Osservatorio di Arcetri Adaptive primary mirrors for ELTs A. Riccardi 1, P. Salinari 1, G. Brusa 1, R. Ragazzoni 1, S. Esposito 1, D. Gallieni 2 and R. Biasi 3 1 Osservatorio Astrofisico di Arcetri, Italy 2 ADS International Srl, Italy 3 Microgate Srl, Italy

Osservatorio di Arcetri Corrector specs for ELTs D=50m gives about 196,000 actuators for the ground layer Target: Multi-conjugate adaptive correction in V band La Palma median seeing: 0.65arcsec (r 0 =0.16cm at 0.55mm) r 01 =22cm (60%) r 02 =33cm (30%) r 03 =64cm (10%) SC Fitting error=0.3 (d/r 0 ) 5/3 rad 2 MC Fitting error=0.3 [(d 1 /r 01 ) 5/3 + (d 2 /r 02 ) 5/3 + (d 3 /r 03 ) 5/3 ] rad 2 MC Fitting error == SC Fitting error with d= r 0 (d 1 /r 01 ) 5/3 = (d 2 /r 02 ) 5/3 = (d 3 /r 03 ) 5/3 = 1/N (N=3) d k = r 0k / N 3/5 = 0.52 r 0k d 1 = 11cm

Osservatorio di Arcetri Corrector specs for ELTs So many actuators need: Low cost per actuator Reliable device

Osservatorio di Arcetri Currently avail. technology Piezo technology Largest unit: Xinetics, 349 act., 7mm spacing (M>14). Position actuators (failure is a major problem) MEMS technology Large number of acts., reduced stroke, 0.3mm spacing (M>333) A  : (50m x 1arcmin) 2 = (150mm x 6deg) 2 Adaptive secondary technology MMT unit: 336 act, 30mm spacing (first light end of June 2002) LBT units: 672 act, 30mm spacing (could be 25mm) Force (electromagnetic) actuators, Large stroke (diam.>11m to match the specs) See Brusa et al. Backaskog Workshop for a general theory

Osservatorio di Arcetri People (some of them) Osservatorio di Arcetri

mirror diameter 642 mm mirror thickness 2 mm membrane in-plane restraint 336 moving magnet actuators nominal air gap ~ 40  m reference body 50 mm thick AL cold plate: actuators support & cooling (7 cooling channels) 24 absolute gap sensors fixed hexapod support frame & interface to Hexapod electronics cooled crates hub interfaces (power, signal & cooling) System Layout

Osservatorio di Arcetri MMT336 ASPHERIC SHELL Assembled unit Magnets 642mm diam. 2mm thick (12mm diam) Cap. sensor armatures (ref.plate)

Osservatorio di Arcetri 3 crates 14 control boards each crate 8 channels controlled by each board (4DSP) capacitive sensor signal conditioning on the actuators (close to variable gap capacitor) Electronics – control system

Osservatorio di Arcetri Resonances in control bw First 270 modes have res < 1kHz + local control Phase lag of 180deg for > res if low damping => unstable P30 “astigmatism” mode TF 37  m gap 115  m gap 37  m gap 115  m gap High damping (18Ns/m => 40  m gap) + local control PD increasing Damping: larger PM

Osservatorio di Arcetri Control loop Commands from WFC - + DM k-th Curr. Driver + coil Capsens DSP of k-th channel 40kHz local loop DSP + Feed-forward + +

Osservatorio di Arcetri Modal stiffness Actuator stiffness 0.2N/  m Local ctrl dominates FF dominates

Osservatorio di Arcetri Step response Only Prop. - Gain=0.2N/  m (40  m gap) Settling time 1.5ms Command Position

Osservatorio di Arcetri Step response FF Force Ctrl+FF Force

Osservatorio di Arcetri Step response

Osservatorio di Arcetri Step response

Osservatorio di Arcetri Turbulence compensation nm Input turbulence Residual of first 200 mode Correction (scale X10) r 0 =550nm, v=6m/s, 8sec 2.3  m rms 140nm rms on acts. Max 0.4N rms (|  T|<1.2 C) (Cap. sensor readings)

Osservatorio di Arcetri Modal analysis (Cap. sensor readings)

Osservatorio di Arcetri MMT336 Flattening The flattening of the shell applying 150 mirror modes: 34nm RMS The central rings are artifacts due to the olographic technique used for testing convex mirrors

Osservatorio di Arcetri Closed optical loop Open loop Closed loop MMT336+SH 12x12 From G.Brusa

Osservatorio di Arcetri LBT672 summary 672 actuators in 14 rings 31mm act-to-act ave dist 642mm 911mm MMT336 LBT672

Osservatorio di Arcetri LBT672 electronics 3x2 crates 14 ctrl board per crate (84 tot) 8 320Mflop/s 32bit-FP DSP per board (ADSP-21160) 2 actuators per DSP (336 tot.) Total computational power: 107 Gflop/s WF reconstruction on-board LBT672 ((32x32x2)x672)x2 op in 0.5ms = 6Gflop/s (5% res.) (8.4  s to compute the FF matrix)

Osservatorio di Arcetri Electronic damping 100  m gap no electronic damping Electronic damping test on P36 G PlantSens DD + - c ff PV: 0.5ms settling time 50Ns/m

Osservatorio di Arcetri Electronic damping Natural vs. electronic damping: Natural damping reduces stroke Electronic damping is limited by delay and capsens noise New capsens for LBT: From 40kHz to 500kHz Reduced aliasing error Analog derivative (reduced delay) G PlantSens DD + - c ff + + +

Osservatorio di Arcetri Actuator spacing limit 30mm Capacitive sensors:3nm resolution, 100  m range (DR propto area) Actuator Efficiency: proportional to sqrt of magnet and coil volume Quilting: shell thickness propto sep 2

Osservatorio di Arcetri Actuator spacing limit Actual spacing 30mm. 4m secondary for 50m telescope: 36cm on turbulent layer Optimal correction: 0.5 r 01 =36cm: =1.6  m

Osservatorio di Arcetri Primary adaptive mirror No more problem of actuator spacing (M=1) Economical advantages Less expensive magnet technology Gravity load is supported in more points: Euro50 segments: 70mm thickness Primary adaptive mirror: 10mm thickness 86% less glass!! High spatial frequency control: larger polishing residual error can be accepted at low and mid-spatial frequencies No Zerodur, borosilicate can be used. Cheaper!!

Osservatorio di Arcetri 50m example 2m diameter segments About 600 segments 2mm gap between segments 10mm thick glass: 77.5kg/segment 46.5tons total

Osservatorio di Arcetri 2m segment layout 10cm/act 397 act/segment Local glass stiffness: 15N/  m (LBT: 672 actuators)

Osservatorio di Arcetri Main components Glass: 77.5kg/seg (46.5tons) Honeycomb: 100kg/seg (60tons) 400 Actuators: 80kg/seg (48tons) Total: 258kg/seg (155tons) Euro50 (70mm thickness): 543kg/seg (326tons). 52% more mass!!

Osservatorio di Arcetri Support and maintenance

Osservatorio di Arcetri Backplate FEA: gravity load 15  m PtV Model

Osservatorio di Arcetri Backplate FEA: dynamics 84Hz 168Hz 201Hz 282Hz

Osservatorio di Arcetri Backplate FEA Deformation of the honeycomb when pushing: 238N/  m (glass: 15N/  m) Oscillation of the ribs: 3.1kHz (out of the bandwidth)

Osservatorio di Arcetri Actuator details Axial and tangential load: astatic levers?

Osservatorio di Arcetri Actuator prototype

Osservatorio di Arcetri Control electronics Control electronics, one box per segment: smaller then LBT672

Osservatorio di Arcetri Pyramid sens. for co-phasing 3 DoF per segment 64x64 or 16x16 signal sampling points Interaction matrix dimension 156x6400 or 156x400 Signals for segment Differential piston Center act. Side actuator S. Esposito

Osservatorio di Arcetri Closed loop simulations rms < /20 Each realization converges to /20 rms after 7 iterations maximum. An input wavefront Piston values of each segment S. Esposito

Osservatorio di Arcetri Primary mirror prototype 1200mm diam. 7x340mm segments 19 acts/segment