Large Aperture Telescope Technology: a design for an active lightweight multi-segmented fold-out space mirror

Transcription

1 Large Aperture Telescope Technology: a design for an active lightweight multi-segmented fold-out space mirror Presenter: Samantha Thompson, University College London

2 UK LATT Team Martin Whalley Ruben Edeson Ian Tosh Olly Poyntz-Wright Samantha Thompson Peter Doel Eli Attad-Ettedgui Dave Montgomery Joe Nawasra Rutherford Appleton Laboratory University College London UK Astronomy Technology Centre ABSL Space Products

3 Large Aperture Telescope Technology (LATT) ESA contract AO5898 Future DIAL mission requiring large 1 aperture 9 month study (Phase 1) Follow-on from a previous study (Advanced Lidar Concepts, ESA 2007) Optical design mostly unchanged Focus on different technologies for the same mission profile Key technologies Lightweight mirrors Amount and type of active control of mirrors (form and co-phasing) Petal deployment mechanisms

4 Mission requirements summary Future DIAL mission 4m diameter primary aperture Launcher fairing 2.1m x 3.5m = nm M1 corrected wavefront error = /6 ( /8 goal) Surface roughness < 10 nm (5 nm goal) Low mass, areal density <16 kgm -2 Modal response f 1 >100Hz, Random 11.12g rms, Shock Thermal: -25 C<T non-op <55 C, 0 C<T op <40 C Power: < 40 W (average)

5 Optical overview Optical design determined from a previous ESA study* Afocal 2 mirror Cassegrain-Mersenne telescope Primary mirror: central hexagon, 6 deployable square petals Reference optical design* M1 mirror segment arrangement (top view) * Lidar executive summary report: LIDAR-RP-CGS-005 (ESA ref doc)

6 Lightweight mirrors Very challenging mass budget (16 kgm -2 ) Includes support structure, mirrors, actuators, launch locks, petal deployment mechanisms Mirror material Require good strength and stiffness to weight ratio Survive launch shocks, deployment, ground-based handling Suitable optics performance at = nm Deformable in range of suitable actuation forces Scalable to meter sizes Active mirrors for low order correction Trade off between mirror stiffness, type and number of actuators, mass and system complexity

7 Carbon-fibre composite mirrors UCL has 5 years+ research into carbon-fibre composite mirror technology Benefits Low density Low thermal expansion High strength High modulus Issues Accuracy of form replication Surface quality Resin stability

8 Carbon-fibre composite mirrors Three constructs investigated FEA and manufacturing feasibility Carbon-fibre honeycomb sandwich panel Thin (nickel coated) carbon-fibre plate Open waffle-backed carbon-fibre plate Discounted waffle-backed for further study UCL initially developed Ni-CFRP mirrors (STFC funding) Limited small-scale trials to manufacture CFRP honeycomb mirrors

9 CFRP-honeycomb test mirror Mirrors are formed on polished glass mandrels Measured average density = 310 kgm -3 Close-up of open-weave CFRP honeycomb core material Aim: to obtain <10nm surface Ra with only resin replication from polished glass tool Result: specifications not reached. Incorrect resin enrichment used, long leadtime to obtain correct system, further tests required.

10 Example of polished Ni surface, Ra ~ 3 nm (time limited) 270 mm mirror form (uncorrected): 3 P-V, < /2 rms ( = 633nm) 1m CFRP plate mirror (prior to coating) UCL 2010 [as part of UK EELT R&D work, STFC funded]

11 Aluminium mirrors Three test mirrors, different alloys ( 100mm approx) Open-backed waffle design (Al 6082) Honeycomb sandwich construction (Al 3003) Thin, solid plate (Al 6061) Manufacturing techniques Conventional machining Diamond turning Optimised support during machining crucial

12 The 3 aluminium test mirrors. From top right: open-waffle back (74% light-weighted), honeycomb sandwich, thin plate (pictures from UKATC)

13 Al mirror results Surface quality (machine mounted) Micro-roughness (RMS, nm) Surface form error : RMS (nm) Waffle back (Al 6082) Honeycomb (Al 3003) Disc (meniscus) (Al 6061) [350 - unmounted] 7.7 n/a [1357] 8.5 n/a [~3000] Need to test better alloy, e.g. RSA6061-T651 where 2-3 nm rms microroughness has been demonstrated. Analysis of cutter forces has lead to new support structure to reduce stresses and improve final form.

14 Actuator design Based on Squiggle Motor (Piezo driven screw device) Positive attributes Relatively high actuation force (5N) Low mass -10g (75g incl. support mechanism and lauch lock) Holds position when unpowered Thermal and vacuum compatibility Commercially available High speed (up to 2 mm/s) Good reported repeatability (20nm) Negative attributes Leadscrew rotates as it translates Requires small preload to work reliably Accuracy TBD

15 Actuation mechanism

16 Actuator arrangement 3x3 square grid on petals 18 actuators, triangular grid on central hexagon

17 Example of mirror modes generated from influence functions obtained by FEA of 3mm CFRP thin plate on 3x3 grid of actuators

18 Deployable telescope system Stow configuration: 3 petals up, 3 down Fairing Diameter (2.1m)

19 Petal deployment Launch locks release and petal deployed Deployed to within actuator correction range Can alter tilt about hinge with stepper motor Petal actuators: Assumes stepper motor and harmonic drive design (~830 g each) Launch locks: Assumes Starsys PP-5501 HOP pinpullers (170g) plus support structure

20 Deployment mechanism Petal structure Position encoder Angular contact bearing Motor, harmonic drive and coupling HOP launch lock

21 Future mission telescope - summary Overall design Central hexagonal mirror, six square petals; three fold up, three down for launch Petal mechanisms deploy, then mirror actuators are used to fine-tune position and shape as required, in conjunction with S-H WFS Petal Mechanism HOP/Thermal knife launch locks Stepper motor/harmonic drive gearbox to drive petals into position Mirror Actuators 3x3 grid for petals, similar spacing for central hexagon Squiggle motor driven mechanism Mirror Materials Aluminium alloy waffle-back mirror CFRP mirror as back-up alternative

22 Hexagonal Mirror Subassembly Subsystem Mass (kg) Mirror 1 off 16.8 (Al waffle) 10.0 (Ni-CFRP) 7.8 (CFRP honeycomb) Actuators 18 off 1.35 Support Structure 1 off 14.3 (CFRP/Al honeycomb panel) 1 off 32.5, 25.7, 23.5 Petal Mirror Assemblies Mirror 1 off 9.6, 5.7, 4.5 Actuators 9 off 0.68 Petal Structure 1 off 3.3 Total 1 off 13.6, 9.7, off 81.6, 58.2, 51 Petal Actuators Actuators 6 off 5 Launch Locks MHRM and Pinpullers 6 off 3 MLI Petals + Hex 1off 3.4 Sum of subsystem masses 125.5, 95.3, 85.9 Margin (~30%) 37.65, 28.6, 25.8 Total 163 (17.3 kg/m 2 ) 124 (13.2 kg/m 2 ) 112 (12 kg/m 2 ) Target Mass 150 (16kg/m 2, Area 9.4m 2 )

23 Future investigation Breadboard development dependent on continuation to Phase 2 of study Breadboard overview: 2 mirrors - concave, spherical form 1 fixed Zerodur Well characterised reference 1 deployable Active mirror Chosen material technology Represents square petal Whole assembly to undergo thermal vacuum tests Lightweight mirror research ongoing at UCL (CFRP) and UKATC (Al)

24 Thank-you for your attention Questions?