Physics – Optics
Scientific paper
Jul 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000spie.4007..620l&link_type=abstract
Proc. SPIE Vol. 4007, p. 620-631, Adaptive Optical Systems Technology, Peter L. Wizinowich; Ed.
Physics
Optics
Scientific paper
In the photon starved regime (which is usually the case in the real world), adaptive optics can benefit from advanced or innovative schemes such as modal control. The concept is quite natural, as one looks for the natural modes (eigen-modes, Karhunen-Loeve) that occur in the atmosphere and that the deformable mirror can produce; once these modes are identified, the loop gain that minimizes the residual phase variance is determined. Because the modes form an orthogonal and independent basis, optimizing each mode individually ensures that the global minimum is found, producing the highest Strehl ratio possible. In the case of Keck Adaptive Optics System, there were two fundamental difficulties that made the modal control scheme difficult. The first one is that the pupil is not circular but hexagonal. The second one is that being a the Nasmyth focus, the (non-circular) pupil rotates on the wavefront sensor. Some sub-apertures are gradually illuminated as others get extinguished, and the natural modes of the system evolve as a function of pupil rotation. In this poster, we describe the first step of building a modal control, which is to determine what the appropriate modes are, and whether they need to be computed in real time, to compensate for pupil rotation. We also show two methods that were tried to optimize the loop gain (random walk and modified Newton-Raphson). These methods were used in zonal control, as a proof of concept. On-sky trials with a modal scheme showed us that the real time mode determination was not practical or necessary, and a set of mode that worked well under any geometry was found: the covariance matrices of these modes had a diagonal that was factors of ten higher than non- diagonal elements indicating that the modes that we computed were in fact independent in the atmosphere. However, the optimizing schemes required too many samples to produce adequate statistics to find the minimum residual phase variance. Changes to the real time code to provide statistics in real time will help to produce a reliable and automatic modal control.
Gendron Eric
Lai Olivier
Stomski Paul J. Jr.
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