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In this paper, we present the design and prototyping of the HARMONI Adaptive Optics Calibration Unit (AOCU). The AOCU consists of a set of on-axis sources (covering 0.5-2.4 μm) with a controllable wavefront shape. It will deploy into the instrument focal plane to inject calibration light into the rest of the system. The AOCU supports all-natural guide-star wavefront sensors for SCAO, HCAO, and LTAO.
The AOCU will be used to calibrate the WFSs, the internal interaction matrices of HARMONI, measure and compensate NCPAs between AO dichroics and the science detectors, and calibrate the pointing model zero position. The illumination assembly of the AOCU will consist of six diffraction-limited sources and a resolved source coupled into fibres. Because of the wide range of wavelengths and the spatial separations requirements, we use two endlessly single-mode fibres and a multimode fibre. In addition, several LED sources need to be coupled efficiently into the single-mode fibres. In this paper, we present the general AOCU design using off-the-shelf with a focus on the illumination and source module.
Tomography requires the knowledge of the statistical turbulence parameters, commonly recovered from the system telemetry using a dedicated profiling technique. For demonstration purposes with the MOAO pathfinder CANARY, this identification is performed thanks to the Learn & Apply (L&A) algorithm, that consists in model-fitting the covariance matrix of WFS measurements dependant on relevant parameters: Cn2(h) profile, outer scale profile and system mis-registration.
We explore an upgrade of this algorithm, the Learn 3 Steps (L3S) approach, that allows one to dissociate the identification of the altitude layers from the ground in order to mitigate the lack of convergence of the required empirical covariance matrices therefore reducing the required length of data time-series for reaching a given accuracy. For nominal observation conditions, the L3S can reach the same level of tomographic error in using five times less data frames than the L&A approach.
The L3S technique has been applied over a large amount of CANARY data to characterize the turbulence above the William Herschel Telescope (WHT). These data have been acquired the 13th, 15th, 16th, 17th and 18th September 2013 and we find 0.67"/8.9m/3.07m.s−1 of total seeing/outer scale/wind-speed, with 0.552"/9.2m/2.89m.s−1 below 1.5 km and 0.263"/10.3m/5.22m.s−1 between 1.5 and 20 km. We have also determined the high altitude layers above 20 km, missed by the tomographic reconstruction on CANARY , have a median seeing of 0.187" and have occurred 16% of observation time.
We have developed a Point Spread Function (PSF)-Reconstruction algorithm dedicated to MOAO systems using system telemetry to estimate the PSF potentially anywhere in the observed field, a prerequisite to deconvolve AO-corrected science observations in Integral Field Spectroscopy (IFS). Additionally the ability to accurately reconstruct the PSF is the materialization of the broad and fine-detailed understanding of the residual error contributors, both atmospheric and opto-mechanical.
In this paper we compare the classical PSF-r approach from Véran (1) that we take as reference on-axis using the truth-sensor telemetry to one tailored to atmospheric tomography by handling the off-axis data only.
We've post-processed over 450 on-sky CANARY data sets with which we observe 92% and 88% of correlation on respectively the reconstructed Strehl Ratio (SR)/Full Width at Half Maximum (FWHM) compared to the sky values. The reference method achieves 95% and 92.5% exploiting directly the measurements of the residual phase from the Canary Truth Sensor (TS).
Stereo-SCIDAR is a generalised SCIDAR instrument which is used to characterise the profile of the atmospheric optical turbulence strength and wind velocity using triangulation between two optical binary stars. Stereo-SCIDAR has demonstrated the capability to resolve turbulent layers with the required vertical resolution to support wide-field ELT instrument designs. These high resolution atmospheric parameters are critical for design studies and statistical evaluation of on-sky performance under real conditions. Here we report on the new Stereo-SCIDAR instrument installed on one of the Auxillary Telescope ports of the Very Large Telescope array at Cerro Paranal. Paranal is located approximately 20 km from Cerro Armazones, the site of the E-ELT. Although the surface layer of the turbulence will be different for the two sites due to local geography, the high-altitude resolution profiles of the free atmosphere from this instrument will be the most accurate available for the E-ELT site.
In addition, these unbiased and independent profiles are also used to further characterise the site of the VLT. This enables instrument performance calibration, optimisation and data analysis of, for example, the ESO Adaptive Optics facility and the Next Generation Transit Survey. It will also be used to validate atmospheric models for turbulence forecasting. We show early results from the commissioning and address future implications of the results.
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