design of a lightweight mounted tip/tilt mirror

MIKE BORDENOPTI 523

4/6 /09

Design of a Lightweight Mounted Tip/Tilt Mirror

Design Problem 4: Large Mirror Mount

To Do List

Type of lightweight

Mirror material and coating

Linear actuator

Mount and flexure design

Lightweighting

Contoured Back • Sandwich, and Open Back

Lightweighting

Lightweighting

Mirror Material

Considerations High stiffness to weight Ease of manufacture Minimal thermal stresses Cost

Main materials being considered ULE, Zerodur, Fused Silica, Beryllium, Aluminum, BK7

Mirror Material

ULE

Zerod

ur

Fused

Silic

a

Beryll

ium

Aluminu

mBK7

0

20000

40000

60000

80000

100000

120000

140000

160000

Substrate Comparison

Stiff

ness

to

Wei

ght

(N/m

m^

2-kg

)

Mirror Materials

0 10 20 30 40 50 60 70 80 90 1000

1

2

3

4

5

6

Open Back Mirror Deflection Curve

ULEZerodurFused SilicaBeryllium

Mirror Angle (deg)

Defl

ecti

on (

nm)

Mirror Material

Possibilities Pure Beryllium mirror

Expensive and difficult to manufacture Thermal considerations non-issue

Fused Silica, BK7, Aluminum substrate w/ silver coating Aluminum w/ silver coating provides smallest ∆𝛂

(23 ppm / K and 18.9 ppm/ K) Thermal expansion / contraction an issue Something crazier – Copper, Brass, Beryllium Copper,

Stainless

Optical SpecificationsSilver Coating

Meets spectrum requirements

Optical Specifications

Error Budget – 30 nm rms WFE Mirror surface roughness (∆S)

20 nm rms Self weight deflection (δ)

20 nm rms

Beam diameter 200 mm diameter mirror accommodates 150 mm

beam diameter

Mounting Concept

Mounting Concept

Mounting Concept

Invar Sleeve

Spherical Bearing

Micrometer

Remaining Tasks

Verify number of supportsReflectance of BeTolerance bearings for thermal loadingFlexure design – dimensions, materialFEA analysis of deflection / thermal loading,

resonant frequencyLinear actuator / micrometer selection