M2 Assemblyand Feed Optics

Ron Price

August 25, 2003

M2 Assembly Functional Requirements

• 60 cm diameter concave reflective optical surface. • M2 surface figure quality 32 nm rms after active optics

correction• Six degree of freedom positioning of M2• Fast tip-tilt motion of M2• Operating Conditions:

– Gravity Orientations - zenith angle of 0° to 80°– Thermal Conditions – solar load and diurnal temp– Wind Loading – wind speeds up to 10 m/sec

• Interfaces:– Optical Support Structure (OSS) of the telescope– Secondary Mirror Lifter– Telescope Control System– Utility Service

M2 Assembly Critical Areas

• Several areas were identified early as exhibiting somewhat higher risk. Consequently, more time and effort has been directed into these areas to resolve the issues as much as possible.– Surface figure change and thermal control of M2 due

to solar loading and diurnal temperature changes– Print-thru of substrate structure due to solar loading– Manufacturability of SiC substrate

M2 AssemblyMajor Components

Hexapod Support

Tip-Tilt Mechanism

Flexure Mounts

M2

Area for thermal control

M2 Blank

• Configuration:– Diameter: 62 cm

nominal– Thickness:

• Central area: 75 mm • Edges: taper to 30 mm

– Lightweighted structure – triangular pockets with 5mm face sheet and rib thickness

M2 Environment Drives Blank Requirements

• Thermal Environment– Print-thru of the ribbed structured onto the optical

surface due to solar load– Surface figure change due to solar load, diurnal

temperature change and thermal control– Desire to have optical surface of M2 track ambient

temperature

• Mechanical Environment– Surface figure change due to changing gravity vector– Surface figure change due to fast tip-tilt motion

Comparison of M2 Candidate Materials

 ρ E k c ν α

  gm/cm³ 10¹º W/m/K J/kg/K - ppm/K Ek/α k/ρ/c E/ρ  N/m²

ULE 2.21 6.76 1.31 767 .170 .03 295.6 .79 30.6Zerodur 2.53 9.06 1.65 812 .120 .05 124.5 .80 35.8Beryllium 1.85 30.40 220.00 1820 .025 11.20 597.1 65.00 164.3SiC 3.10 46.60 200 700 .280 2.40 3883.3 89.00 150.3 

  Ek/α is a measure of resistance to bending due to thermal transients k/ρ/c is called thermal diffusivity E/ρ is referred to as specific stiffness

  ρ – density c – specific heat   E - Elastic modulus ν – Poisson’s ratio k - thermal conductivity α – thermal expansion coefficient

  

   

M2 Thermal & Mechanical Analysis

• Thermal analysis of M2 is being performed to:– Quantify the level of rib structure print-thru on optical

surface due to thermal gradients caused by solar loading

– Evaluate the baseline cooling methods for their ability to control M2 surface temperature under solar loading and track diurnal temperature changes

– Evaluate global surface figure changes

• Baseline cooling uses ambient temp air jets into pockets on back of M2

M2 Analysis – Finite Element Model

M2 Thermal & Mechanical Analysis - Preliminary Results

• Print-thru – less than 10 nm with SiC or Zerodur

• Tracking of ambient air temperature – within 0.9 to 1.3 deg C for SiC depending on cooling flow rates

24

22

20

18

16

14

12

10

Te

mp

era

ture

(C

)

121086420Time (Hours)

Mirror Bulk Average Temperature Ambient Temperature

SiC M2 Thermal Profile-no air jet under flexure

M2 temperature profile for SiC substrate during peak solar load - 0.14 C range; 0.9 C above ambient

SiC M2 Surface Profile –no air jet under flexure

Global figure change for SiC substrate during peak solar load - 680 nm P-V

SiC M2 Surface Profile – uniform air jet cooling

Global figure change for SiC substrate during peak solar load - 40 nm P-V

M2 Substrate Material

• Although other materials have excellent performance in specific areas, silicon carbide appears to have the best overall performance in all of the required areas

• Further analysis will be performed, including dynamic performance under tip-tilt conditions, to confirm silicon carbide as the best substrate material

• Demonstrated capability by several vendors in silicon carbide at the 65 cm scale

SiC Fabrication Methods

• CVD (Chemical Vapor Deposition) - Gaseous chemicals react on a heated surface (usually graphite mandrel) to form solid SiC.

• Reaction Bonded SiC - SiC slurry is poured into a mold, freeze dried, sintered to form a porous SiC structure, then subjected to a high temperature process that introduces silicon and results in high density.

• Direct Sintered SiC- Very small SiC particles are Cold Isostatically Pressed into shape, machined, then sintered at 2500 deg C.

• Hot Pressed SiC - Very small SiC particles are Hot Isostatically Pressed (HIgh temp and Pressure) into shape.

• C/SiC - Carbon felt made of short randomly oriented carbon fibers is machined to shape. This “green body” is heated in a vacuum and infiltrated with liquid silicon; this results in a silicon carbide matrix.

M2 Blank Status

• Currently evaluating vendors and processes for SiC substrates– Boostec (CoorsTek USA) - Direct Sintered SiC– ECM (GE Power System Composites USA) - C/Sic– POCO Graphite - Reaction Bonded– Xinetics - Reaction Bonded

• Obtaining material properties and ROM cost and schedule estimates

M2 Polishing Specifications

• Surface Parameters– Surface Shape: Off-axis Ellipsoid– Conic Constant: K= -0.53936– Radius of Curvature: -2,081.259– Surface Roughness: 20 A rms or better

• Preliminary specifications are being developed that meet the error budget allocation for surface figure yet allow large spatial frequency errors correctible by the active optics system

M2 Support System Functional Requirements

• Functional Requirements– Mirror Support - Support M2 weight

and minimize surface figure changes over operational zenith angles

– Mirror Defining - Control the position and orientation of M2

– Provide fast tip-tilt motion

• Configuration– Commercial hexapod to provide

basic positioning motion– Custom Tip-Tilt mechanism will

mount on hexapod to provide fast image motion compensation

– M2 will attach to tip-tilt mechanism kinematically via three flexures

M2 Support System Performance Requirements

• Performance Requirements– Positioning

• Six degrees of freedom – 1 micron accuracy• Range of motion: 5 to 10 mm

– Tip-Tilt• Amplitude: 10 arc seconds• Rate: 10 hz; goal of 25 hz

Safety Restraint System

• Requirement - the M2 Restraint System provides protection of the primary mirror in the event of shock and vibration due to seismic activity.

• Configuration: safety restraints between rear of M2 structure and tip-tilt mechanism

M2 Control System

Functional Requirements:• Control M2 positioning system• Control M2 thermal management system• Control M2 fast tip-tilt system• Interface to AOCS• Interface to TCS

Feed Optics

M4

M3

M5

M6

To coude

From M2

Feed Optics

• M3 Flat Fold– 12 cm– Heat Load: 27.2 watts

• M4 Concave Ellipsoid– 34 cm– Heat Load: 24.5 watts

• M5 - Deformable Mirror Device

– 33 cm– Heat Load: 22.1 watts

• M6 Flat Fold– 26 cm– Heat Load: 19.9 watts

M2 Thermal and Mechanical Analysis will be extended to Feed Optics to determine optimum substrate and configuration for each optic