Prototype Development for the GMT FSM Secondary - Off-axis Aspheric Mirror Fabrication -

  • cc icon
  • ABSTRACT

    A prototype of the GMT FSM has been developed to acquire and to enhance the key technology – mirror fabrication and tip-tilt actuation. The ellipsoidal off-axis mirror has been designed, analyzed, and fabricated from light-weighting to grinding, polishing, and figuring of the mirror surface. The mirror was tested by using an interferometer together with CGHs, which revealed the surface error of 13.7 nm rms in the diameter of 1030 mm. The SCOTS test was employed to independently validate the test results. It measured the surface error to be 17.4 nm rms in the diameter of 1010 mm. Both tests show the optical surface of the FSMP mirror within the required value of 20 nm rms surface error.


  • KEYWORD

    GMT , secondary , mirror , off-axis , aspheric , fabrication

  • 1. Ahn KB, Kim YS, Lee S, Park K, Kyeong J 2011 Sensitivity analysis of test methods for aspheric off-axis mirrors [Advances in Space Research] Vol.47 P.1905-1911 google doi
  • 2. Biasi R, Gallieni D, Salinari P, Riccardi A, Mantegazza P 2010 Contactless thin adaptive mirror technology: past, present and future [Proc. of SPIE] P.7736-7781 google doi
  • 3. Esposito S, Riccardi A, Pinna E, Puglisi A, Quiros-Pacheco F 2011 Large Binocular Telescope Adaptive Optics System: New achievements and perspectives in adaptive optics [Proc. of SPIE 8149 Astronomical Adaptive Optics Systems and Applications IV] google doi
  • 4. 2006 Giant Magellan Telescope Conceptual Design Review google
  • 5. 2014 Giant Magellan Telescope System Level Preliminary Design Review google
  • 6. Huang R, Su P, Horne T, Zappellini GB, Burge JH 2013 Measurement of a large deformable aspherical mirror using SCOTS (Software Configurable Optical Test System) [Proc. of SPIE] Vol.8838 google doi
  • 7. Huang R, Kim D, Su P 2014 FSMP SCOTS Test Report, Contract No. G201300048 google
  • 8. 2010 K-GMT FSMP PDR (Preliminary Design Review), P641_KA_Review_100506v1.0 google
  • 9. 2012 Korean Giant Magellan Telescope Project, Annual Report P.14-17 google
  • 10. 2012 K-GMT FSMP TPR (Test Progress Review), P641_KA_Review_121112v1.0 google
  • 11. 2013 K-GMT FSMP FTR (Final Test Review), P641_KA_Review_130717v1.0 google
  • 12. Kim HS, Lee D-C, Lee K-D, Kim Y-S 2014 Performance Evaluation of the Tip-tilt Actuator in Fast Steering Secondary Mirror for Large Telescope [Journal of the Korean Society for Precision Engineering] Vol.31 P.403-409 google doi
  • 13. Kim Y-S, Ahn K-B, Park K, Moon IK, Yang H-S 2009 Accuracy Assessment for Measuring Surface Figures of Large Aspheric Mirrors [Journal of the Optical Society of Korea] Vol.13 P.178-183 google doi
  • 14. Lloyd-Hart M 2000 Thermal performance enhancement of adaptive optics by use of a deformable secondary mirror [PASP] Vol.112 P.264-272 google doi
  • 15. 1998 Technical Specifications for Polishing the #2 Magellan Project f/11 Secondary Mirror, Document No. 98SE0023 google
  • 16. Riccardi A, Brusa G, Salinari P, Gallieni D, Biasi R 2003 Adaptive secondary mirrors for the Large Binocular Telescope [Proc. of SPIE] Vol.4839 google doi
  • 17. Su P, Parks RE, Wang L, Angel RP, Burge JH 2010 Software configurable optical test system: a computerized reverse Hartmann test [Applied Optics] Vol.49 P.4404-4412 google doi
  • 18. Su P, Wang S, Khreishi M, Wang Y, Su T 2012 SCOTS: a reverse Hartmann test with high dynamic range for Giant Magellan Telescope primary mirror segments [Proc. of SPIE] Vol.8450 google doi
  • 19. Su T, Wang S, Parks RE, Su P, Burge JH 2013 Measuring rough optical surfaces using scanning long-wave optical test system. 1. Principle and implementation [Applied Optics] Vol.52 P.7117-7126 google doi
  • 20. Wildi F, Brusa G, Riccardi A, Lloyd-Hart M, Martin HM 2002 Towards 1st light of the 6.5m MMT adaptive optics system with deformable secondary mirror [Proc. of SPIE] Vol.4839 P.155-163 google
  • [Fig. 1.] Schematic drawing of the GMT. The primary mirror consists of seven segments of 8.4 m in diameter.
    Schematic drawing of the GMT. The primary mirror consists of seven segments of 8.4 m in diameter.
  • [Table 1.] Optical prescription of the GMT.
    Optical prescription of the GMT.
  • [Fig. 2.] Optical ray traces of the GMT. The star lights reflected by the primary segments will be converged first to a plane before the secondary.
    Optical ray traces of the GMT. The star lights reflected by the primary segments will be converged first to a plane before the secondary.
  • [Fig. 3.] A drawing of the GMT FSM segments. Among the seven segments, the surrounding six segments are placed at off-axis positions.
    A drawing of the GMT FSM segments. Among the seven segments, the surrounding six segments are placed at off-axis positions.
  • [Table 2.] Comparison of the GMT FSM and the Magellan secondary.
    Comparison of the GMT FSM and the Magellan secondary.
  • [Fig. 4.] FSM mirror segments drawn according to the Zemax data of the GMT.
    FSM mirror segments drawn according to the Zemax data of the GMT.
  • [Fig. 5.] Finite element analysis of the FSMP mirror model. Surface deformation by gravity is expected to be 7.6 nm rms in zenith position (a), and 7.7 nm rms in vertical position (b).
    Finite element analysis of the FSMP mirror model. Surface deformation by gravity is expected to be 7.6 nm rms in zenith position (a), and 7.7 nm rms in vertical position (b).
  • [Fig. 6.] Manufacturing drawing for the light-weighting.
    Manufacturing drawing for the light-weighting.
  • [Fig. 7.] Back side of the mirror after light-weighting. 91 holes were made from the spherical back side.
    Back side of the mirror after light-weighting. 91 holes were made from the spherical back side.
  • [Fig. 8.] A surface map during grinding. It was the measured data on Oct. 29, 2012. The peak to valley was about 5 μ excluding the redundant edges which were cut down after grinding.
    A surface map during grinding. It was the measured data on Oct. 29, 2012. The peak to valley was about 5 μ excluding the redundant edges which were cut down after grinding.
  • [Fig. 9.] The surface error map of the FSMP mirror announced on July 17, 2013. The rms error was 13.7 nm for the diameter of 1030 mm.
    The surface error map of the FSMP mirror announced on July 17, 2013. The rms error was 13.7 nm for the diameter of 1030 mm.
  • [Fig. 10.] Test results of the off-axis mirror by using the SCOTS and the interferometer. The SCOTS result is 17.5 nm rms, and the interferometer result is 12.9 nm rms in the diameter of 1010 mm. Both removed the Zernike 11 terms.
    Test results of the off-axis mirror by using the SCOTS and the interferometer. The SCOTS result is 17.5 nm rms, and the interferometer result is 12.9 nm rms in the diameter of 1010 mm. Both removed the Zernike 11 terms.