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1 AN ALIGNMENT STRATEGY FOR THE ATST M2 Implementing a standalone correction strategy for ATST M2 Robert S. Upton NIO/AURA February 11,2005

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2 BACKGROUND NSO hired ORA to perform a sensitivity analysis of the ATST 1.Optical performance is most sensitive to M2 misalignments 2.Image and pupil boresight error correctible with M3 and M6 tilts 3.ORA have defined an alignment strategy using their AUT optimization routine in CODE V 4.NSO would like a “standalone” reconstruction/optimization control strategy that can restore optical performance subject to M2

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3 OUTLINE 1.Statement of work 2.Analysis a.Pupil and image boresight b.Zernike coefficients c. Linear mathematical analysis. 3.Correction strategy 4.Summary 5.Other thoughts

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4 STATEMENT OF WORK Develop an understanding of the problem Develop a suitable optical model Perform analysis to develop suitable alignment strategy Test the strategy Comment of potential future areas of analysis and development

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5 ATST OPTICAL MODEL Use CODE V macro capability to perturb ATST, develop boresight relations, Zernike sensitivity analysis, and test correction strategy

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6 ANALYSIS Characterize the pupil and image bore sight sensitivities Characterize the higher-order optical sensitivities M2 Determine ATST system linearity ATST linear analysis Alignment strategy. Linear reconstruction and optimization

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7 M3 AND M6 MOTION SENSITIVITY ANALYSIS: Maintaining pupil and image boresight Determine the angular motions of M3 and M6 that maintain pupil and image alignment subject to changing M2 Used CODE V optimizer with gut ray position constraint Determined that and rotation are most sensitive for M3 and M6 Second-order angular contributions and cross-term contributions have significance c subscript denotes compensator motions

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8 PREALIGNMENT TEST Apply boresight equations to actual perturbation test Data arranged to provide all combinations of decenters and tilts, except rotation about Z ZZ ZZ YY XX XX … ZZ ZZ YY XX XX

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9 DEFINE THE PRE-ALIGNMENT CORRECTION Pupil and Image motion Perturb the telescope by a total of 400 m in decenters and 0.4 degrees in tilts

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10 HIGHER-ORDER OPTICAL SENSITIVITY FOR M2 Perturb the M2 through its 6 DOF and calculate the resulting Zernike (rms) coefficients at three field locations The rms Zernike coefficients Z4, Z5, Z6, Z7, Z8, Z9, and Z10 are calculated These Zernike coefficients quantify astigmatism, focus, trefoil and coma M2 is decentered through 20 values from 0 to 2 mm M2 is decentered through values from 0 to 0.2 degrees (0, 1.5 arc-min) (-1.5 arc-min, 0)(1.5 arc-min, 0) Perturb telescope Perform pre-alignment Determine Zernike coefficient Renew telescope prescription The Zernike coefficient sensitivities are determined whilst correcting the boresight error

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11 HIGHER-ORDER OPTICAL SENSITIVITY FOR M2 The Zernike coefficients are fit to a second-order vector polynomial resulting in matrix coefficients C0, C1, C2 Linear algebraic analysis is performed on the linear matrix coefficient C1. Determines linear independence M2 reconstruction is demonstrated in the linear limit

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12 DETERMINE ATST SYSTEM LINEARITY ATST system linearity is encapsulated in C1 If system is largely linear then a large range of elegant linear algebraic tools can be used to restore optical performance for the perturbed ATST In other words, ? WFS Modes

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13 ATST SYSTEM LINEARITY Z4(y) Z4( ) Z5(y)Z5(z) Z5( ) Z6(x) Z6( ) Most dominant aberrations and DOF are linear

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14 ATST LINEAR ANALYSIS ATST M2 System linearity for dominant contributions provides an elegant solution space for analysis and reconstruction (correction) Classical solution to linear problem is least-square fit Should work. RIGHT? Not quite. The LSQ solution requires C1 to be full rank (i.e. columns in C1 are linearly independent). The ATST does not have linear independence in WFS modes or DOF Use Moore-Penrose pseudo-inverse (Barrett and Myers Foundation of Image Science) Pseudo-inverse algorithms make use of singular value decomposition (SVD)

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15 ATST LINEAR ANALYSIS What SVD does for you … SVD is a matrix factorization scheme The matrix V contains orthonormal columns that define a vector subspace in WFS space The matrix U contains orthonormal columns that define a vector subspace in DOF space The matrix contains singular values along its diagonal in decreasing magnitude. The number of values equals the rank of C1 ATST M2 has a rank of 5 (6 M2 DOF) SVD reconstructs d in a non-unique way (minimum norm solution) Uncoupled representation of ATST One M2 DOF is a combination of 5 others

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16 ATST LINEAR ANALYSIS What SVD does for you … Linear reconstruction d` is a minimum norm solution The mirror DOF are reconstructed from d` The ATST optical performance is reconstructed in a non- unique way

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17 CORRECTION STRATEGY Linear reconstruction and optimization Perturb telescope Boresight correction (pre-alignment) WFS measurement Zernike sensitivity data Perturb telescope WFS measurement Align telescope Boresight correction (pre-alignment) Define sensitivity dataReconstruction

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18 CORRECTION STRATEGY Monte Carlo results Plot merit function for 101 trials with random perturbations Linear reconstruction restores the optical performance of the ATST to diffraction limited performance

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19 CORRECTION STRATEGY Limitations of correction The linear reconstruction technique requires C0 to be known every time the control loop is used For T & g the telescope prescription changes resulting in C0 C0 results in C1 and d Reconstructor requires updating Use of simplex optimizer can help provide a least-squares solution even if C0 is not well known Other wavefront sensors/fiducials are required to distinguish the motions of mirrors from changes due to T & g

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20 SUMMARY 1.Statement of work has been completed a.Understand the problem b. Develop a suitable model c.Define standalone correction strategy d.Correction strategy works over large range of motions. Average rms error is corrected by a factor of 400 2.The ATST is linear for the dominant aberrations (Focus, Astigmatism & Coma) 3.The 6 M2 DOF are not linearly independent. Rank(C1)=5. Suspect the non-full rank C1 has to do with Z-rotation of M2 about center of parent vertex 4.Simplex development 5.Report delivered by COB 02/11/05

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21 FURTHER STUDY Finish the simplex optimizer in MATLAB Study the field dependence of aberrations that arise from ATST perturbation. Zernike polynomials are not the most appropriate basis set. Use SVD … For bending modes of mirror the reconstructor has to be extended Investigate the effects of T and g on C0. Efficacy of linear solution Effects of noise on WFS. Atmospheric effects. Local mirror seeing and etc Develop a true alignment test using focus of primary, Gregorian focus, and pupil masks to establish the optical axis w.r.t. mechanical axis Extend optical model to incorporate control architecture. Mapping between mirror modes and actuator modes

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