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11 July 2018 Effects of the telescope spider on extreme adaptive optics systems with pyramid wavefront sensors
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The secondary mirror and associated support structure (spider) will obscure entire rows and columns of sub- apertures of the wavefront sensors (WFS) on the extremely large telescopes. Consequently, piston discontinuities between the segments created by the spider can arise in the estimated wavefront. By optimising the amount of regularisation, the number of modes corrected and the illumination threshold for active subaperture selection, we achieve a closed-loop Strehl of 0.9467 with a spider versus 0.9681 for a case with no spider in K band. The European Southern Observatory’s end-to-end Monte Carlo simulator, OCTOPUS, is used for the numerical evaluation. We also investigate the orientation of the spider relative to the prism and find that when the spider arms are aligned with the edges of the WFS prism, the closed-loop Strehl is at its highest. In our test case of a 2-fold spider and an unmodulated pyramid WFS, we found the difference between best and worst case Strehl to be approximately 18%. We found that an illumination threshold of 20-50% is optimum when a spider arm is paraxial to the wavefront sensor. We also consider the effects of the spider on the reconstruction algorithm, using maximum a posteriori (MAP) reconstructors, one using wavefront slopes and another using the conjugate aperture plane images directly. At high flux both MAP reconstructors perform similarly.
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Byron Engler, Steve Weddell, Miska Le Louarn, and Richard Clare "Effects of the telescope spider on extreme adaptive optics systems with pyramid wavefront sensors ", Proc. SPIE 10703, Adaptive Optics Systems VI, 107035F (11 July 2018);

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