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

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Presentation on theme: "Evaluation of Astrometry Errors due to the Optical Surface Distortions in Adaptive Optics Systems and Science Instruments Brent Ellerbroek a, Glen Herriot."— Presentation transcript:

1 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 a TMT Observatory Corporation, b NRC Herzberg, c National Astronomical Observatory 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 1

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 2

3 Astrometry Requirements for TMT [REQ-1-ORD-3650] NFIRAOS shall enable precise differential astrometry measurements, –where one-dimensional time-dependent rms astrometric 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 3

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 4 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… 5TMT.AOS.PRE.13.087.REL01 AO4ELT3, Florence, 05/28/13

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

7 Modeling assumptions 7 Optical surface errorsRandom, with shift-invariant statistics  Defined by PSDs Induced wavefront errorsLinear sum of contributions from “phase screens” at each surface Resulting image distortionRMS best-fit tilt to exit pupil wavefront Random boresight errorsNormally distributed in 2 dimensions Intentional field-of-view offset or rotation Linear displacement of optical path through some or all optical surfaces Distortion calibration by reference sources/stars Measures image distortion map up to the Nyquist rate defined by reference source spacing Distortion calibration by field stars Removes low-order (polynomial) modes of image distortion map Other simplifying assumptions Circular, unobscured aperture Circular field-of-view Circularly symmetric error PSDs 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 8 Domain of aliasing Translation filter Error PSD Tip/Tilt filter Low-order Mode removal filter TMT.AOS.PRE.13.087.REL01 AO4ELT3, Florence, 05/28/13

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

10 TMT+NFIRAOS Optical Layout 10 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 Total76.5 nm RMS Win 1 (-191.1 km)30.9 Win 2 (-198.2 km)31.0 OAP1 (49.2 km)25.1 OAP2 (-36.6 km)24.6 OAP3 (35.2 km)24.9 Sci B/S (-9.7 km)27.5 OAP4 (-43.6 km)24.3 Inst. Fold (-85.2 k m)27.1 TMT.AOS.PRE.11.123.REL01 AO4ELT, Victoria, September 26 2011 11 Uniform tolerance of 25 nm RMS assumed for initial astrometry budgeting 9.0 5.5 Input window specifications tightened as needed to achieve astrometry budget

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

13 IRIS Optical Surface Specifications #Surf. Name DA,mDA,m h, km PSD pwr , nm #Surf. Name DA,mDA,m h, km PSD pwr , nm 1Win f0.1028902.302.813ADC 1f0.10-19.82.123.2 2Win b0.1025202.292.714ADC 1c0.10-21.52.120.7 3Col 1f0.1212902.232.615ADC 1b0.10-22.82.124 4Col 1b0.1211902.222.616ADC 2f0.10-25.72.124 5Fold 10.128952.2110.617ADC 2c0.10-272.120.7 6Fold 20.121562.159.618ADC 2b0.10-28.72.123.2 7Col 2f0.1263.52.146.919Filt. F0.08-61.52.122.0 8Col 2b0.12612.146.920Filt. B0.08-63.52.122.0 9Col 3f0.1250.82.132.121Cam 10.16-1712.149.7 10Col 3b0.1247.32.132.122Cam 20.16-3702.1610.0 11 Col 4f0.1245.32.132.223Cam 30.24-6202.1710.5 12Col 4b0.1240.22.132.2 13 TMT.AOS.PRE.13.087.REL01 AO4ELT3, Florence, 05/28/13

14 Distortion Calibration Errors vs. Reference Source Spacing All optics except NFIRAOS Windows Spacing of 0.7 arc sec yields 5  arc sec error NFIRAOS windows, original specs Errors too large! NFIRAOS windows, revised specs Spacing of 5 arc sec yields 6  arc sec error 14 Source Spacing arc sec 0.1 10.01.0 Calibration Error,  arc sec 0.1 1.0 10.0 100.0 TMT.AOS.PRE.13.087.REL01 AO4ELT3, Florence, 05/28/13

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

16 Differential Image Distortion Due to Iris/NFIRAOS Rotation Fourier model approximates field rotation by a global translation: 16 Line-of-sight shift in NFIRAOS, arc sec 0.1 10.01.0 Field-averaged differential distortion,  arc sec 1.0 100 10 1000 Red: Original window tolerances Blue: Revised tolerances Solid: Global tip/tilt calibration using reference stars Dashed: Plate scale calibration TMT.AOS.PRE.13.087.REL01 AO4ELT3, Florence, 05/28/13

17 Sensitivities for Quasi-Static DM Figure Errors on DM11.2 in NFIRAOS D=30m 30” FoV 11.2 km DM conjugate range 0.5m actuator pitch Max. sensitivity of ~2.5  as/nm with global tip/tilt calibration ~0.15 with plate scale calibration 17 Spatial frequency, cycles/m 1.0 0.1 0.01 0.001 1.0 10.0 0.1 0.01 Sensitivity,  as/nm 0.001 0.0001 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  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. 18 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. 19 TMT.AOS.PRE.13.087.REL01 AO4ELT3, Florence, 05/28/13

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