Optomechanical Technologies for Astronomy, SPIE 6273 (2006)1 Manufacture of a 1.7 m prototype of the GMT primary mirror segments Buddy Martin a, Jim Burge a,b, Steve Miller a, Bryan Smith a, Rene Zehnder b, Chunyu Zhao b a Steward Observatory, University of Arizona b College of Optical Sciences, University of Arizona

Optomechanical Technologies for Astronomy, SPIE 6273 (2006)2 Outline 1.Giant Magellan Telescope and New Solar Telescope 2.Polishing of 1.7 m NST primary mirror 3.Measurement of NST mirror 4.Status of NST mirror

Optomechanical Technologies for Astronomy, SPIE 6273 (2006)3 Giant Magellan Telescope Seven circular 8.4 m segments 22 m collecting area resolution of a 24 m telescope f/0.7 primary mirror (18 m focal length) Allows compact, stiff structure Lightweight truss supports adaptive Gregorian secondary Segments are honeycomb sandwich mirrors. ~same dimensions as LBT primaries Six outer segments are ~off-axis paraboloids. 14 mm p-v aspheric departure This manufacturing challenge is addressed by design, analysis and manufacture of prototype.

Optomechanical Technologies for Astronomy, SPIE 6273 (2006)4 UA heritage for polishing large mirrors GMT builds on University of Arizona experience making 8-m class mirrors. We have polished hundreds of m 2 at ~f/1. Polishing GMT introduces new difficulties: Efficient polishing and testing of steep off-axis asphere Optical testing for such long focal lengths (36-m ROC) We have addressed these issues directly with the 1/5 scale prototype 6.5-m MMT Twin 6.5-m Magellan telescopes LBT, combining two 8.4-m telescopes

Optomechanical Technologies for Astronomy, SPIE 6273 (2006)5 New Solar Telescope at Big Bear Solar Observatory 1.7 m off-axis telescope f/0.7 parent (3.85 m focal length) Mirror is 100 mm thick Zerodur 2.7 mm aspheric departure approximately 1/5 scale model of GMT segment mirrordiameter focal length off-axis distance parent focal ratio NST primary 1.7 m3.85 m1.84 m0.7 GMT segment 8.4 m18 m8.71 m0.7

Optomechanical Technologies for Astronomy, SPIE 6273 (2006)6 Approach to manufacturing NST mirror Methods we’re using on NST mirror are same as those we’re developing for GMT segments. Polish with stressed-lap system. Full-aperture interferometric test with hybrid reflective-diffractive null corrector. Error analysis includes active control of alignment and low order bending modes in the telescope. Use laser tracker to measure mirror figure in early stages and to control alignment of optical test. We now have high confidence in general approach, and we gained experience that benefits GMT Prototype systems for polishing and measuring were successfully implemented. We’ve identified many details we can improve before building these systems for GMT segments.

Optomechanical Technologies for Astronomy, SPIE 6273 (2006)7 NST Mirror support Passive support for polishing and testing matches active support used in telescope. Tolerance analysis for optical test assumes active control of low-order aberrations.

Optomechanical Technologies for Astronomy, SPIE 6273 (2006)8 Polishing system for NST mirror 30 cm stressed lap shown polishing the 1.7 m off-axis mirror. The lap bends actively to follow the varying curvature of the aspheric surface. The lap is stiff enough to smooth out small-scale structure as if the mirror were spherical. 1.2 m stressed lap shown polishing an 8.4 m LBT primary mirror. Same system will be used for GMT off-axis segments. Stressed-lap polishing of an off-axis segment or mirror requires only minor changes in software.

Optomechanical Technologies for Astronomy, SPIE 6273 (2006)9 Stressed-lap polishing of off-axis surface Stressed lap bends actively to follow curvature variations of any aspheric surface. Works for off-axis as well as symmetric aspheres. Performance depends only on amount of bending required. Mild curvature variation of off-axis mirrors (mostly astigmatism) means lap sees only small fraction of mirror’s asphericity. Asphericity of 1.7 m off-axis mirror (nm) Small circles represent 0.3 m lap. Bending of 0.3 m lap (nm) Only ~1/10 of mirror’s asphericity

Optomechanical Technologies for Astronomy, SPIE 6273 (2006)10 10 cm CGH 50 cm spherical mirror 4D interferometer 1.7 m primary mirror Off-axis piece of f/0.7 parent 15 cm lens Optical test is similar to GMT test. Aspheric test wavefront is produced by oblique reflection off spherical mirror and diffraction by hologram. Alignment tolerances are ~10 μm. Optical test for NST mirror

Optomechanical Technologies for Astronomy, SPIE 6273 (2006)11 Null CGH for test of mirror with ~20 µm pitch Design showing every 350 th line Define positions of tooling balls at spherical mirror Provide references for aligning the NST mirror Pattern for aligning CGH to interferometer CGH for optical test Actual CGH

Optomechanical Technologies for Astronomy, SPIE 6273 (2006)12 Alignment of null corrector Alignment of interferometer, CGH and spherical mirror are critical: null corrector induces 2.7 mm asphericity to accuracy of < 1 µm. Ball at focus of un-diffracted beam serves as reference to position spherical mirror. CGH focuses light on balls at mirror surface. Reflected wavefront is used to position balls in lateral direction. Metering rods with LVDTs are used to control distance from these balls to ball at focus. See poster by Zehnder et al., “Use of computer-generated holograms for alignment of complex null correctors”, in this conference Tuesday 30 May, 6:00. Ball at focus Ball at mirror CGH Metering rod w/ LVDTs Lens Spherical mirror

Optomechanical Technologies for Astronomy, SPIE 6273 (2006)13 Surface measurements with laser tracker Initial measurements of rough surface are made with laser tracker used as 2-D optical profilometer. Tracker combines distance-measuring interferometer with angular encoders to measure points in 3 D. Special calibration of tracker, and use of real- time references for alignment and low order seeing, will give even better accuracy for GMT measurements. See poster by Burge et al., “Alternate surface measurements for GMT primary mirror segments”, 6:00 in this conference Tuesday, May m mirror on polishing machine

Optomechanical Technologies for Astronomy, SPIE 6273 (2006)14 Laser tracker measurements of ground surface Tracker is mounted above mirror. Retroreflector is scanned over mirror surface. Tracker has sub-μm accuracy in radial direction. With tracker located near COC, surface measurement is not very sensitive to angular errors. Accuracy requirement is to guide figuring within range of optical test. Tracker mounted over a 1 m mirror

Optomechanical Technologies for Astronomy, SPIE 6273 (2006)15 Tracker measurements of NST mirror Laser tracker measurements of NST mirror surface near beginning of loose-abrasive grinding and near end. Plots show departure from ideal off-axis paraboloid, with only piston, tip and tilt removed. Power is measured and controlled. Initial surface 31 March μm rms surface mm After loose abrasive grinding 22 April μm rms surface

Optomechanical Technologies for Astronomy, SPIE 6273 (2006)16 Initial measurements with optical interferometer Tracker map is a 6 th - degree polynomial fit to tracker data. Ignoring alignment aberrations, tracker and optical measurements agree to 0.5 μm rms. ( Focus, coma and astigmatism depend strongly on alignment of mirror in the optical test, were removed from both maps.) Note excellent fringe contrast, and severe image distortion, in optical measurement. optical measurement: 630 nm rmstracker measurement: 590 nm rms nm surface tracker - optical: 520 nm rms

Optomechanical Technologies for Astronomy, SPIE 6273 (2006)17 Optimal use of active optics for off-axis mirrors Coupling of bending modes and alignment aberrations for off axis mirror. Coma and astigmatism can be eliminated by combination of alignment and forces But out of infinite number of combinations that do this, only one is optimum in terms of minimizing correction forces and residual error. Low-order aberrations have very different stiffness. For a given magnitude of aberration, coma requires 14 x greater correction force than astigmatism. We have developed methodology of optimizing alignment and bending that minimizes the required force For 1 mm radial shift, wavefront gets 0.96 µm rms astigmatism µm rms coma

Optomechanical Technologies for Astronomy, SPIE 6273 (2006)18 Tolerance analysis for optical test: Method Tolerance analysis for alignment follows same method used for GMT test. Simulates procedure that will be used when mirror is installed in telescope: 1.Mirror will be aligned to optimize aberrations as measured using star light. 2.Low-order aberrations will be corrected by mirror’s active support system. Tolerance analysis simulates this procedure as follows. For each alignment parameter (e. g., tilt of 0.5 m spherical mirror): 1.Perturb parameter, causing wavefront error that would be imprinted on NST mirror, including power, astigmatism, and coma. 2.Adjust alignment of mirror in telescope (in ray-trace program) to optimize, given this wavefront error. 3.Optimize support forces to minimize remaining wavefront error. Keep track of displacement of mirror from nominal, correction forces, and residual wavefront error.

Optomechanical Technologies for Astronomy, SPIE 6273 (2006)19 Tolerance analysis for optical test: Results Componentparameter tolerance radial shift compensation clocking compensation rms correction force rms wavefront error μmmmmradNnm hologram (100 mm diameter) axial position tilt in x tilt in y spherical mirror (500 mm diameter) axial position tilt in x tilt in y radius of curvature temperature1 K net change (sum in quadrature) Correction forces are small fraction of nominal load (160 N at zenith).

Optomechanical Technologies for Astronomy, SPIE 6273 (2006)20 Current status of NST mirror Optical test is now aligned, appears to exceeds specifications. Continued corroborative metrology is underway. Polishing was stopped in December, ready to continue. Figure as of December 2005: 32 nm rms surface error over 1.6 m clear aperture. Several low-order aberrations that will be corrected by active supports have been subtracted. nm surface

Optomechanical Technologies for Astronomy, SPIE 6273 (2006)21 Summary We are figuring the 1.7 m off-axis primary mirror for the New Solar Telescope as part of the technology development for manufacture of the 8.4 m off-axis segments of the GMT. Key developments are:. stressed-lap polishing off-axis surface interferometric measurement with a reflective-diffractive null corrector alignment techniques that involve the hologram, metering rods and the laser tracker optimal coupling of alignment and bending in the active optics accurate measurement of the ground surface with a laser tracker The NST mirror is near completion.