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

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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

    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. 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. 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

    18. Metering rod calibration bench

    19. 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. Alignment of spherical mirror relative to CGH

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

    21. Initial testing results

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

    23. Summary 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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