GISMO. (Giant IR and SubMm Observatory) Tim Hawarden. UK Astronomy Technology Centre, Royal Observatory, Edinburgh

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1 GISMO (Giant IR and SubMm Observatory) by Tim Hawarden UK Astronomy Technology Centre, Royal Observatory, Edinburgh

2 ORIGIN OF PROPOSAL: Invited Technology Review for lightweight space FIR telescopes (ESA workshop, Madrid, 1-5 Sept 2003) PERSONAL GOAL OF REVIEW: Outline design for a telescope that is: Large (>>10m) Cold (<<30K) To work in the FIR-submm (~20 to ~700µm),.. and has a chance of flying in my lifetime (I m ~60, go figure..)

3 ORIGIN OF PROPOSAL: Invited Technology Review for lightweight space FIR telescopes (ESA workshop, Madrid, 1-5 Sept 2003) RESULTS Membranes have plenty of problems (cusps, wrinkles, uniformity, how to tension, shape control ) Stiff technologies unlikely to offer >>10m aperture* 2 Fresnel lenses being explored at LLNL for optical applications must be x easier in FIR-submm! *But see TRW plans for 30m JWST-style telescope for 7-17µm: C.F. Lillie et al., Proc SPIE, 4860, 84 (2003)

4 Diffractive Fresnel lenses Bulls-eye ring pattern, radii R M = (2Fλ/M) ½ ; M = Zone [ring] number, F = Focal length of lens. φ = 2π φ = 0 h Refractive Index = n t Zones shaped like segments of a larger lens (quadratically curved) = perfectly blazed grating. For n ~ 1.5, thickness averages ~ (λ + t).

5 Diffractive Fresnel lenses Each zone inserts phase delay φ = 2πh(n-1)/λ φ = 2π at inside zone edge where h = λ/(n-1) φ = 0 at outer zone edge. Spherical wavefront results. This last is true for any λ such that φ = N x 2π (Integral N) i.e. the lens works at harmonics as well as the fundamental λ.

6 Advantages of lenses Lenses have relaxed surface tolerances: Wavefront error δ = (n-1)δt (surface error), so if n ~ 1.5, λ min = 20µm, for δ λ min /8 (~ D.L. perf.) δt λ min /4 = 5µm Slow lenses have relaxed position tolerances: In-plane posn. error tolerance f/ratio Out-of-plane posn. error tolerance f/ratio 2 For a 30m f:100 lens, n ~ 1.5, λ min = 20µm : Radial zone position tolerance ~ 300µm Out-of-plane tolerance ~ 40cm

7 ACHROMATIC DIFFRACTIVE FRESNEL TELESCOPES Primary (L1) is a diffractive Fresnel lens Focal length F 1/λ, so lens is highly dispersive But can correct using Schupmann design:

8 Dispersed colours Fresnel Corrector Lens (FC) Off-axis ray Achromatic Focal Plane Primary Fresnel Lens (L1) Collector Optics Re-imaging optics Schupmann achromatic Fresnel telescope: Primary lens L1 is demagnified by factor η and imaged onto Fresnel Corrector (FC) Adapted from Fig. 2 of R. A. Hyde, Eyeglass. 1. Very large aperture difractive telescopes Appl. Optics 38, 4198 (1999)

9 ACHROMATIC DIFFRACTIVE FRESNEL TELESCOPES L1 is re-imaged on Fresnel Corrector lens (FC) of identical but opposite dispersion. FC has the same number of zones and the same material dispersion (preferably very small!) Optics re-imaging L1 must be diffraction limited and achromatic

10 ACHROMATIC DIFFRACTIVE FRESNEL TELESCOPES Wavelength range of achromatic correction R = λ/ λ = 2α where α = D collector /D L1 (light not collected can t be corrected). Dimensions of collector also determine FOV of system (it s a field lens too). With finite FOV, bandpass determined by filter

11 ACHROMATIC DIFFRACTIVE FRESNEL TELESCOPES c.f. Eyeglass proposal from LLNL (R.A. Hyde, 1999: Appl. Opt. 38, 4198) 80 cm and 5 m glass demonstrators nearly diffraction limited in visible. Achromatism demonstrated Moon in white light taken with 200mm LLNL demonstrator.

12 Designing GISMO: Mission Size Corrector FC must be << Collector re-imager; with instruments this must fit in ~4m launcher fairing D FC <<1m We choose D FC = 30cm. For manufactureability, f FC 1 (zones λ) We choose f FC = 1.0 Let η = D FC /D L1 = 0.01 for D L1 = 30m. To match FC and L1 dispersions, F FC = η 2 F L1, SO F L1 = 3000m GISMO must be TWO SPACECRAFT

13 Solar arrays Sunward 5-layer sunshade Service Module GISMO Sunshades here c.f. SAFIR and Lens Achromatic Fresnel Telescope Spacecraft 2: Lens re-imager, Fresnel Corrector and instruments 3000m Spacecraft 1: 30-m Primary Lens In two Spacecraft

14 Designing GISMO: The Orbit Two spacecraft: Holding formation 3km apart with Position accuracy ~ 10 cm, ~10" and must be Strongly (radiatively) cooled Sun-Earth L2 Halo Orbit

15 The Orbit L4 Second Sun- Earth Lagrangian Point L2: L3 L1 L2 Sun, Earth and moon all close on sky, good for sun/earth shields Sun Earth Small gravity gradients, easy station-keeping L5 Distance 1.5X10 6 km, good data communications

16 The Orbit Moon s orbit Halo Orbit Around L2 P ~ 180 d Earth Days after launch L Sun 1.5 x 10 8 km x 10 6 km Sun-Earth L2 Halo Orbit

17 Designing GISMO: The Lens D = 30.1m, F = 3000m Lens material must be: Transparent through FIR and sub-mm Moderate refractive index n: surface reflectivity = [(n-1)/(n+1)] 2 = 4% per surface for n = 1.5 Low density (ρ = 1 mass = 700 kg/mm thickness) Flexible (will need to be folded/ rolled up) Easily fabricated High Molecular-Weight Polyethylene (HMW-PE)

18 Polyethylene in general HDPE: n = ±.0001 from ~1.9mm to ~300 µm (Birch et al. 1981) so very little dispersion likely δn < 1% for UHMW-PE but must check Reflection losses 2x (n-1) 2 /(n + 1) 2 ~ 8% : not serious Density ρ ~ 0.92 ~ 640 kg mm -1 for 30-m diameter Transmittance (need more data: coming!): ~80% to ~90% from 20 to 100 µm absorption ~140 µm (narrow at low T) 90 to 95% from 100 to 1000 µm

19 Working range ~20 to ~600 µm Transmittance Wavelength (µm) Transmittance of a 0.7mm UHMW-PE sample at room-temperature

20 PERFORMANCE RELATIVE TO IDEAL LENS R = angular resolution E = total efficiency Performance of GISMO: R~7.5 bandpasses at harmonic λ values Design wavelength = 1200 µm, continuous coverage ~20 - ~700µm [Derived from Fig. 7 of R.A.Hyde, Eyeglass. 1. Very large aperture difractive telescopes, Applied Optics 38, 4198 (1999)]

21 Lens Details: Spacecraft 1: The Lens (L1) 63 zones, outer 24 cm wide; h (step size) = 2.4mm Average thickness ~2.2mm, ρ ~ 0.95 ~1500 kg For launch: Folded and rolled into cylinder 6m x ~2.3m. Thickness set by rolling radius of UHMW-PE, sizes of CFRP struts and Shape Memory Alloy hinges Deployment: Inflatable stretcher tubes and outer ring CFRP struts unfolded by SMA hinges

22 Deployment arms, Umbilical for gas and power Inflatable Deployment tubes Folded lens unrolls Rim tension tube (inflates) Fold lines Struts SMA self-opening hinges (on all fold lines) L1: Fresnel lens ø = 30.1m L1 deployment sequence: 1. Before launch, lens is folded (5 layers) and rolled into 6-m cylinder, axis along launch direction, boxes secure 2. On orbit, deployment arms rotate cylinder 90º and latch 3. Deployment tubes inflate, unroll lens 4. Inner, then outer, SMA hinges heated to unfold lens 5. Rim tension tube inflated to bring lens into plane 6. Sunshade deployed, lens moved into shade to cool and stiffen

23 Sunshade LENS Spacecraft: systems 5 layers, 10m x 40m, layers tapered for sun-avoidance angle ± ~7º 1st layer second-surface-silvered Teflon (SSST) for solar radiation rejection, deployed from SVM rear face edges. 2 intermediate shields (on 40K cooled box?) 2 rear shields (on 15K cooled box, c.f. SAFIR?). Rear shield stabilises at ~20K (model for radiation only, but can cool conductive elements so should be about right). Service Module (SVM) Solar arrays, array shield, power, attitude control & orbital maintenance, cryo-coolers (+ 4 He liquefier & SFLHe pumps to cool lens?).

24 Lens (L1) Shield deployment Boxes (Cryo-cooled?) SVM T= 7.6K T= 7.1K CCS T= 7.3K Sunward face, SSS-mylar, T~230K T~120K T~65K T~36K T~20K GISMO End view, with approximate thermal performance. Radiation-only, assuming 7K equivalent energy density at L2, ε=0.1 ε(λ); detailed properties of UHMW-PE not included COOLING TIMES (very approx!) To 50K ~60h 25K ~20d 10K ~250d (It s very big, very thin, very well coupled to space!)

25 Designing GISMO: Corrector & Instruments Spacecraft 2: Collector-Corrector Spacecraft (CCS) Collector ( Field lens ) optics: 6m x 3m near-field off-axis* Ritchey-Chretien images 30-m f/100 L1 onto 30-cm f/1.0 Fresnel Corrector lens (FC) Instrument optics refocus the (achromatic) emerging light to focal plane for instruments Sunshades, coolers, Focal Plane instrumentation Like SAFIR: but smaller optics = cheaper! * Central obstructions obscure some wavelengths: Need off-axis CCS optics.

26 Fresnel CORRECTOR Sunshades here Deployment truss and hinge (c.f. JWST, SAFIR) Re-imaging optics and Instrument Bay (4K) segment hinge lines COLLECTOR M1 (3 segments) COLLECTOR M2 CCS: Collector-Corrector Spacecraft Multi-layer sunshade c.f. SAFIR COLLECTOR: Near-field RC design 6 x 3m off-axis oval aperture in (only) 3 segments: 3 x 3m centre panel 2 x 1.5m radius semicircles 2m colour correction aperture (defined by bandpass filters) plus 1 x4 FOV T optics = 4K (c.f. SAFIR) F RC = 30m CORRECTOR: F:1.0 Fresnel corrector lens forms cryostat window FP radius=50 cm; maybe HDPE field-flattener in front INSTRUMENTS: Re-imaging optics and focal plane instruments in cryostat behind M1

27 GISMO TECHNICAL SUMMARY D L1 ~ 30m, F = 3000m. Resolution = 3."4 at 500 µm, 0. "14 at 20 µm L1 tolerances set by λ min = 20 µm: ~ 40 cm out of plane, 0.3mm in plane L1 and FC material Ultra High Molecular Weight Polyethylene Telescope (L1) temperature assumed ~10K, ε ~ 0.1 Passive Cooling times: ~60h to 50K, ~20d to 25K, ~250d to 10K Total Mass ~5000 kg (L1 ~ 2400, CCS ~ 2600) Ariane 5 to L2 ~80% mass margin, but volume?) Length: Lens SC ~8.5m, CCS ~5.7m, Total ~14.2m OK for Delta IV-H fairing, single long Ariane 5?

28 GISMO Fresnel optics scales & tolerances Fresnel lens feature sizes Set by longest (fundamental) wavelength (1200 µm) Manufacturing and adjustment tolerances Set by shortest working wavelength (20 µm assumed) L1: outer zone is 24 cm wide In-plane tolerance ~300µm Out-of-plane tolerance ~40cm! Surface quality tolerance ~10 µm Check with mfrs? Refractive Index homogeneity ~10-3 Check with mfrs? (prob.ok) Cumulative CTE homogeneity ~10-5 Check with mfrs? FC: outer zone is 2.4 mm wide In-plane and out-of-plane tolerances ~2 µm No showstoppers??

29 GISMO Performance: IMAGING Relative to a cold 10m telescope (curves from SAFIR, for which ε = 0.05 assumed): GISMO ~7x raw sensitivity (ε = 0.10, T=10K, D=30m) 10x confusion limited sensitivity x gain in speed for λ < ~ 100 µm 10x gain in potential spatial information

30 GISMO Performance: SPECTROSCOPY Temperature vs. Spectroscopic Sensitivity T-dependence from GISMO SAFIR models GISMO performance: 10K curve lowered by (D/10) 2 / ε, ε = 0.10 assumed for UHWM-PE, all λs

31 SPECTROSCOPY (continued): 30m GISMO at 10K: GISMO (10K) Outperforms 10m at 4K for 20 < λ < 150 µm..and for λ > 550 µm Up to ~6x less sensitive for 150 < λ < 550 µm (if confusion not important for spectroscopy).

32 PERFORMANCE COMPARISONS: GISMO Relative to SAFIR: Active cooling of optics? 10x more spatial information per unit solid angle 10x more sensitive for imaging at all confused wavelengths, 7x at others 10x more sensitive for spectroscopy at wavelengths below 100µm, maybe 6x LESS sensitive from ~150 to ~500 µm IF THIS IS IMPORTANT, COOL THE LENS? To cool 700 m 2 of surface, with ε 0.1, to 4K in a radiation field of equivalent energy density 7K (~ 3 x 10-5 Wm -2 ), requires a total of ~25 mw heat removal POSSIBLE?!? (Caveat: UHMW-PE absorptivity high at short wavelengths where Zodiacal Dust emits!) Lateral conductance of L1 probably enough to allow effective cooling by pumped superfluid 4 He in small tubes 2m apart.

33 ASTRONAUT/ ROBOT ASSEMBLY: 100m lens: 6 x 16m, 16 tonne, segments ( slices ) two launches Delta IV-H Assemble on-orbit (too large even for lowthrust boost?) CCS: Aperture ~10m (cf SAFIR) one more Delta IV Investment now large: servicing is economic necessity

34 ASTRONAUT/ ROBOT SERVICING: Challenges servicing cryogenic missions: Cryostat warm-up capability (temperature controlled 4 He supply needed (build in? bring on repair truck? Telescope warm-up capability: Roll optic into sunlight? ( thermal shock!) provide viewfactor to warm/ sun-reflecting object (radiator/ mirror in sunlight, start far, end near..?) Begin mission well ahead of astronaut arrival!

35 A GIANT FRESNEL TELESCOPE MAY BE THE MOST COST-EFFECTIVE WAY FORWARD FOR FIR and SUBMILLIMETRE ASTRONOMY

36 ACKNOWLEDGEMENTS Lawrence Livermore National Laboratory: James Early s SPIE 03 paper on a 20-m OPTICAL diffractive Fresnel telescope provided the idea (if its thinkable at 0.5 µm its easy at FIR!) Roderick A Hyde s 1999 paper (Appl. Opt. 38, 4198) and demonstrator models explained diffractive achromatic Fresnel telescopes JPL and GSFC Hal Yorke (JPL) and Dominic Benford (GSFC) provided SAFIR material and discussions of performance UK Astronomy Technology Centre: Tully Peacocke assisted in the first FIR conceptual design Eli Atad and Dave Henry did preliminary models confirming the basic LLNL concept (since many were sceptical of achromatism) Mark Clift produced excellent images at very short notice University of Wales, Cardiff: Peter Ade measured and provided transmission curves for ultra-high molecular weight polyethylene (UHMW-PE) and other materials