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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 College of Optical Sciences The University of Arizona Outline The mirror to be tested Testing system design and assembly Optical alignment Initial testing result Summary College of Optical Sciences The University of Arizona 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 College of Optical Sciences The University of Arizona Surface profile College of Optical Sciences The University of Arizona P-V: 2.767mm RMS: 508um RMS asti: 497um RMS coma: 108um RMS trifoil: 9um RMS spherical: 5.8um Residue: 2.5um 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 College of Optical Sciences The University of Arizona Requirement for testing system Amount of lower order mode: Astigmatism: 170nm Coma: 15nm Trefoil: 42nm Quatrefoil: 17nm Spherical: 20nm Residual: 20nm rms College of Optical Sciences The University of Arizona 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. College of Optical Sciences The University of Arizona Lens + CGH + Spherical Mirror College of Optical Sciences The University of Arizona Error Budget in n m CGH dz tilt x Spherical mirror tilt y 7@ Edge dz tilt x ROC Therma l tilt y Actuator RSS 7 7@ Edge 7@ Edge 20 1deg C Amount Correctable By 15 N Forces Needed (N) um 7 7@ Edge Z5 0.2 0.0 -5.6 0.0 0.0 -37.6 0.0 0.0 38.1 346.0 1.6 Z6 133.9 -7.0 1.6 93.0 -149.5 3.1 63.0 206.5 309 346.0 13.4 Z9 -16.1 1.7 -0.3 -4.3 19.1 -0.5 -3.0 -12.2 28.4 87.0 4.9 Z10 0.0 0.0 -1.8 0.0 0.0 -15.0 0.0 0.0 15.1 87.0 2.6 Z11 5.9 1.2 0.0 1.9 -4.2 0.0 1.2 4.7 9.0 43.0 3.1 Z14 1.1 3.1 0.0 0.0 -0.6 0.0 -0.1 0.2 3.3 35.0 1.4 Z15 0.0 0.0 2.8 0.0 0.0 -0.5 0.0 0.0 2.9 35.0 1.2 RSS College of Optical Sciences The University of Arizona 15.0 Residual wavefront errors Total allowed: 40nm Testing system budget: 20nm Errors in lens, CGH and spherical mirror will be backed out in nm CGH dz tilt x Spherical mirror tilt y dz tilt x ROC Thermal tilt y RSS um/md eg 7 7 7 7 1.4 1.4 20 1degree RMS Fit error 2.0 2.6 1.7 1.2 2.6 2.5 0.7 3.5 6.4 Z12 -6.3 -2.0 -0.1 -0.9 4.4 -0.2 -0.6 -3.2 8.6 Z13 0.0 0.0 -0.5 0.0 0.0 3.2 0.0 0.0 3.2 RSS subtotal Lens+CGH 8 Mirror 8 RSS Total College of Optical Sciences The University of Arizona 11.2 15.9 Optical Bench and Testing Tower College of Optical Sciences The University of Arizona NST testing system System assembled and aligned in lab College of Optical Sciences The University of Arizona System mounted and aligned in test tower (looking up) 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 College of Optical Sciences The University of Arizona The CGHs 3-segment CGH creating crosshair Main CGH Substrate alignment CGH 4 CGHs creating beams for spherical mirror alignment CGH creating clocking line College of Optical Sciences The University of Arizona 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. Alignment of CGH With the reflection fringes from the alignment CGH controlled to 0.5 in power, the CGH substrate is aligned within 7μm. College of Optical Sciences The University of Arizona Alignment of spherical mirror Ball at mirror CGH Metering rod w/ LVDTs Lens Ball at focus 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. College of Optical Sciences The University of Arizona 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. College of Optical Sciences The University of Arizona 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. College of Optical Sciences The University of Arizona 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. College of Optical Sciences The University of Arizona Reflection fringes From the ball @focus-> Aligning the test mirror: projected crosshair and clocking line A crosshair and a clocking line are generated by CGHs to align the Usage of crosshair and NST mirror to the test optics. Line Line to get clocking Crosshair to get x-yposition Intensities plotted in logscale College of Optical Sciences The University of Arizona Initial testing results College of Optical Sciences The University of Arizona Morphed surface map College of Optical Sciences The University of Arizona P-V: 8.6 wave RMS: 1.5 wave 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 knowhow acquired will be applied to testing GMT mirrors which are 5x scale off-axis parabolas College of Optical Sciences The University of Arizona GMT Telescope