Physics – Optics
Scientific paper
Sep 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002esoc...58..343l&link_type=abstract
Beyond conventional adaptive optics : a conference devoted to the development of adaptive optics for extremely large telescopes.
Physics
Optics
Scientific paper
High dynamic range imaging is defined as the search for very faint objects around bright ones. This pushes requirements on every aspect of telescope and instrument design. Coronography (lyot and phase), high order adaptive optics, apodised pupils may all have a role to play in this endeavor. High order adaptive optics systems have conventionnaly been thought of as Shack-Hartman/piezostack devices because, in theory they should simply scale with the number of actuators, as long as the interactuator spacing remains constant on the pupil. However, empirical evidence seems to show that this behavior is not found in practice, and as systems get more and more complex, i.e. have more and more degrees of freedom, their efficiency drops to unduly low levels. It has therefore been suggested that instead of increasing the number of actuators indefinitely, it might prove more interesting to try to increase their efficiency. This has advantages in terms of robustness and reliablity as well as cost. One of the principal reasons why conventionnal curvature adaptive optics looses its advantage in efficiency for large numbers of degrees of freedom is because of the different scaling laws between the measurement of curvatures inside the pupil and measurements of the radial phase derivative. The aliasing properties increase faster inside the pupil, while noise propagation increases faster on the edge derivative. This implies that for large degrees of freedom, the radial order of the subapertures needed to measure the radial derivative should be smaller than the highest radial order of curvatures. Monte Carlo simulations of such systems have been performed in view of upgrading PUEO, the CFHT adaptive optics bonnette, as a test bench for igh dynamic range imaging, and for visible adaptive optics. Simulations show that 104 subapertures/electrode are sufficient to produce a Strehl ratio of 92% in K band (S(I)= %, FWHM(I)= mas; S(R)= %, FWHM(R)= mas; S(V)= %) FWHM(V)= mas), while it is estimated that at least 900 actuators would be needed in a conventionnal system to produce similar performance, with obvious drawbacks in cost and reliability. One fundamental drawback of the current implementation of curvature sensors is that the edge derivative measurement relies on a null measurement (which is achieved in closed loop), where a zero phase error produces zero photons in the given subaperture. Due to photon noise statistics, this implies that the signal to noise ratio decreases with the actual signal, providing less and less accuracy on the measurement as it gets smaller and smaller, preventing extremely high determination of fundamentally noisier modes that have non-zero radial derivatives. A new scheme to measure the radial phase derivative, based on a radial shearing interferometer inside the pupil is therefore introduced, although no simulations of its implmentation have yet been performed. There might indeed be a fundamental limit to the increase of degrees of freedom for curvature sensors due to the fact that the number of integration constants (provided by the boundary condition measurement) has to be larger than the number of independant curvature measurements across the pupil, if phase reconstruction is intended. It still remains to be determined what this number is on a real system and to be demonstrated that this number is not sufficient for existing 8 meter class telescopes or even some of the smaller ELTs.
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