Theoretical and Applied Optics
The Homer test bench
Adaptive optics and multi-conjugation
Adaptive optics: see further, see smaller
Nowadays, Adaptive optics (AO) is a technique that is widely used in the astronomy community. It is used to correct in real-time the deterioration of telescope images due to atmospheric turbulence. Figure 1 shows the functional diagram for an AO system and its key elements. A more detailed description can be found on the site of
NAOS, the AO system that equips the very large telescopes (VLT) in Chile, and which Onera has contributed to.
Figure 1: Adaptive optics functional diagram
AO is today used on the majority of large terrestrial telescopes. However, the original concept of AO only corrects a small zone of the field of view: as the distance from the analysis source increases, the performance of the system degrades. This is due to the phenomenon of anisoplanatism linked to the difference between analyzed and corrected turbulent volumes. Figure 2 illustrates this problem.
Figure 2: Diagram representing the phenomenon of anisoplanatism. It can be clearly seen that the path taken by the analyzed light and that of the correction is different. A part of the turbulence is not seen and so cannot be corrected. (Acknowledgements to E. Marchetti, ESO)
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Figure 3: Image of the galactic center taken with NAOS on the VLT. In this image we see the effect of anisoplanatism, causing a deterioration in the image away from the center, where the AO loop has been closed.
Multi-conjugation: increasing the observed field
To overcome this effect, the concept of multi-conjugate adaptive optics (MCAO) was proposed by Dicke then taken up by Beckers (see figure 4). To increase the field of correction, estimation and correction of the turbulence are performed in a volume by using several directions of analysis and several deformable mirrors optically conjugated to different altitudes. This concept can be used to significantly increase the corrected field, typically from several arc seconds up to fields of the order of an arc minute. Consequently, it is particularly interesting for numerous astrophysics programs.
Figure 4: Diagram of the concept of MCAO. Three WFSs are used to analyze the light from three analysis directions. The correction is made by two deformable mirrors conjugated to equivalent altitudes.
Figure 5: Image of the galactic center, free from anisoplanatism. The stars in the field are here well-resolved and not deformed.
