MSFC’s Heritage in Segmented MirrorControl Technology

John RakoczyAdvanced Optical Systems Development Group

NASA Marshall Space Flight Centerjohn.rakoczy@msfc.nasa.gov

MSFC’s Segmented Mirror Control Technology Heritage

• SELENE (1991-94)

• PAMELA (1993-Present)

• SIBOA (1998-Present)

• HET SAMS (1999-Present)

• NGST (1996-Present)

MSFC Program Relevance to GSMT:SpacE Laser ENErgy (SELENE)

• Background•$5M advanced concept defintion and technology development• funded by NASA Headquarters 1991-94• transmit electrical power from the ground to satellites and spacecraft via high energy laser illumination of photovoltaic array

•Relevant Products•Post-Keck, 12 meter-class ground imaging telescope design•Broad exposure to state-of-the-art• 1000 meters of highest tow, pultruded graphite composite tubes•12 meter primary mirror truss engineering drawings - checked.•Prototype tetrahedron including nodes & tube-end fittings•Experimentally verified finite element models

SELENE Trade Studies Relevant to GSMT•Control algorithms for huge numbers of segments•Optimum segment size and keystone families•Optimum actuator stroke and resolution at segment vs. cluster level•Telescope performance for parabolic vs spherical primary•Adaptive optics reqmts vs, altitude, geography, and high zenith angle•Cost vs. performance of low(angle iron), moderate(stainless steel), and high (composite) tech primary mirror truss materials. •Sources and size of error off homology in truss assemblages vs. tip angle•Fixtures/instruments/procedures for precise assembly of large telescope trusses•Roles of edge sensing and image-based wavefront control techniques •Evaluation of several prototype mirror flexures•Damping qualities of composite components in a large telescope•Distributeds processor architectures for highly segmented active mirrors•Comparison of various wavefront control techniques and instruments•Self-sensing, high resolution, long stroke,low power, linear actuators•Industrialization of small segment production capacity•Advanced mirror materials and fabrication techniques

PAMELA: Phased Array Mirror Extendible Large Aperture

• 36-segment adaptive spherical primary mirror

• Shack-Hartmann wavefront sensor

• Inductive edge sensors

• 5 kHz sample rate

• Tip/tilt/piston control via voice coil actuators

• Closed-loop bandwidth exceeding 100 Hz

PAMELA Control Challenges

• Utilized Shack-Hartmann sensor for local tip/tilt feedback and edge sensors for nearest neighbor edge-matching

• More than 100 modes within control bandwidth

• Segment dynamics coupled through primary mirror backplane

SIBOA TestbedSystematic Image Based Optical Alignment (SIBOA) testbed to demonstrate quasi-deterministic image-based alignment and phasing techniques at low temporal bandwidths•Seven spherical segments•Aspheric secondary•Broadband, multiwavelength and monochromatic sources•PC/LabVIEW/MATLAB software interface•New Focus picomotor actuators for tip/tilt/piston control•Blue Line HET-grade edge sensors

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3-segment Aperture Mask for quasi-deterministic phasing

Next slide shows PSFs when one segment is pistonedout of phase from 0 to 2 in 1/8 wave increments

SIBOA Quasi-deterministic Phasing

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Segment Alignment Maintenance System (SAMS)for theHobby-Eberly Telescope (HET)

Correct thermoelastically induced misalignment of primary mirror segments using inductive edge sensors

SAMS’s Inductive Edge Sensors

Accuracy ~ 50 nm RMSNoise < 25 nm RMS

•480 edge sensors on HET’s 91 mirror segments•1 Hz sample rate•Give PMC updates every 10 seconds•Control software in LabVIEW for Solaris on Sun UltraSparc 5•Successful demonstration on 7-segment sub-array in April 2001

Reference Stack

SAMS StackEE50 = 1.57 arcseconds

M20EE50 = 1.15 arcseconds

On-sky Image 5-April 02:00

Sub-array SAMS On-sky Performance

74 hours after last stack

NGST: Next Generation Space Telescope

• Contributed to preliminary design and government “yardstick” concept

• Hands-on experience in integrated modeling utilizing JPL’s IMOS (integrated modeling of optical systems) MATLAB toolbox

• Studied application of edge sensor architecture for aligning NGST segments (rigid or flexible)

• Managed lightweight mirror development and advanced cryogenic actuator development contracts

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NGST Simulink Example

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NGST Simulink Optics Block Diagram Example

MSFC’s Unique Capabilities• Operation of 2 unique active/adaptive optics testbeds

– PAMELA: adaptive, high temporal bandwidth, 36 segments, lots of dynamic coupling– SIBOA: active, low temporal bandwidth, 7 segments, relatively benign disturbance environment

• Developed a MATLAB toolkit for analyzing segmented mirror control, including edge sensor configurations, radius of curvature control, and image point spread functions

• Utilized LabVIEW and MATLAB for rapid software development of segmented mirror control systems

• Over 40 nights of engineering-time experience on-site as PI on HET

• Integrated thermal, structural, optics, controls modeling of telescope structures

Recent Bibliography   J. Rakoczy, D. Hall, R. Howard, J. Weir, E. Montgomery, G. Ames, T. Danielson, P. Zercher, “Demonstration of a

segment alignment maintenance system on a seven-segment sub-array of the Hobby-Eberly Telescope,” No. 4494-10, SPIE: Adaptive Optics Systems and Technology II, July 30-August 1, 2001, San Diego, California.     

J. Rakoczy, E. Montgomery, J. Lindner, “Recent Enhancements of the Phase Array Mirror Extendible Large Aperture

(PAMELA) Telescope Testbed at MSFC,” No. 4004-61, SPIE: Astronomical Telescopes and Instrumentation 2000, March 27-31, 2000, Munich, Germany.

       J. Booth, M. Adams, G. Ames, J. Fowler, E. Montgomery, J. Rakoczy, “Development of the Segment Alignment Maintenance System (SAMS) for the Hobby-Eberly Telescope,” No. 4003-20, SPIE: Astronomical Telescopes and Instrumentation 2000, March 27-31, 2000, Munich, Germany.

        J. Rakoczy, “An Edge Sensor Architecture Concept for Coarse Figure Initialization of the Next Generation Space Telescope,” NASA/MSFC Internal Memo ED11(12-98-124), June 9, 1998.

       G. Mosier, M. Femiano, K. Ha, P. Bely, R. Burg, D. Redding, A. Kissil, J. Rakoczy, “Fine Pointing Control for a Next Generation Space Telescope,” No. 3351-06, SPIE: Astronomical Telescopes and Instrumentation, March 20-28, 1998, Kona, Hawaii.

   G. Mosier, M. Femiano, K. Ha, P. Bely, R. Burg, D. Redding, A. Kissil, J. Rakoczy, L. Craig, “Integrated Modeling Environment for Systems-Level Performance Analysis of the Next Generation Space Telescope,” No. 3356-08, SPIE: Astronomical Telescopes and Instrumentation, March 20-28, 1998, Kona, Hawaii.

       D. Redding, S. Basinger, A. Lowman, A. Kissil, P. Bely, R. Bur, G. Mosier, M. Femiano, M. Wilson, D. Jacobson, J. Rakoczy, J. Hadaway, “Wavefront Sensing and Control for a Next Generation Space Telescope,” No. 3356-47, SPIE: Astronomical Telescopes and Instrumentation, March 20-28, 1998, Kona, Hawaii.

       G. Ames, R. Howard, J. Lindner, E. Montgomery, A. Patterson, J. Rakoczy, G. Zeiders, H. Waites, “Phase 1 Testing and Verification on a 0.5 Meter Diameter Telescope with a 36 Segment Adaptive Primary Mirror,” No. 2376-22, SPIE: Laser Power Beaming II, February 4-10, 1995, San Jose, California.