04/22/23 Hans J. Kärcher1

SOFIA Telescope Manufacturing-

Lessons Learned

Hans J. Kaercher, Senior Telescope EngineerMT Mechatronics GmbH, Mainz, Germany

hans.kaercher@mt-mechatronics.de

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Experience with Telescopes at Exposed SitesRadio Telescopes

30m IRAM Spain

9m O‘HigginsAntarctica

50m LMT Mexico

15m IRAM France

APEX, ALMA

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Experience with Telescopes at Exposed SitesOptical Telescopes

2.7m Airborne Telescope SOFIA

1.5m Solar Telescope GREGOR

1m Solar Balloon Telescope Sunrise

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SOFIA Main Telescope Components

Primary Mirror

Secondary Mirror

TertiaryMirror

ScienceInstrument

MeteringStructure

Counter Weights

SuspensionAssembly

Imagers

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Access to the Science Instrument during flight

Research flight hours per year 960 hours

Time per flight at 12.5 km altitude 6 hours

Unvignetted elevation range 20 - 60 degrees

Effective telescope aperture 2.5 m

Telescope optical image quality 1.5 arcsec

Telescope pointing stability rms 0.2 (0.8) arcsec

SOFIA Main System Requirements

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SOFIA Telescope Optics will be in Cold Environment (- 50° C, 0.2 bar)

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SOFIA Telescope Science Instrument FlangeAccessible during Operations from the Cabin Side

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Thermal Barrier

Pressure Barrier

+ 20° C - 50° C0,8 bar 0,2 bar

Cabin

Cavity

Steady State Type Environmental Influences on the Telescope

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Metering Structure

Mirror Cell

Nasmyth Tube

Bearing Sphere

SOFIA Structural SystemCarbon Fiber Composites

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SOFIA Primary Mirror System

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Suspension Assembly

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Secondary Mirror Assembly Attitude Controlwith fiber-optical gyros

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M2 Mirror

SiC/C 35cm Ø

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M2 Focus-Center MechanismHexapod Qualified for – 50°C Operational Environment

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Environmental Influences on the Telescope

Science Instrument

Shear Layer at Cavity Opening

Standing Waves Excited by the Shear Layer

Wind in Cavity Induced by the Shear Layer

Aircraft Vibrations

Cabin

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End-to-end Simulation ModelSOFIA end-to-end simulation

1image motion

unbalance

torque drive

raytrace

position controller

mirror 2image motion

estimator for M2

image motionestimator for

position controller

gyro

feed forward filter M2

bearing

accelerometers

Vibration IsolationAssembly (VIA)

Telescope Assembly (TA)Structure

2aircraft acceleration

1aerodynamic disturbance

estimatedimage motion

positioncommands angular displacement of M2

56 f orces

u,v ,w acceleration

torque (as f orces to rotor segments)

torque to telescope

torque to cradle

all VIA f orces

torque commands

v elocity and displacement

all VIA displacementsand v elocities

cradlerotational displacement

and v elocity

telescoperotational displacement

and v elocity

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Servo Domain Aero-acoustic Domain

Image Quality

„Low“ structural damping (0,5 – 1 %)

„High“ structural damping (1 – 2 %)

RMD for 32,2 Hz mode (5 %)

32,2 Hz mode

22,4 Hz dumbbell

Influence of SMMvia FBC

~ 75 Hz rocking mode

End-to-end Simulation Results

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Summary of SOFIA Experience

• System Level– Thermal System Design– Pointing System Design

• Subsystem Level– Optics, Structure– Suspension Assembly– Secondary Mirror Mechanism– Servo Control System

• Material Selection etc.– Structures in Carbonfiber Composites– Dedicated Metals for Mechanisms– Same for Motors, Drives etc.

• QA, RAMS– Quality Control absolute essential– Reliability, Maintainability same– Methods qualified in Aircraft and

Space Business are adequate– Management has to accept

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Recommendations for Arena

< 2m

- Material Choice- Low Temperature qualified

Mechanical Component- Low Temperature qualified

Motors, Encoders- Thermal Concept for Optics,

Controllers- QA, RAMS

CFRP Tube elements with

integrally manufactured stringers

Tube Elements

Primary Mirror Cell Outer Ring

Spokes

Inner Ring

GluedCleat

GluedCleat

Spiders / Headring

A-AA

AB

B

B-B

A-AA

AB

B

B-B

Sunrise CarbonfiberTechnology

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Thermal Concept for Sunrise

PrimaryMirrorCooling

IR Radiation and Reflection from „Warm“ Earth

IR Radiationto Cold Sky

IR Radiationto Warm Earth

Cold Sky < 100 K ??

Surrounding Air Temperature ~200 K / 290 K ?

Antarc tic Ice ~250 K

Balloon-Gondola-

TelescopeTemperature

~200 K~0° - 70° el

HeatRejectionCooler

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Recommendations for Arena

> 2m

Combination of O’Higgins Pedestal with SOFIA Mirror

Compare with exposed design

(e.g. Gregor)

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Conclusions from Engineers Point of View

• Challenging!

• Interesting!

• Nice, Funny!

Let’s do it!