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Use of a commercial laser tracker for optical alignment

Laser Tracker: Optical coordinate measuring machine. Projects a laser beam. Use two-axis gimbals to track the reflection from a corner cubeMeasure 3-space position:Two pointing angles Radial distance ADM (Absolute Distance Measurement)DMI (Distance measuring interferometer)SMR

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Use of a commercial laser tracker for optical alignment

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    1. Use of a commercial laser tracker for optical alignment James H. Burge, Peng Su, Chunyu Zhao, Tom Zobrist College of Optical Sciences Steward Observatory University of Arizona

    2. Laser Tracker: Optical coordinate measuring machine What is a Laser Tracker?

    3. Laser tracker components

    4. Leica Geosystems (Switzerland) FARO (USA) API (USA) Three manufacturers of Laser Trackers

    5. Laser tracker accuracy Assume advertised performance (all values are 2s) Define z as line of sight direction for tracker Uncertainty in position using ADM is

    6. Calibration of laser tracker Distance Measuring Interferometer gives < 0.1 m/m accuracy Typically limited by air temperature (1C gives 1 m/m error.) Tracker repeatability is typically < 1 m/m for all dimensions The tracker can be calibrated for specific measurements using the DMI. Radial : use DMI mode, moving the tracker ball Lateral : use a second tracker in DMI mode So it is possible to get micron level accuracy Need thermal control Control of geometry Careful calibration Average out noise

    7. Special advantages of the laser tracker Can achieve micron accuracy (so can CMM) Portable Measure over very large distances Can use optical tricks Measure through fold mirrors Measure through windows Measure angles

    8. New Solar Telescope

    9. Use tracker to align mirrors in telescope

    10. Measurement of NST secondary mirror

    11. Measurement of angle with tracker

    12. Test of tracker through fold mirrors Use high quality 12 flat mirror. Compare SMR measurements (actual and apparent). Calculate mirror normal Measure mirror surface directly by touching the mirror with the SMRs The two methods agree to within the 1 arcsec stability of the mirror

    13. Measurement of objects 3D orientation Fix 2 mirrors to the object at known angles Determine mirror normal directions using the tracker Determine objects 3D orientation in space

    14. Definition of mirror angles 4 measurements : 2 normals, 2 DoFs each We get no information about rotation about the mirrors normal 3 unknowns (three space orientation) Use least squares fit

    15. Sensitivity vs angle between mirrors

    16. Interferometric testing primary mirror segments for the Giant Magellan Telescope

    17. Reference CGH

    18. Defining CGH orientation in tracker coordinates

    19. Measure mirror normals wrt CGH

    20. Use of laser tracker for system alignment

    21. Using tracker through window

    22. Test of tracker looking through window An SMR was measured directly at ~1 m 1 cm thick window was inserted between the tracker and the SMR The apparent SMR position was measured with the tracker This was corrected for the refraction of the window These tests showed agreement to 20 ppm, which is consistent with the noise levels of this test

    23. Conclusion The laser tracker is great for general purpose metrology It has some special capabilities that make it especially useful for optical alignment Follows the light through fold mirrors Can be calibrated to very high accuracy Can be used for measuring angle as well as position Can be used to measure through a window

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