Optique théorique et appliquée
The Homer test bench
The steps of the integration and the results
First experimental results obtained in GLAO-MCAO (June 2008)
We describe here the first experimental results obtained on the Homer test bench. The loop was closed for AO (figure 2), GLAO [1] (figure 3) and MCAO (figure 4) with an integrator type conventional control. The observed and corrected field is 280 x 301 lambda/D. The turbulence is placed at the conjugation altitudes of the deformable mirrors (since the mirrors themselves generate turbulence due to the application of voltages that deform the mirrors) in accordance with a well defined turbulent phase. The first turbulent layer is situated in the pupil plane of the telescope and the second layer is situated at the conjugation altitude of the second mirror. The footprint relative separation in altitude, a ratio of alpha x h/D, is of the order of 0.3. The distribution of the Cn2 is 50% in each one of the turbulent layers and the D/r0 equals 7. The Strehl ratio (SR) is the criterion quantifying the quality of the image in the frame of our study.
The GLAO is a very simple system of multi-analysis and mono-correction. We analyze the information from several guide stars, average these measurements, then apply an average correction using the pupil conjugate deformable mirror of the system. This technique enables us to correct the low altitude layer in particular, the latter contributing greatly to performance degradation.
Figure 1: Open loop long exposure image with the turbulence in the field
Figure 2: Long exposure image in a closed conventional AO loop with an integrator type control. The direction of analysis and correction is circled in green. We see the phenomenon of anisoplanatism that causes off-axis performance degradation.
Figure 3: Closed loop long exposure image with a GLAO type control. The guide stars have a red dotted circle. The correction is quasi-uniform in the field but the quality is poor.
Figure 4: MCAO [2] long exposure image, the guide stars are the same as previously. The correction is uniform in the field.
These first results are very encouraging for the rest of our work which is to test the LQG [3] type control. For each card, we have indicated the Strehl ratio corresponding to the stars of interest. We have also indicated the guide stars and the correction star for the AO.
With these results we clearly see the phenomenon of anisoplanatism in the AO correction. We obtain a very good Strehl ratio for the guide star, then the performance deteriorates in the field. The GLAO control is used to average the measurements taken in different directions but corrects with a single mirror only. So a SR is obtained that is quite uniform in the field, but low. Finally, the MCAO control helps to correct the turbulence at altitude. A quasi-uniform SR is obtained in the field and almost as good as for AO. These very good results, in terms of uniformity in MCAO, were obtained in very favorable turbulence conditions. We have deliberately decided on a low footprint relative separation for this first attempt in order to simplify some of the adjustments.
[1] GLAO: Ground Layer Adaptive Optics: Wide field OA system comprising several WFS and a pupil conjugate deformable mirror. It is used to correct the average turbulence in a wide field. The performance of such a system is not very high but is homogeneous in the field.
[2] MCAO: Multi-Conjugate Adaptive Optics: Wide field OA system comprising several WFSs to analyze the volume of turbulence and several MDs to correct the turbulence in a wide field.
[3] LQG: Linear Quadratic Gaussian: Control law of a reconstructed state feedback form based on a Kalman filtering. In AO without mirrors dynamics, the predicted value of the turbulent phase is obtained with a Kalman filtering then simply projected onto the deformable mirror(s).
