Development of the Primary Mirror Segment Support Assemblies for ...

Development of the Primary Mirror Segment Support Assemblies for the Thirty Meter Telescope Eric Ponslet*a, Dan Blancob, Myung Chob, Terry Mastc, Jerry Nelsonc, RJ Ponchionea, Mark Sirotad, Vince Stephensa, Larry Steppd, Alan Tubba, Eric C. Williamsa a HYTEC, Inc., 110 Eastgate Drive, Los Alamos, NM, USA 87544; b NOAO, 950 N. Cherry Avenue, Tucson, AZ, USA 85719; c Center for Adaptive Optics, University of California, Santa Cruz, USA 95064; d TMT Project, 2632 East Washington Blvd, Pasadena, CA, USA 91107 ABSTRACT This paper describes the studies performed to establish a baseline conceptual design of the Segment Support Assembly (SSA) for the Thirty Meter Telescope (TMT) primary mirror. The SSA uses a combination of mechanical whiffletrees for axial support, a central diaphragm for lateral support, and a whiffletree-based remote-controlled warping harness for surface figure corrections. Axial support whiffletrees are numerically optimized to minimize the resulting gravityinduced deformation. Although a classical central diaphragm solution was eventually adopted, several lateral support concepts are considered. Warping harness systems are analyzed and optimized for their effectiveness at correcting second and third order optical aberrations. Thermal deformations of the optical surface are systematically analyzed using finite element analysis. Worst-case performance of the complete system as a result of gravity loading and temperature variations is analyzed as a function of zenith angle using an integrated finite element model. Keywords: TMT, segment, mirror, support, whiffletree, warping-harness, active, optics, optimization, FEA

1. INTRODUCTION 1

The Thirty Mirror Telescope (TMT ) project, a partnership between ACURA, AURA, Caltech, and the University of California, is currently planning a thirty meter diameter optical-infrared, ground based telescope. The telescope will be used for research in astronomy at near-ultraviolet, visible and near infra-red wavelengths. The optical design is an Aplanatic-Gregorian with a 30-meter diameter, f/1, segmented primary mirror; a 3.6-meter diameter, concave secondary mirror; and a flat tertiary mirror. These will deliver an f/15 beam to the adaptive optic systems and science instruments located on two Nasmyth platforms. During observation, the telescope structure moves 360 degrees in azimuth and 0-65 degrees in zenith angle. A space frame mirror cell carries the segmented primary mirror. The segmented Primary Mirror (PM) will be comprised of 738 independent, Low Expansion (LE) glass segments, separated by 2 mm gaps. Each segment is hexagonal, cut from an aspherical meniscus, with a 0.6 meter nominal side length, and a thickness of 40 mm. To achieve the required surface accuracy and stability (less than 10 nm surface RMS figure error from support-induced deformations), each segment will be supported by a multi-point, passive, nearkinematic system of levers and flexures, actively controlled in piston, tip and tilt by a set of three linear actuators, and figure-controlled by an automated warping harness. The TMT segment support design is largely based on technologies developed for other segmented mirrors. It is an evolution of the Keck2,3 designs, with some features adopted from the Southern African Large Telescope (SALT4,5,6,7), which was recently inaugurated. Even though large segmented telescopes are only about 20 years old, the technological approaches to segment support are relatively well established. Almost every segmented telescope project since Keck has used mechanical whiffletrees for axial support, and a central diaphragm lateral support. In contrast with earlier segmented telescopes of the 10-meter class such as Keck, the much larger size of the TMT primary mirror will result in increased gravity-induced deflections of the PM cell, requiring larger actuator strokes to maintain segment positioning. The TMT primary mirror segments will also be thinner than previous large telescopes, which makes control of gravity-induced deflections a more difficult problem. These characteristics drove us to adopt a *

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SPIE Paper 6273-45 Proc. SPIE, Vol. 6273, pp. 379-397, Optomechanical Technologies for Astronomy, Eli Atad-Ettedgui, Joseph Antebi, and Dietrich Lemke, Eds, July 2006. Presented at the SPIE Astronomical Telescopes and Instrumentation Symposium, Orlando, Florida, May 24-31, 2006.

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moving frame support concept7 first introduced by the designers of the Southern African Large Telescope (SALT). In this approach, an intermediate, stiff moving frame is used to isolate the segment from the relatively large flexure reactions which result from the large actuator motions. The moving frame, guided by a separate set of flexures, absorbs these loads. 3

8

Keck

HET

7

SALT

GTC

9

10,11

LAMOST

TMT baseline

PM Error budget

EE80 < 0.32 arcsec

EE80 < 0.90 arcsec

Number of Segments

36 + 6 spares (Keck I or II, each)

91 + 3 spares

91 + 3 spares

36 + 6 spares

37 (Spherical Primary)

738 + 123 spares

EE80 < 0.24 arcsec

Segment size (circumscribed Ø) Segment thickness Bending stiffness, (EI)

1.8 m

1.15 m

1.15 m

1.9 m

1.1 m

1.2 m

75 mm (EI ~ 7)

52 mm (EI ~ 2)

50 mm (EI ~ 2)

80 mm (EI ~ 8)

75 mm (EI ~ 7)

40 mm (EI ~ 1)

Axial support

36-pt whiffletree

9-pt whiffletree

9-pt whiffletree

36-pt whiffletree

18-pt whiffletree

27-pt whiffletree

Lateral support

Central diaphragm

Central diaphragm

Central diaphragm

Central diaphragm

Central diaphragm

Central diaphragm

Warping harness

Manual, 30 DOF

None

None

Automated, 6 DOF

None

Automated, 18 DOF

†

Elevation

Variable

Fixed (55°)

Fixed (55°)

Variable

Fixed (-25°)

Variable

Actuation

Direct

Direct

Moving frame

Direct

Direct

Moving frame

Segment gaps

3 mm

6 to 19 mm

3 mm

6.5 mm

2 mm

Accommodation for segment geometry variations

Weights on segments

None (all segments identical)

None (all segments identical)

Weights on WT

First light

May 1993 (Keck I), Oct. 1996 (Keck II)

December 1996

September 2005

Late 2006 (expected)

Customize WT joint locations 2007 (expected)

2014 (planned)

Table 1: Comparison of existing and planned primary mirror segments for large ground-based telescopes.

An automated warping harness with 18 actuators per segment will provide the ability to remotely alter the surface figure of each segment as frequently as several times per night, if required, to correct for effects such as coating stresses, figuring errors, lateral position errors, and through-the-thickness variations of the coefficient of thermal expansion of the glass. Finally, given the unprecedented number of segments in TMT (738 + 123 spares), cost control is a major consideration in the design process. The target cost of TMT is less than half of the scaled-up Keck cost. This places very stringent requirements on fabrication costs in every subsystem. In the case of the SSA, the relatively large number of replications of an identical assembly gives us an opportunity to consider mass production approaches, which will help control costs.

2. SYSTEM OVERVIEW AND REQUIREMENTS Each mirror segment will interface with the mirror cell (primary truss) through a Segment Support Assembly (SSA), which will provide several functions:

†

•

Support the segment in the axial (piston, tip, and tilt) and lateral (two in-plane directions and clocking) degrees of freedom in a way that maintains segment position within the required accuracy and minimizes gravityinduced distortions of the segment as the elevation angle changes.

•

Maintain vibration mode frequencies above specified levels to minimize disturbances from external sources such as wind & machinery.

•

Accommodate a precision tip/tilt/piston position control capability, as provided by three precision linear actuators and twelve edge-mounted displacement sensors per segment.

•

Provide the ability to re-figure the segment in a controlled manner.

•

Provide a means to accurately and permanently align each SSA during initial installation

•

Provide a registration feature that allows removal and replacement of a segment with itself or a spare segment with specified repeatability without realignment.

•

Provide a means of decoupling a segment assembly from its base and lifting it out of the PM array.

DOF (degree of freedom)

SPIE Paper 6273-45 Proc. SPIE, Vol. 6273, pp. 379-397, Optomechanical Technologies for Astronomy, Eli Atad-Ettedgui, Joseph Antebi, and Dietrich Lemke, Eds, July 2006. Presented at the SPIE Astronomical Telescopes and Instrumentation Symposium, Orlando, Florida, May 24-31, 2006.

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•

Provide a way to accommodate variable segment geometry with a single support system design.

•

Protect the segment during any anticipated non-operating events such as shipping, handling, seismic, or similar.

2.1. Primary mirror segmentation The primary mirror will be segmented into six identical sectors, each containing 123 unique segments. Because segment-to-segment gaps will be nominally constant at 2 mm, each of the 123 segments within a sector has a unique, slightly irregular geometry (in shape, area, or both). YM1 component of Gravity

YM1 YSSA

YSSA

XSSA

ZSSA

ZSSA

XSSA

R 60m XM1

ZSSA

40 mm

YSSA

XSSA

1.20 m

Figure 1:

TMT Primary mirror segmentation, nominal segment geometry, and definition of global (M1) and local (SSA) reference frames.

Details of segmentation and segment geometry for TMT are not yet completely established. For the purpose of the conceptual design studies presented in this paper, the nominal segment (Figure 1) is assumed to be a constant-thickness spherical (the slight asphericity is neglected) meniscus, with a 60 m radius of curvature, a 40 mm thickness, and a regular hexagonal outline with a side length of 0.6m. The mirror substrate material is to be a low expansion glass or glass ceramic. For each segment, a local SSA frame of reference is defined as shown in Figure 1. The design of the SSA is performed in this coordinate system. Throughout the remainder of this paper, and unless otherwise indicated, all reference is to this coordinate system. 2.2. Segment support assembly A schematic of the current baseline concept for the TMT SSA is shown in Figure 2. Although several concepts were considered, most of the techniques used in the current baseline design were eventually adopted from previous telescopes, primarily the Keck telescopes and the Southern African Large Telescope. The essential components of the SSA, shown in Figure 2 are: •

Axial support: provides axial support of the segment (piston, tip, and tilt DOFs). The baseline concept is a 27point mechanical whiffletree. The whiffletrees include stiff load spreaders, flexural joints, rod flexures and low-expansion glass interface blocks bonded between the flexure and the segment glass.

•

Lateral support: provides lateral support to the segment (two in-plane and one clocking DOFs). considering several concepts, we decided on a classical central diaphragm flexure.

•

Warping harness: a remote-controlled warping harness using a set of instrumented leaf springs driven by stepper motor and screw actuators.

•

Sub-cell: collection of structures, flexures, and alignment & registration systems that serve as a foundation for the axial and lateral support systems. It is composed of the following elements: o Reference frame: a rigid structure that will be accurately aligned to serve as a reference base for the SSA. This frame interfaces with the cell through a precision registration system.

SPIE Paper 6273-45 Proc. SPIE, Vol. 6273, pp. 379-397, Optomechanical Technologies for Astronomy, Eli Atad-Ettedgui, Joseph Antebi, and Dietrich Lemke, Eds, July 2006. Presented at the SPIE Astronomical Telescopes and Instrumentation Symposium, Orlando, Florida, May 24-31, 2006.

After

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o

o

o

•

Moving frame: a rigid structure that is attached to the reference frame through a set of flexures to allow piston/tip/tilt motion, and driven by the three segment position control actuators. This structure serves as a common base for both the axial and lateral support systems. It isolates the segment from the loads that would otherwise be induced by the actuation system (a concept that was used in the SALT7 design) Alignment system: a set of three, 2-degree-of-freedom manual adjusters that allow the reference frame to be aligned accurately, in six degrees of freedom, with respect to the telescope as a whole. This alignment will be performed once, during initial assembly of the telescope. Registration system: a kinematic system of ball-in-groove locators that maintain the reference frame in an accurate and repeatable location relative to the adjusters. When a segment is removed from the array for maintenance/replacement, it separates from the cell at this level.

Lifting jack: raises and lowers the SSA in a precisely controlled manner to allow removal and installation of segment assemblies, and hand-off to the segment lifting crane. Segment lateral support Axial support rod flexures Warping harness actuator Segment

Moving frame guide flexures Axial support whiffletrees Moving Frame

Actuator decoupling flexure Actuator coupling clamp

Reference Frame

Kinematic registration 6-dof alignment system Position actuator M1 cell truss Segment lifting jack (removable)

Figure 2: Schematic description of current baseline SSA concept.

2.3. Design requirements 2.3.1. Segment surface figure The primary function of the SSA is to support the mirror segment while minimizing deformations of the optical surface under variable environments, from its reference figure, as produced during final figuring at the optics shop. It is envisioned that the SSAs will be assembled to the segments before final figuring and optical testing. What matters after final figuring are any changes in surface figure, induced by environmental effects such as changes in temperature and the orientation of the gravity vector relative to the segment. Our performance requirements are expressed as zenith-angle-dependent upper bounds on both the RMS‡ surface error and the spot size (EE80). Since the magnitudes of the support-induced figure errors are a small fraction of a wavelength, correct estimation of the spot size requires the use of physical optics calculations. Preliminary calculations have shown that the current design will meet the encircled energy requirement at all operating wavelengths. The segments are assumed to be final-figured at zenith§ (more precisely, with local ZSSA axis vertical), and at a nominal temperature equal to the mean operating temperature at the telescope site (Tref), with a tolerance of ±2°C. Since a site for

‡

Throughout this paper we refer to surface RMS and P-V as opposed to wavefront. Other figuring zenith angles could in theory be considered to further minimize gravity-induced figure errors in observation (such as the mean observing zenith angle, for example); that approach has been considered but rejected for reasons of increased complexity and cost in optical testing of the segments.

§

SPIE Paper 6273-45 Proc. SPIE, Vol. 6273, pp. 379-397, Optomechanical Technologies for Astronomy, Eli Atad-Ettedgui, Joseph Antebi, and Dietrich Lemke, Eds, July 2006. Presented at the SPIE Astronomical Telescopes and Instrumentation Symposium, Orlando, Florida, May 24-31, 2006.

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TMT has yet to be selected, the operating (observing) temperature is assumed be to 0±5°C** for conceptual design purposes. Combining the tolerances on figuring and observing temperatures, we arrive at the design temperature excursion, ±7°C about Tref. Telescope error budget allocations for the primary mirror produced the following support-induced surface figure requirements: RMS (ζ ) ≤ 9.2 nm× sec(ζ ) 0.5 , and EE 80 (ζ ) ≤ 0.041 arcsec× sec(ζ ) 0.6 ,

where ζ is the segment zenith angle (angle between ZSEG and the local vertical††). The zenith angle dependence accounts for the degrading image quality when observing through increasing atmosphere as the telescope rotates from zenith to horizon. At zero segment-zenith-angle, the support-induced RMS surface error is almost entirely dominated by thermal effects (besides manufacturing and assembly errors) since the segments are tested on the support system at approximately this zenith angle, eliminating the gravity-induced component. As the zenith angle increases toward the horizon, the surface RMS increases as axial spring-back and in-plane gravity distortions both increase. 2.3.2. Warping harness Warping harness performance is specified in terms of a Zernike12 expansion of segment figure. Both second and third order Zernike terms are to be controllable. For each Zernike term, both the amplitude of the error to be corrected, and the required reduction factor are specified, as summarized in Table 2. In addition, warping harness actuators must be capable of correcting the worst case combination of all terms and amplitudes shown in the table. The required force accuracy (