Tomographic reconstruction at ELT scales requires knowledge of the high resolution optical turbulence profile Cn2(h). While in most cases this will be obtained through analysis of the AO telemetry, it is also important to have an independent site monitor that is capable of verifying these profiles, or providing profiles to "cold start" the system at the beginning of an observation. Here, we present the initial results of the pt5m-SCIDAR, a generalised-SCIDAR instrument operating on the robotic point-5 metre (pt5m, D=0.5m) telescope in La Palma. We show that despite the small aperture we are able to extract around 15-layer Cn2(h) profiles and explore the results of measurement campaigns with comparison to other site monitors around the observatory. We believe SCIDAR on smaller telescopes such as the pt5m presents a cost-effective solution as a site monitor for the Cn2(h) profile with sufficient resolution to support ELT tomography.
TURBO (TURBulence MOnitoring and forecasting Equipment) is the first robotic, 24-hour, urban turbulence monitoring station. Situated on the roof of the D4 building at Campus Nord, Universitat Politècnica de Catalunya (UPC) in Barcelona, TURBO represents a joint effort by CommSensLab-UPC, the European Space Agency and Durham University. The system is centred around two 24-hour Shack-Hartmann Image Motion Monitors (24hSHIMM). The 24hSHIMM utilises a small telescope and Shack-Hartmann wavefront sensor to observe single stars in the short-wave infrared and derive key optical turbulence (OT) parameters continuously, throughout the day and night. We present novel data on seasonal, 24-hour urban turbulence conditions and the effects of fully continuous data assimilation for improving the accuracy of mesoscale forecasting tools. We discuss the first few months of results from the experiment and the implications for current adaptive optics activities in astronomy and optical communications.
A six-night optical turbulence monitoring campaign has been carried at Cerro Paranal observatory in February and March, 2023 to facilitate the development and characterisation of two novel atmospheric site monitoring instruments - the ring-image next generation scintillation sensor (RINGSS) and 24-hour Shack Hartmann image motion monitor (24hSHIMM) in the context of providing optical turbulence monitoring support for upcoming 20-40m class telescopes. Both instruments offer the potential to replace the outdated MASS-DIMM, utilising new technologies to produce detailed, real-time characterisation of optical turbulence conditions at a site. In addition, the well-characterised Stereo-SCIDAR and 2016-MASS-DIMM were both active throughout the campaign providing further data for comparison. Contemporaneous measurements of both the integrated turbulence parameters - including seeing, free atmosphere seeing, coherence time and angle – and the vertical optical turbulence profile are examined along with the first 24-hour, day and night turbulence measurements at the site.
Understanding the vertical distribution of atmospheric optical turbulence is essential for the global-scale implementation of free-space optical communications (FSOC). Maintaining communications with satellites in low-Earth orbit (LEO) requires tracking over changing elevation angles. Decreasing elevation angles in optical communication links due to a satellite’s orbit attributes to significant signal losses due to increased propagation lengths and strong turbulence. Here we present the variance in atmospheric optical turbulence measurements in the form of scintillation index and Fried parameter measured on the Island of La Palma. These measurements are taken between elevation angles of 90° and 0° with reference measurements being taken concurrently at zenith to remove temporal variations. The results are compared with the existing theory.
Atmospheric optical turbulence causes signal loses in laser propagation. Here we present vertical measurements of optical turbulence taken in London’s financial district. Additionally, we demonstrate a method of modelling atmospheric states in simulation from the measured data. From this we derive the predicted system performance of an optical downlink from a satellite in low Earth orbit (LEO) to ground in the atmospheric conditions observed on the night. We also present the improvements in performance with the addition of adaptive optics at the receiver end.
We present the 24-hour Shack-Hartmann Image Motion Monitor (24hSHIMM), the first truly continuous, 24-hour optical turbulence monitor. Atmospheric optical turbulence is a significant limitation for free-space optical communications and other technologies. Knowledge of the turbulence conditions allows for the selection of favourable sites for optical ground stations. It also enhances operations though providing data for assimilation into turbulence forecasting models and real-time monitoring of conditions. The 24hSHIMM uses a Shack-Hartmann wavefront sensor to measure a low-resolution vertical optical turbulence profile, from which the coherence length, angle and Rytov variance are calculated. Additionally a vertical wind speed profile from meteorological forecast data is used to calculate the coherence time. Due to its portability, the instrument can operate in a wide variety of locations, even urban, to provide continuous information about the atmospheric turbulence. To demonstrate this, we show parameters recorded at the astronomical observatory in La Palma for a continuous 36-hour period. With its wide array of capabilities, the 24hSHIMM offers strong support for future research in free-space optics.
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