Solar orbiter – EUS instrument mechanical design

Tim Froud and Doug Griffin

Talk outline

• Mechanical-thermal-optical considerations for the Primary Mirror• Silicon Carbide Testbed Study• Structural layout

Primary mirror WFE under Solar IR Loads

• The Solar radiation load will introduce thermal gradients in the Primary Mirror

– Thermal stresses

• Internal mirror temperature gradients

• Possibly also from CTE mismatches

• The thermal load will vary by roughly an order of magnitude during each orbit

• Goal is to meet the optical performance during the encounter and during down link phases of the orbit

• Mirror distortions will introduce distortions in the surface of the mirror which will reduce the spectral and spatial resolution of the instrument

Primary Mirror WFE Tolerance Estimates

RMS Irregularity l/X @ 633nmRMS

IrregularityApprox PTV

IrregChange in RMS

Spot SizeTotal spot

size(in waves at test

wavelength)(in nm)

(with mirror irregularity)

0.5 X = 2 317 nm 448 nm 67.0 um 74.6 um0.2 X = 5 127 nm 179 nm 23.0 um 30.6 um0.1 X = 10 63 nm 90 nm 8.9 um 16.5 um0.05 X = 20 32 nm 45 nm 3.2 um 10.8 um

0.0333 X = 30 21 nm 30 nm 1.7 um 9.3 um0.025 X = 40 16 nm 22 nm 1.1 um 8.7 um

• Calculation based on encircled energy and are to be considered provisional • Given the Spectral and Spatial resolution requirements:

– Derived requirement on PSF ~ 10um:• Mirror rms WFE: ~l/30 i.e. 21nm rms

NI Primary Mirror Distortion EstimatesModel Geometry

Uniform heat load B/C35kW/m2

Axis of symmetry

5mm

10

mm

5m

m5

mm

25

mm

40

mm

Fixed Temperature

B/C

NI Primary Mirror Distortion EstimatesMaterial Assumptions

• Assume that the mirror is made from homogeneous Silicon Carbide– E=249GPa– Poission ratio: 0.16– CTE: 2.7x10-6 m/m– Thermal conductivity: 127 W/m/K– Mirror is absorbing in the IR

• Thermal interfaces have identical mechanical properties as Mirror– No interface stresses

NI Primary Mirror Distortion EstimatesTemperature Distribution

TemperatureT (K)

5.280

4.752

4.224

3.696

3.168

2.640

2.112

1.584

1.056

0.528

-0.000

NI Primary Mirror Distortion EstimatesThermal distortions

Axial displacement

-800

-700

-600

-500

-400

-300

-200

-100

0

0 5 10 15 20 25 30 35 40 45

Distance from C/L (mm)

Dis

pla

cem

ent

(nm

)

NI Primary Mirror Distortion EstimatesConclusions

• The analysis has to be considered as a preliminary estimate

– Optical

– Thermal

– Mechanical

– Layout

• Nonetheless: The conclusion is that the thermal control of the Primary Mirror will have to be considered in tandem with the overall instrument optical performance

– It could be one of the largest challenges for the instrument design

– Rastering of the instrument via the Primary mirror could be very difficult with the complexity of the thermal control

Primary Structure Material SelectionTest Bed Study

• A study was carried out on a SiC based Optical Bench

– Looked at the practical details of designing structural components in SiC

– Very useful in terms of understanding:• Detailed manufacturing processes • Achievable tolerances• Mounting and interface designs• Lightweighting• Non-destructive Testing

– Procurement of test-bed did not proceed due to cost

– Promising outcome for Flight-build

Primary Structure Material SelectionTest Bed Study

Components of system

Components of system

Optical bench

Primary mirror

Slit mechanism

Grating

Detectors and electronics

Enclosure and baffling