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Optical Turbulence Profiling with Stereo-SCIDAR for VLT And MNRAS 000,1{11 (2015) Preprint 8 June 2018 Compiled using MNRAS LATEX style file v3.0 Optical turbulence profiling with Stereo-SCIDAR for VLT and ELT J. Osborn,1? R. W. Wilson1, M. Sarazin2, T. Butterley1, A. Chac´on2, F. Derie2, O. J. D. Farley1, X. Haubois2 D. Laidlaw1, M. LeLouarn2, E. Masciadri3, J. Milli2, J. Navarrete2 and M. J. Townson1, 1Centre for Advanced Instrumentation, University of Durham, Durham, UK 2European Southern Observatory, Karl-Schwarzshild-Str.2, 85748 Garching bei Muenchen, Germany 3INAF Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, I-50125 Florence, Italy Accepted XXX. Received YYY; in original form ZZZ ABSTRACT Knowledge of the Earth's atmospheric optical turbulence is critical for astronomical instrumentation. Not only does it enable performance verification and optimisation of existing systems but it is required for the design of future instruments. As a minimum this includes integrated astro-atmospheric parameters such as seeing, coherence time and isoplanatic angle, but for more sophisticated systems such as wide field adaptive optics enabled instrumentation the vertical structure of the turbulence is also required. Stereo-SCIDAR is a technique specifically designed to characterise the Earth's atmospheric turbulence with high altitude resolution and high sensitivity. Together with ESO, Durham University has commissioned a Stereo-SCIDAR instrument at Cerro Paranal, Chile, the site of the Very Large Telescope (VLT), and only 20 km from the site of the future Extremely Large Telescope (ELT). Here we provide results from the first 18 months of operation at ESO Paranal including instrument comparisons and atmospheric statistics. Based on a sample of 83 nights spread over 22 months covering all seasons, we find the median seeing to be 0.6400 with 50% of the turbulence confined to an altitude below 2 km and 40% below 600 m. The median coherence time and isoplanatic angle are found as 4.18 ms and 1.7500 respectively. A substantial campaign of inter-instrument comparison was also undertaken to as- sure the validity of the data. The Stereo-SCIDAR profiles (optical turbulence strength and velocity as a function of altitude) have been compared with the Surface-Layer SLODAR, MASS-DIMM and the ECMWF weather forecast model. The correlation coefficients are between 0.61 (isoplanatic angle) and 0.84 (seeing). Key words: atmospheric effects { instrumentation: adaptive optics { site testing { telescopes arXiv:1806.02585v1 [astro-ph.IM] 7 Jun 2018 1 INTRODUCTION 500 m (see for example Neichel et al. 2008; Basden et al. 2010; Vidal et al. 2010; Gendron et al. 2014). Turbulence within the Earth's atmosphere imposes a limita- The next generation of 40 m class ELTs that are cur- tion upon astronomical observations. Wavefront distortions rently under construction will enable new discoveries in all blur the image and can be compensated with Adaptive Op- areas of astronomy and push forwards the boundaries of hu- tics (AO) systems. The future of this correction technique man knowledge. They will look further back in space and requires comprehensive knowledge of the dynamics of the time to explore the early universe and shed light on cur- Earth's atmosphere. This is critical for future sophisticated rently unanswered questions such as the physical basis of AO systems on existing very large and the future extremely dark energy and dark matter, as well as their evolution in large telescopes (ELTs). These telescopes will be sensitive the time scales from early universe to present time. They to variations in turbulence altitude of the order of 100 to will discover and characterise extra-solar planets and poten- tially find distant habitable worlds. More details of ELT sci- ? E-mail: [email protected] (JO) ence cases can be found in European Southern Observatory © 2015 The Authors 2 J. Osborn et al. (2009); Skidmore & Committee(2015). To fulfil these ambi- and validation. This will require sequences of turbulence pro- tious objectives these giant telescopes will be equipped with files and corresponding astro-atmospheric parameters, such highly sophisticated AO in order to counteract the detri- as seeing, coherence time and isoplanatic angle. mental effects of the Earth's atmosphere. • Astronomical instrument design. All future instrumen- SCIDAR (Scintillation Detection and Ranging) (Vernin tation needs to be designed for the specific atmospheric con- & Roddier 1973) is a technique often used for profiling the ditions they are expected to encounter. The Stereo-SCIDAR Earth's atmospheric turbulence. Stereo-SCIDAR is an ex- will provide distributions of astro-atmospheric parameters tension of the SCIDAR technique. Stereo-SCIDAR is a sen- and the median optical turbulence profile, which is critically sitive, high-altitude resolution turbulence monitor capable important for the future generation of wide-field AO. of returning the vertical profile of Earth's optical turbu- • Real-time instrument optimisation. lence strength and velocity in real-time. The Stereo-SCIDAR • General site monitoring. instrument has been described previously (Shepherd et al. • Meso-scale atmospheric turbulence forecasting calibra- 2014). It has been routinely and reliably used at the Obser- tion and validation (Masciadri et al. 2017). vatorio del Roque de Los Muchachos, La Palma, Spain, (Os- There has been a lot of previous work on characterising born et al. 2015a) and more recently at ESO Paranal. There the Earth's turbulent atmosphere above ESO Paranal. is also a version under development for Mount Stromlo, Aus- The DIMM has been in regular operation on site since tralia (Korkiakoski et al. 2016). 1990 and therefore provides a large sample from which to The wind velocity profiles from Stereo-SCIDAR have derive integrated turbulence statistics (Sarazin et al. 2008). been validated with both balloon borne radiosonde and Gen- Other studies such as the multi-instrument campaign eral Circulation numerical weather forecast models from the of 2007 (Dali Ali et al. 2010; Sarazin & Roddier 1990; Ziad Global Forecast System (Osborn et al. 2017). This multi-way et al. 2004; Rami´oet al. 2008; Kornilov et al. 2003; Tokovinin comparison shows that the numerical models are capable et al. 2010; Maire et al. 2007) and the surface layer charac- of forecasting wind profiles for astronomical instrumenta- terisation campaign of 2010 (Lombardi et al. 2010; Sarazin tion optimisation on average but if high-temporal resolution & Roddier 1990; Kornilov et al. 2003; Osborn et al. 2010; variations are of interest then the optical monitor is still Tokovinin et al. 2010), were extremely useful for understand- required. ing the atmosphere and the various instruments. However Recent applications of Stereo-SCIDAR include support- due to the limited nature of the campaigns they do not at- ing the AO testbed, Canary (Morris et al. 2014), where it tempt to present a statistical representation of the site. was used to validate the tomographic reconstructor as well Cute-SCIDAR, another SCIDAR instrument was oper- as to validate the Linear Quadratic Gaussian smart AO con- ational at ESO Paranal during November / December 2007 troller (Sivo 2014). (Rami´oet al. 2008; Masciadri et al. 2011). This SCIDAR op- Here we specifically discuss the Stereo-SCIDAR com- erated for 20 nights and was used as part of the Paranal 2007 missioned by ESO to operate at Cerro Paranal, Chile, the multi-instrument campaign (Dali Ali et al. 2010). The data site of the Very Large Telescope (VLT). The VLT comprises from this instrument has proved extremely useful to further of four 8 m unit telescopes (UT) and four 1.8 m auxiliary the understanding of the behaviour of the MASS (Masciadri telescopes (AT), and only 20 km from the site of the future et al. 2014; Lombardi & Sarazin 2016). ELT. Stereo-SCIDAR was commissioned at ESO Paranal in With the exception of the MASS-DIMM these cam- April 2016 and has been in regular operation since this date. paigns provide a limited dataset with which to compare our The Stereo-SCIDAR profiles are of particular and current data. Here, we present the first 20 months of Stereo-SCIDAR interest at Paranal due to the development of wide-field of operation which significantly increases the volume of high- view AO system on the VLT and the planned AO systems on altitude resolution and high-sensitivity turbulence profiles the ELT. The optical turbulence profiles will enable perfor- at ESO Paranal. mance estimation as well as performance validation of these Section2 describes the instrument, the data analysis complicated tomographic AO systems. pipeline and the data archive. Section3 shows the distri- We compare the results from Stereo-SCIDAR with the bution of the turbulence statistics as measured by the first existing Paranal suite of dedicated site monitors, including phase of the Stereo-SCIDAR operation at Paranal. The com- the Surface-Layer Slope Detection And Ranging (SLODAR) parisons of the parameters as estimated by Stereo-SCIDAR instrument(Wilson 2002; Osborn et al. 2010; Butterley et al. are compared to other existing instrumentation in section4. 2015), and the Multi-Aperture Scintillation Sensor - Differ- The conclusions are in section5. ential Image Motion Monitor (MASS-DIMM) (Sarazin & Roddier 1990; Sarazin et al. 2011; Kornilov et al. 2003). We also compare the wind velocity profiles from the Stereo- 2 STEREO-SCIDAR SCIDAR with those from the European Centre for Medium Range Weather Forecasts (ECMWF) (Osborn et al. 2017). The Stereo-SCIDAR method has been described in detail Using the three instruments and the model in this way it is several times before (see for example Shepherd et al. 2014; possible to validate the performance of the recently commis- Osborn et al. 2015a, 2017; Derie et al. 2016).Briefly, the sioned Stereo-SCIDAR instrument. Stereo-SCIDAR uses the triangulation technique by cross- After instrumentation cross-validation we present the correlating the spatial intensity pattern (scintillation) from results, statistics and temporal variations from the Stereo- two stars.
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