1. TMT.OPT.PRE.11.046.DRF011 Thirty Meter Telescope Secondary and Tertiary Mirror Systems International Symposium on Photoelectronic Detection and Imaging 2011 International Colloquium on Thirty Meter Telescope Virginia Ford 25 May 2011

2. TMT.OPT.PRE.11.046.DRF012 Secondary Mirror (M2) System M2 and M3 Systems Tertiary Mirror (M3) System Primary Mirror (M1) System

3. TMT.OPT.PRE.11.046.DRF013 M2 System M3 System M2 and M3 Systems M3 Positioner and Control Electronics M3 Cell Assembly M2 Cell Assembly M2 Positioner and Control Electronics

4. TMT.OPT.PRE.11.046.DRF014 M2 and M3 Cell Assemblies: Passive Passive support systems – non-actuated optical surfaces M2 and M3 optical surface errors will be corrected by M1 segments – Low-spatial frequency errors are correctible by M1 Residual fitting errors are within error budget – Image blur from beam footprint motion on M3 is acceptable – Off-axis image shear errors are acceptable SURFACE ERROR CORRECTION nm RMS UNCORRECTED M2 SURFACE 120 CORRECTED WITH M1 SEGMENTS: PISTON, TIP AND TILT (PTT) 11 CORRECTED WITH M1 SEGMENTS: WARPING HARNESS AND PTT 0.8 EXAMPLE: M2 GRAVITY PRINT-THROUGH PATTERN CORRECTED BY M1 SEGMENTS UNCORRECTED M2 M1 SEGMENT PISTON, TIP, TILT ONLY Surface Peak to Valley = 530 nmSurface Peak to Valley = 54 nm

5. TMT.OPT.PRE.11.046.DRF015 M2 and M3 Mirror Substrates Substrate requirements: – Low-speed movements → ultra light weight substrate not required – High thermal stability required Thermally-induced surface distortions should cause spatially smooth errors – Resistant to chemicals used to clean and strip the coating Substrate selected: – Very-low expansion glass or glass ceramic – Smoothly varying, low CTE – Resistant to cleaning and stripping chemicals – Meniscus-style substrate form – gravity print-through creates low spatial frequency, smooth, correctable surface errors – Existing technology – no development, low-cost, low-risk TMT.OPT.TEC.11.067.DRF01

6. TMT.OPT.PRE.11.046.DRF016 M2 and M3 Mirror Polishing Surface Error Requirements Surface error requirements are controlled as a function of spatial frequency – Low-spatial-frequency allowances are generous to ease polishing and metrology Well correctible by M1 segments – Mid-spatial-frequency allowances have tighter limits Partially correctible by M1 segments Ability to correct decreases as spatial frequency increases – High-spatial-frequency allowances are tightly controlled Not correctible by M1 segments or the Adaptive Optics (AO) system

7. TMT.OPT.PRE.11.046.DRF017 M2 and M3 Mirror Polishing State-of-the-Art TMT M2 and M3 mirrors are larger than existing equivalent mirrors that have been produced – Recent advances in polishing techniques and metrology enable their fabrication M2 State of the art: – MMT convex 1.7 m diameter secondary mirror: f/1.3; asphericity 330 μm; radius of curvature 5151mm; conic constant 2.7 – TMT convex 3.0 m diameter secondary mirror: f/1.0; asphericity 885 μm; radius of curvature 6228mm; conic constant 1.3 M3 State of the art: – 2 m diameter flat (University of Arizona – reported SPIE 2007) Metrology: subaperture Fizeau interferometry and scanning pentaprism resulting in 3nm rms measurement uncertainty – TMT: 3.5m x 2.5m ellipse

8. TMT.OPT.PRE.11.046.DRF018 Secondary Mirror Polishing Challenges Since M2 is convex, it cannot be tested from its center- of-curvature – Must fabricate very large test optics or use smaller test optics with subaperture stitching M2 is highly aspheric – Polishing tool fitting challenges Especially at the edge of the mirror, the polishing tool size must be small The tight requirements on high spatial frequency optical surface errors provides challenges to small tool polishing

9. TMT.OPT.PRE.11.046.DRF019 M2 Mirror Polishing Challenges Asphericity causes tool fit issues: – M2 optical surface is extremely non-spherical (for tool misfit < 1 μm) large spherical tool aspherical surface small spherical tool

10. TMT.OPT.PRE.11.046.DRF0110 M3 Polishing Challenges General issues with large flats: – Edge roll-off Created by area of polishing tool that is unsupported at edge of part Elliptical shape issues – Polish mirror as roundel then cut ellipse Must remove stress relief springing errors caused by cutting or – Cut ellipse then polish Asymmetrical forces during polishing cause surface errors 3.594 m 2.536 m

11. TMT.OPT.PRE.11.046.DRF0111 M2 and M3 Metrology Challenges Full aperture acceptance testing requires impractical large test optics or subaperture stitching – M2 possibilities for acceptance test: Fizeau Interferometry Hindle Sphere or Hindle Shell or Aspheric test plate system – M3 possibilities for acceptance test Ritchey-Common test Fizeau Interferometry

12. TMT.OPT.PRE.11.046.DRF0112 M2 and M3 Mirror Cell Assemblies Axial Support Systems Have explored many axial support designs including from 9 to 60-point active and passive – 18-point passive whiffletree supports are sufficient provided some axial print-through can be polished out – 60 point active support: better than required and costly

13. TMT.OPT.PRE.11.046.DRF0113 M2 and M3 Mirror Cell Assemblies Lateral Support Systems Passive lateral whiffletree concept study – one concept – 3 meter diameter, 100mm thick mirror Lateral Support Performance 3 point12 point RMS surface error11.7 nm4.6 nm Peak to Valley surface error583 nm171 nm Gravity direction NELSON & CABAK LOAD DIRECTIONS (3 PLACES)

14. TMT.OPT.PRE.11.046.DRF0114 Support Print-through Polish-out Polishing-out a portion of support gravity print-through improves performance Optimize the portion that is polished out – the bias – wavefront variance through the zenith angle operating range – atmospheric degradation effects With the polish-out portion (bias) optimized for a zenith angle ~30°, surface errors are reduced significantly Bias Implementation: – During the surface polishing of M2 and M3 with use of small tools – After polishing is completed by sending M2 and M3 to a specialized vendor Large facility is needed: – Large enough Ion Beam Finishing (IBF) facility – Large enough Magneto Rheologic Fluid (MRF) facility

15. TMT.OPT.PRE.11.046.DRF0115 M2 18-point axial support performance – polish-out biased or non-biased JERRY NELSON

16. TMT.OPT.PRE.11.046.DRF0116 M2 Positioner Requirements: – Travel Range of Motion: Translation: ±15 mm Rotation: ±413 arcsec – Positioning accuracy: Translation: 3 μm Rotation: 0.1 arcsec – Jitter: Translation: 1 μm RMS Rotation: 0.05 arcsec RMS Similar performance to: – Subaru Prime Focus Unit Hexapod – Blanco DECam Hexapod MOVING PLATFORM HEXAPOD ARM FIXED PLATFORM POSITIONER CONTROL SYSTEM

17. TMT.OPT.PRE.11.046.DRF0117 M3 Positioner Tilt Axis (Φ) Tilt Cradle Tilt Actuator Counter- balance weights Rotation Cradle Rotation Bearing Cable Wrap Not shown: Positioner Control Electronics Rotation Axis (θ)

18. 18 M3 Positioner Requirements: – Positioning accuracy: Rotation: 0.2 arcsec Tilt: 0.2 arcsec – Jitter: Rotation: 0.1 arcsec rms Tilt: 0.1 arcsec rms – Motion of M3 during tracking is a function of instrument location and zenith angle: Graphs show instrument locations on +X Nasmyth platform only Instrument locations

19. Summary TMT M2 and M3 Systems are challenging but can be fabricated Conceptual designs have been developed that meet requirements Polishing and metrology are challenging but several facilities in the world are capable of meeting these challenges TMT.OPT.PRE.11.046.DRF0119

20. TMT.OPT.PRE.11.046.DRF0120 Acknowledgements The TMT Project gratefully acknowledges the support of the TMT partner institutions. They are the Association of Canadian Universities for Research in Astronomy (ACURA), the California Institute of Technology and the University of California. This work was supported as well by the Gordon and Betty Moore Foundation, the Canada Foundation for Innovation, the Ontario Ministry of Research and Innovation, the National Research Council of Canada, the Natural Sciences and Engineering Research Council of Canada, the British Columbia Knowledge Development Fund, the Association of Universities for Research in Astronomy (AURA) and the U.S. National Science Foundation.