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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 Slide 2 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 Slide 3 Osservatorio di Arcetri Corrector specs for ELTs So many actuators need: Low cost per actuator Reliable device Slide 4 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 Slide 5 Osservatorio di Arcetri People (some of them) Osservatorio di Arcetri Slide 6 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 Slide 7 Osservatorio di Arcetri MMT336 ASPHERIC SHELL Assembled unit Magnets 642mm diam. 2mm thick (12mm diam) Cap. sensor armatures (ref.plate) Slide 8 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 Slide 9 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 Slide 10 Osservatorio di Arcetri Control loop Commands from WFC - + DM k-th Curr. Driver + coil Capsens Linearization @625Hz DSP of k-th channel -3dB@56kHz 40kHz local loop DSP + Feed-forward + + Slide 11 Osservatorio di Arcetri Modal stiffness Actuator stiffness 0.2N/ m Local ctrl dominates FF dominates Slide 12 Osservatorio di Arcetri Step response Only Prop. - Gain=0.2N/ m (40 m gap) Settling time 1.5ms Command Position Slide 13 Osservatorio di Arcetri Step response FF Force Ctrl+FF Force Slide 14 Osservatorio di Arcetri Step response Slide 15 Osservatorio di Arcetri Step response Slide 16 Osservatorio di Arcetri Turbulence compensation nm Input turbulence Residual of first 200 mode Correction (scale X10) r 0 =15cm @ =550nm, v=6m/s, 8sec 2.3 m rms 140nm rms on acts. Max 0.4N rms (| T|