects of atmospheric turbulence, it is necessary to measure the turbulence moment. It is well known that zero-order moments of turbulence (μ0) and five-thirds-order moments of turbulence (μ5/3), which correspond to the seeing and the isoplanatic angles, respectively, have been monitored as routine parameters in astronomical site testing. However, the direct measurement of second-order moments of turbulence (μ2) of the whole layer atmosphere has not been reported. Using a star as the light source, it has been found that μ2 can be measured through the covariance of the irradiance in two receiver apertures with suitable aperture size and aperture separation. Numerical results show that the theoretical error of this novel method is negligible in all the typical turbulence models. This method enabled us to monitor μ2 as a routine parameter in astronomical site testing, which is helpful to understand the characteristics of atmospheric turbulence better combined with μ0 and μ5/3.

Contribution of statistical site learning to improve optical turbulence forecasting

ABSTRACT The forecast for atmospheric and turbulence conditions above astronomical observatories is of interest to the astronomical community because it allows observations to be planned with maximum efficiency, a process called flexible scheduling. It can also be used to simulate long-term site testing to provide local information useful for the conception of focal and post-focal instrumentation. We have presented our forecasting tool in previous publications, but in this paper we focus on the importance of using local measurements to improve the predictive turbulence model and to better consider the local specificities of a given site, a process we call site learning. For this study, we use a local data base provided by the Calern Atmospheric Turbulence Station, which has been operational since 2015 at Calern Observatory. In addition, we use a set of several months of predictions to feed the turbulence model, taking into account daytime and nighttime conditions. This upgrade improves the quality of our forecasting by reducing the absolute bias between measurements and predictions from 25 to 50 per cent for each layer of the $C_n^2$, by 25 per cent for the seeing, and by 70 per cent for the isoplanatic angle.

A reliable model for estimating the turbulence intensity and integrated astroclimatic parameters from sounding data

ABSTRACT The vertical distributions of atmospheric turbulence ($C_n^2$) are key factors in determining the performance of ground-based telescopes and interferometers. As site-testing campaigns are extremely expensive, atmospheric modelling may represent the estimated results of astronomical sites. We propose a new outer-scale model to estimate refractive index structure constant ($C_n^2$) profiles containing potential temperature and wind shear. The integrated astroclimatic parameters (Fried parameter r0, seeing ε, isoplanatic angle θ0, and coherence time τ0) can also be calculated using this outer-scale model. Radiosonde measurements from a filed campaign at Tibet (Lhasa) and Rongcheng (Shandong) are utilized to quantify the performance of this model. Compared with the Dewan model, HMNSP99 model and Thorpe model, the new model gives better results as a profile and an astroclimatic parameter estimator. Compared with the measured median profile, the correlation coefficient reaches 0.88 in Lhasa and 0.87 in Rongcheng.

Spectral Performance Optimization of Small-Diameter Telescopes for Space Object Detection

Successful tracking and detection of resident space objects (RSOs) requires selection of spectral filters that are capable of compensating for the effects of wavelength dependent atmospheric scattering, and material reflectance properties. When operating in an urban environment such as Atlanta, GA the emitted city light pollution is also an important factor. In this study, we perform a multi-spectral optimization for the Georgia Tech Space Object Research Telescope across the visible through infrared spectrum that accounts for atmospheric turbulence, atmospheric transmission, and background sky radiance. The first contribution of this work is the development of a wavelength-dependent performance metric for the detection of RSOs under different atmospheric conditions. The second contribution is the derivation of a simplified model for light pollution that is based on existing astronomy models. This simplified model accounts for double scattering of light and can be used for adding a light pollution component to sky background results that are generated by complex atmospheric simulation tools. The final contribution is the derivation of novel spectral filters that are tuned to the sub-optimal atmospheric seeing conditions of Atlanta, GA and the RSO materials that are considered in this study. While this study is developed for the geographic region of Atlanta, GA and uses simplified bidirectional reflectance distribution models models for spacecraft materials, the framework is generalizable to different geographic regions, weather conditions, and satellite orientations.

Predictive control for adaptive optics using neural networks

Adaptive optics (AO) has become an indispensable tool for ground-based telescopes to mitigate atmospheric seeing and obtain high angular resolution observations. Predictive control aims to overcome latency in AO systems: the inevitable time delay between wavefront measurement and correction. A current method of predictive control uses the empirical orthogonal functions (EOFs) framework borrowed from weather prediction, but the advent of modern machine learning and the rise of neural networks (NNs) offer scope for further improvement. Here, we evaluate the potential application of NNs to predictive control and highlight the advantages that they offer. We first show their superior regularization over the standard truncation regularization used by the linear EOF method with on-sky data before demonstrating the NNs’ capacity to model nonlinearities on simulated data. This is highly relevant to the operation of pyramid wavefront sensors (PyWFSs), as the handling of nonlinearities would enable a PyWFS to be used with low modulation and deliver extremely sensitive wavefront measurements.

AGN Jets and a Fanciful Trio of Black Holes in the Abell 85 Brightest Cluster Galaxy

A new radio map of the Abell 85 Brightest Cluster Galaxy (BCG) was obtained with the Karl G. Jansky Very Large Array. With a resolution of 0.″02, this radio image shows two kiloparsec-scale bipolar active galactic nucleus jets emanating from the active galactic nucleus of the A85 BCG. The galaxy core appears as a single entity on the new radio map. It has been assumed that the A85 BCG contained a binary black hole in its core. However, Chandra X-ray data and the new high-resolution radio map show no evidence that the A85 BCG harbors a binary black hole. The assumption that this galaxy contains a binary black hole was based on the analysis of its optical surface brightness profile obtained under poor seeing conditions. We demonstrate how the well-known blurring effects of atmospheric seeing can mimic the effects of a binary supermassive black hole (SMBH). Likewise, SDSS J004150.75–091824.3 was postulated to be “a third” SMBH associated with the BCG. In the optical and X-rays, SDSS J004150.75–091824.3 is a point-like source located ∼14″ away from the nucleus of the A85 BCG. A new spectrum of SDSS J004150.75–091824.3, obtained with the 10.4 m Gran Telescopio Canarias, reveals that this source is a background quasar at a redshift of z = 1.5603 ± 0.003 and not associated in any way with the A85 cluster.