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Testing an off-axis parabolic mirror with a CGH and a spherical mirror as null lens

Testing an off-axis parabolic mirror with a CGH and a spherical mirror as null lens. Chunyu Zhao Rene Zehnder Jim Burge Buddy Martin College of Optical Sciences, University of Arizona. Outline. The mirror to be tested Testing system design and assembly Optical alignment

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Testing an off-axis parabolic mirror with a CGH and a spherical mirror as null lens

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  1. Testing an off-axis parabolic mirror with a CGH and a spherical mirror as null lens Chunyu Zhao Rene Zehnder Jim Burge Buddy Martin College of Optical Sciences, University of Arizona

  2. Outline • The mirror to be tested • Testing system design and assembly • Optical alignment • Initial testing result • Summary

  3. The mirror • Off-axis parabolic mirror with 1.6m diameter of clear aperture • Parent: f/0.7 parabola with 7.7m ROC. • Offset from the parent vertex: 1.84m

  4. Surface profile • P-V: 2.767mm • RMS: 508um • RMS asti: 497um • RMS coma: 108um • RMS trifoil: 9um • RMS spherical: 5.8um • Residue: 2.5um

  5. Surface quality spec • Lower bending mode can be subtracted by the following amount: • Astigmatism: 200nm • Coma: 17nm • Trefoil: 50nm • Quatrefoil: 20nm • Spherical: 25nm • Residual: • 40nm rms

  6. Requirement for testing system • Amount of lower order mode: • Astigmatism: 170nm • Coma: 15nm • Trefoil: 42nm • Quatrefoil: 17nm • Spherical: 20nm • Residual: • 20nm rms

  7. System Configuration • A spherical mirror removes most of astigmatism and some coma – residual 0 astigmatism 22um and coma 46um • A CGH removes rest of the aberrations.

  8. Lens + CGH + Spherical Mirror

  9. Error Budget

  10. Residual wavefront errors • Total allowed: 40nm • Testing system budget: 20nm • Errors in lens, CGH and spherical mirror will be backed out

  11. Optical Bench and Testing Tower

  12. NST testing system System assembled and aligned in lab System mounted and aligned in test tower (looking up)

  13. Alignment • Using CGH patterns to align the CGH itself • Using CGH patterns and metering rods to align the spherical mirror • Additional CGH patterns to create cross hairs for position the test mirror

  14. 3-segment CGH creating crosshair Main CGH Substrate alignment CGH 4 CGHs creating beams for spherical mirror alignment CGH creating clocking line The CGHs • 10 segments create 8 wavefronts. • Main CGH creates the testing wavefront. • The ring type CGH aligns the substrate to the interferometer. • 3 segments create a crosshair and 1 segment creates a clocking line to align the NST mirror to the test optics. • 4 circular CGHs send beams to align the lateral positions of the 4 balls mounted on the surface of the fold sphere.

  15. Alignment of CGH With the reflection fringes from the alignment CGH controlled to 0.5 in power, the CGH substrate is aligned within 7μm.

  16. Ball at mirror CGH Metering rod w/ LVDTs Spherical mirror Lens Ball at focus Alignment of spherical mirror • Position a ball at focus of the 0th order diffraction beam after CGH, use it as a reference to position the spherical mirror. • Put a few balls at the mirror surface, patches of CGH direct spherical beams toward the ball and the reflection fringes are used to position the balls accurately in lateral direction, and metering rod with LVDTs are used to control the distance from these balls to the ball at focus. The mirror is adjusted so that all the balls are at proper position.

  17. Metering rod calibration • The metering rods are made of low CTE carbon fiber tubes with invar tips glued on both ends • Metering rods are calibrated between two balls separated by known distance. • Since the tip of the rod has proper curvature slight misalignment does not alter calibration.

  18. Metering rod calibration bench • The calibration bench serves as master reference for the metering rods. • The distance between the balls was measured by a laser tracker. • In order to minimize measurement errors the tracker was aligned to direction of motion. • The calibration bench is made of ULE and mechanically mounted to minimize environmental influence.

  19. Alignment of spherical mirror relative to CGH • Spherical mirror is aligned with metering rods. • 4 balls mounted on the mirror surface. Their lateral positions are controlled with beams from the CGHs. Small stages position the balls laterally to give retroreflection. • Initial alignment scheme based on reflected wavefront did not work. New scheme based on reflected image is being implemented. • 1 ball mounted on the 0th order beam focus. Its position is controlled by nulling the reflection fringes from its surface. • Metering rods lengths are calibrated to 3 μm. Reflection fringes From the ball @focus->

  20. Aligning the test mirror: projected crosshair and clocking line • A crosshair and a clocking line are generated by CGHs to align the NST mirror to the test optics.

  21. Initial testing results

  22. Morphed surface map • P-V: 8.6 wave • RMS: 1.5 wave

  23. Summary GMT Telescope • We have built a system for interferometrically testing an off-axis parabolic mirror • A CGH and a spherical mirror is used as null lens • Initial testing results are encouraging • Experience and know-how acquired will be applied to testing GMT mirrors which are 5x scale off-axis parabolas

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