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Brent Ellerbroek a , Glen Herriot b , Ryuji Suzuki c , and Matthias Schoeck a

Evaluation of Astrometry Errors due to the Optical Surface Distortions in Adaptive Optics Systems and Science Instruments. Brent Ellerbroek a , Glen Herriot b , Ryuji Suzuki c , and Matthias Schoeck a

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Brent Ellerbroek a , Glen Herriot b , Ryuji Suzuki c , and Matthias Schoeck a

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  1. Evaluation of Astrometry Errorsdue to theOptical Surface DistortionsinAdaptive Optics Systems and Science Instruments Brent Ellerbroeka, Glen Herriotb, Ryuji Suzukic, and Matthias Schoecka aTMT Observatory Corporation, bNRC Herzberg, cNational AstronomicalObservatory of Japan Adaptive Optics for Extremely Large Telescopes 3 Florence, Italy May 28, 2013 TMT.AOS.PRE.13.087.REL01 AO4ELT3, Florence, 05/28/13

  2. Presentation Outline • Current astrometry requirements and error budget for TMT • Objectives of this exercise • Simplified model for astrometric observations • Simplified modeling assumptions • Summary of analytical results • Application to NFIRAOS+IRIS for TMT • Summary and future plans TMT.AOS.PRE.13.087.REL01 AO4ELT3, Florence, 05/28/13

  3. Astrometry Requirements for TMT • [REQ-1-ORD-3650] NFIRAOS shall enable precise differential astrometry measurements, • where one-dimensional time-dependent rmsastrometric positional uncertainties, after fitting distortion measured with field stars, and over a 30 arcsecond field of view • shall be no larger than 50 µ-arcseconds in the H band for a 100s integration time. • Errors should fall as t-1/2. • Systematic one-dimensional rms position uncertainties shall be no more than 10 µas. • [REQ-1-ORD-3652] The AO system should provide sufficient calibration information to not degrade the astrometric capabilities beyond the limits set by the atmosphere. TMT.AOS.PRE.13.087.REL01 AO4ELT3, Florence, 05/28/13

  4. Astrometry Error Budget Organization for TMT • Reference catalogue errors • Atmospheric refraction correction • Other atmospheric effects • Opto-mechanical errors: • Telescope optics calibration • Guide probe position • Imager calibration • Optical surface calibration • Rotator calibration • Quasi-static errors • Stuck actuators, diffraction spikes • Vibrations • Coupling with other effects • Focal plane errors • More than 30 (currently 34) terms grouped in 5 categories • Organization derived in part from MICADO budget • Values of many terms are scenario-dependent • Many terms remain work in progress Driven by optical surface errors in IRIS and NFIRAOS TMT.AOS.PRE.13.087.REL01 AO4ELT3, Florence, 05/28/13

  5. Objectives of this Exercise • Develop engineering formulas for estimating astrometry errors due to optical surface errors in instruments and AO systems • Intended as a practical tool to support development of error budgets and optical surface specifications • Apply to current optical designs and surface specifications for IRIS + NFIRAOS to confirm that TMT astrometry requirements can be met • Begin to iterate designs and surface specifications as necessary… TMT.AOS.PRE.13.087.REL01 AO4ELT3, Florence, 05/28/13

  6. Observing Sequence Model Quasi-static error #2 Boresight error #2 Field rotation or dither Quasi-static error #1 Boresight error #1 Distortion Calibration w/ Stars Distortion Calibration w/ Ref. Grid Science Exposure #1 Science Exposure #2 + + + Distortion map, pre-focal optics Distortion map, post-focal optics S S S - - - + + S S - - Position Measurement #1 Position Measurement #2 Calibration by Field Stars Differential Position Measurement TMT.AOS.PRE.13.087.REL01 AO4ELT3, Florence, 05/28/13

  7. Modeling assumptions TMT.AOS.PRE.13.087.REL01 AO4ELT3, Florence, 05/28/13

  8. Formulation of Results • Mean-square error a sum of contributions from each surface: • For quasi-static errors • For dither/rotation errors (random boresight errors similar) • For calibration errors Tip/Tilt filter Low-order Mode removal filter Error PSD Translation filter TMT.AOS.PRE.13.087.REL01 AO4ELT3, Florence, 05/28/13 Domain of aliasing

  9. Optical Train Schematic for NFIRAOS (Facility AO System) + IRIS (Near IR Imager/Spectrograph) TMT Distortion Calibration via Stars NFIRAOS Windows (Reference Source Grid) NFIRAOS Optics Rotates in IRIS Coordinate System (Rotation Bearing) Distortion Calibration via Reference Sources IRIS Window IRIS Optics Focal Plane TMT.AOS.PRE.13.087.REL01 AO4ELT3, Florence, 05/28/13

  10. TMT+NFIRAOS Optical Layout 2-5: Input windows 1,16: Input/output focus 7,9,10,14: OAPs 8,11: DMs 12: Science beamsplitter 15: Instrument selection fold TMT.AOS.PRE.13.087.REL01 AO4ELT3, Florence, 05/28/13

  11. (Initial) NFIRAOS Optical Surface Specifications • Developed based upon overall NFIRAOS wavefront error budgets • Science field • Off-axis guidestars • Compensation by NFIRAOS deformable mirrors permitted • Values specify transmitted wavefront errors over surface clear aperture with tip/tilt/focus removed • Power law error PSD specified • p=-2.5 9.0 5.5 Uniform tolerance of 25 nm RMS assumed for initial astrometry budgeting Input window specifications tightened as needed to achieve astrometry budget TMT.AOS.PRE.11.123.REL01 AO4ELT, Victoria, September 26 2011

  12. IRIS Imaging Channel (ApT Collimator + TMA Camera) Fold mirror Entrance window Detector Camera TMA Pupil Collimator lens NFIRAOS focal plane Filter ADC TMT.AOS.PRE.13.087.REL01 AO4ELT3, Florence, 05/28/13 Fold mirror Collimator lenses

  13. IRIS Optical Surface Specifications TMT.AOS.PRE.13.087.REL01 AO4ELT3, Florence, 05/28/13

  14. Distortion Calibration Errors vs. Reference Source Spacing 100.0 All optics except NFIRAOS Windows Spacing of 0.7 arc sec yields 5 m arc sec error NFIRAOS windows, original specs Errors too large! NFIRAOS windows, revised specs Spacing of 5 arc sec yields 6 m arc sec error 10.0 Calibration Error, m arc sec 1.0 0.1 1.0 10.0 0.1 Source Spacing arc sec TMT.AOS.PRE.13.087.REL01 AO4ELT3, Florence, 05/28/13

  15. Distortion Calibration Error Contribution by Surface NFIRAOS windows, original specs NFIRAOS windows, revised specs Camera mirrors 2 and 3 Fold mirror 1 IRIS input window Collimator lens 1 Calibration Error, m arc sec Surface number in optical train TMT.AOS.PRE.13.087.REL01 AO4ELT3, Florence, 05/28/13

  16. Differential Image Distortion Due to Iris/NFIRAOS Rotation 1000 Fourier model approximates field rotation by a global translation: Red: Original window tolerances Blue: Revised tolerances Solid: Global tip/tilt calibration using reference stars Dashed: Plate scale calibration 100 Field-averaged differential distortion, m arc sec 10 1.0 1.0 10.0 0.1 Line-of-sight shift in NFIRAOS, arc sec TMT.AOS.PRE.13.087.REL01 AO4ELT3, Florence, 05/28/13

  17. Sensitivities for Quasi-Static DM Figure Errors on DM11.2 in NFIRAOS 10.0 • D=30m • 30” FoV • 11.2 km DM conjugate range • 0.5m actuator pitch • Max. sensitivity of ~2.5 mas/nm with global tip/tilt calibration • ~0.15 with plate scale calibration 1.0 0.1 Sensitivity, mas/nm 0.01 0.001 0.0001 1.0 0.01 0.001 0.1 Spatial frequency, cycles/m TMT.AOS.PRE.13.087.REL01 AO4ELT3, Florence, 05/28/13

  18. Summary • A Fourier model for evaluating the magnitude of image distortions due to optical surface errors has been applied to develop astrometry error budget terms for observations with IRIS+NFIRAOS on TMT • Results are preliminary, but confirm intuition and are encouraging: • Calibration of static distortions to 5-7 m arc sec is feasible, but… • Tolerances on surfaces near focal planes are tight • Even so, many references sources are needed • IRIS optical design may be iterated to adjust surface conjugates • As with K-mirrors, a consistent image rotator angle must be used for repeated observations of the same field • Image distortion due to quasi-static errors on DM11.2 are small, provided that overall plate scale changes can be calibrated using reference stars • Further modeling/simulation is planned to using more realistic models of static errors, calibration procedures, quasistatic errors, etc. TMT.AOS.PRE.13.087.REL01 AO4ELT3, Florence, 05/28/13

  19. Acknowledgements • The TMT Project gratefully acknowledges the support of the TMT partner institutions. • They are • the Association of Canadian Universities for Research in Astronomy (ACURA), • the California Institute of Technology • the University of California • the National Astronomical Observatory of Japan • the National Astronomical Observatories and their consortium partners • And the Department of Science and Technology of India and their supported institutes. • This work was supported as well by • the Gordon and Betty Moore Foundation • the Canada Foundation for Innovation • the Ontario Ministry of Research and Innovation • the National Research Council of Canada • the Natural Sciences and Engineering Research Council of Canada • the British Columbia Knowledge Development Fund • the Association of Universities for Research in Astronomy (AURA) • and the U.S. National Science Foundation. TMT.AOS.PRE.13.087.REL01 AO4ELT3, Florence, 05/28/13

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