Adaptive telescope mirror developments in Arcetri A. Riccardi

Adaptive secondary concept Reference plate Heat-sink and act. support plate Electronics boxes deformable shell Concept: Substitution of conventional M2 telescope mirror with a thin ( mm) deformable shells controlled in position with large-stroke (~0.1mm) electromagnetic (voice-coil like) force actuators and using internal capacitive sensors as position feedback Central membrane for lateral support

Control electronics Communication Board (1x backplane) DSP control Board (14x backplane) Reference Signal Generator Board (1x backplane) Communication Board (1x backplane) DSP control Board (14x backplane) Reference Signal Generator Board (1x backplane) Communication Board (1x backplane) DSP control Board (14x backplane) Reference Signal Generator Board (1x backplane) Communication Board (1x backplane) DSP control Board (14x backplane) Reference Signal Generator Board (1x backplane) Communication Board (1x backplane) DSP control Board (14x backplane) Reference Signal Generator Board (1x backplane) Communication Board (1x backplane) DSP control Board (14x backplane) Reference Signal Generator Board (1x backplane) Liquid cooled crates, each comprehending 2 backplanes (3x) Distribution boards Actuators Gap Thin mirror Reference signal Power ± 48V, 35 A Real time comm link 2.9 Gbit/s ( MMT160Mb/s) Daisy chain connection DSP control Board (14x backplane) Coil 3 cooled electronics boxes 2 crates/box 84 custom DSP boards 2 DSP/board - 8 acts/board 32-bit floating-point 470Mmac/s (MMT: 16-bit integer 40Mmac/s) Gigabit Ethernet Switch Diagnostic communication link To the AO supervisor 400Mbit/s Total computational power: 78 Gmac/s (32bit fp) Real-time reconstructor on-board WFS: 30x30 => s (z-m) Slope comm time: 20 s

Why and adaptive telescope mirror? WFS Sci. Camera DM Coll. TTM BS Conventional Secondary Adaptive Secondary Sci. Camera WFS Less warm surfaces K band: shorter exp. time (MLH, PASP) Adaptive Secondary

Advantages Un solo correttore per tutti i fuochi (es. LBT: 4 fuochi/pup) Maggiore riflettività (5 riflessioni eliminate: =0.90) Minore emissività IR (1/3-1/4 exp.time K-N bg-limited) Compattezza della parte di sensing (maggiore stabilità) Attuatori elettromagnetici con feedback capacitivo: Grande stroke: TTM+DM+chopper+FS in ununica unità Unità robusta rispetto malfunzionamento di attuatori Tecnologia estendibile a specchi adattivi per ELTs Grande stroke (wind bufferting) >10 4 att., grande numero attuatori grandi specchi adatt.

Overview of developments MMT Adaptive Secondary (on sky 2000) Joint venture OAA-Steward MG-ADS contract CAAO LBT Adaptive Secondary (integration phase, on-sky 2008) VLT Adaptive Secondary (design phase, on-sky 2015) INAF under OPTICON-JRA1 MG-ADS subcontract with ESO TEC0-TEC1: EU funded under the ELT-DS project MG-ADS subcontract INAF M4-ARU EELT: ESO funded development MG-ADS proposal for contract INAF subcontract of MG-ADS Magellan Adaptive Secondary (copy of LBT, on-sky ???)

MMT on sky m V 8.0 (B0V) Credits:

Existing adaptive mirror in hardware MMT: 336 act 640mm diam 2.0mm thick 31 mm/act (Jan 2003) 640mm P45proto LBT (2 units): 672 act 911mm diam 1.6mm thick 31 mm/act (in production) Integration of final unit 911mm INAF, Steward Obs, Microgate Srl, ADS Int. Srl

LBT integration progress

Next ASM generation: VLT-DSM VLT-DSM 1.1m 1170 act. 29 mm pitch 1 ms response VLT-DSM ESO, Microgate Srl, ADS Int. Srl, INAF

Current technology: a comparison

Current limitation in BW (or stroke) 7kHz Capsens-coil crosstalk Currently it limits derivative gain Some level of natural damping is still req. 70um gap: ms settling time 100um gap: ms settling time CL Actuator transfer function (with deriv gain=0) Item to solve in the commissioned studies especially for TEC0 (larger mass, larger gap, larger derivative gain required) In case of glass, keeping constant g-quilting: Mass per actuator: ~6.5 g (LBT,TEC1-30mm) : ~350g (TEC0-100mm) : ~30g (TEC0-50mm)

Noise vs gap (i.e. stroke) 40um gap70um gap 130um gap

TEC0 and TEC1 target Feasibility study of 2.5m DM with actuator spacing of: DM-TEC 0: woofer corrector, medium stroke field stabilizer mm actuator pitch ( acts) 200 m PtV stroke (±2as on-sky for 42m telesc.) (300 m goal) with high efficiency actuator prototype DM-TEC 1: tweeter corrector, low stroke field stabilizer corrector25-30mm actuator pitch ( acts) 100 m PtV stroke (±1as on-sky for 42m telesc.) (200 m goal) with scale down prototype ( ~ 100act)

Towards an Adaptive ELT Part of the technological solutions currently used cannot directly transferred to Adaptive ELT, in particular: Production of optical flat/concave/convex thin (~2mm) glass shell with diam>1m is not proven. See FP6 studies by SESO and INAF-Brera. See studies with other material like CFRP Lateral support from central membrain could induce too large stresses ( diam) on glass and for membrane buckling. Alternative lateral support shall be studied also to avoid holes in case of segments Current reference+cold plate scheme is not applicable for large mirrors: More favorable stiffness-to-mass ratio backplate will be studied. Larger stroke required (70um -> um): larger dynamical range capacitive sensor shall be studied with reduced noise at large gap. Crosstalk has to be reduced to increase electronic damping (large gap with large BW).

Comparison among alternative materials

CFPR by Composite Mirror applications Tucson, AZ