Beyond Conventional Adaptive Optics Venezia 7-10 May 2001 The adaptive secondary mirror for the 6.5...

Beyond Conventional Adaptive Optics Venezia 7-10 May 2001

The adaptive secondary mirror for the 6.5

conversion of the MMT

Presented by A. RiccardiA. Riccardi1, G. Brusa1, C. Del Vecchio1, R. Biasi2, M. Andrighettoni2, D. Gallieni3, F. Zocchi4, M. Lloyd-Hart5, F. Wildi5, H. M. Martin5

1 – Osservatorio Astrofisico di Arcetri, Firenze, Italy2 – Microgate, Bolzano, Italy3 – ADS, Lecco, Italy4 – Media Lario, Lecco, Italy5 – Steward Observatory, Tucson, AZ, U.S.A.

Osservatorio di Arcetri

Beyond Conventional Adaptive Optics, Venezia 7-10 May 2001

People (some of them)



Beyond Conventional Adaptive Optics, Venezia 7-10 May 2001Adaptive Secondary

Concept

WFS

Sci. Camera

DM

Coll.

TTMBS

ConventionalSecondary

AdaptiveSecondary

Sci. Camera

WFS

Less warmsurfaces

K band: 2-2.6shorter exp.time(MLH, PASP)

AdaptiveSecondary



Large stroke, no hysteresis

240

mm

P36 prototype

36 act. prototype (=633nm) Astigmatism PtV=12m



• 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



MMT336 ASPHERIC SHELL

Assembled unit

Magnets

642mm diam.2mm thick

(12mm diam)

Cap. sensor armatures (ref.plate)



• 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



Resonances in control bwFirst 270 modes have res< 1kHz + local control

Phase lag of 180deg for >res if low damping => unstable

P30 “astigmatism” mode TF

37m gap

115m gap

37m gap

115m gap

High damping (18Ns/m => 40m gap) + local control PD

increasingDamping:larger PM



Control loopCommands from WFC

kc

DsG -

+

DM

k-th Curr. Driver + coil

CapsensLinearization

@625Hz

DSP of k-th channel

-3dB@56kHz

40kHz local loop

DSP

ii

ki cK

336

133621, cccc k

+

Feed-forward

++


Beyond Conventional Adaptive Optics, Venezia 7-10 May 2001Feed-forward matrix -

zonal

336

1

336

1

x

x

K

f

f

FF matrix

(Cap. sensor readings)


Beyond Conventional Adaptive Optics, Venezia 7-10 May 2001Feed-forward matrix -

modal


Modal excitation:improved SNR



The FF matrix fitting



Modal stiffness

Act stiffness 0.2N/m

Local ctrl dominates

FF dominates



Step response

Only Prop. - Gain=0.2N/m (40m gap) Settling time 1.5ms

CommandPosition



Step response

FF Force

Ctrl+FF Force



Step response



Less Bandwidth wrt P36

G=0.8M/m S.T.=0.7ms G=0.2M/m S.T.=1.5ms

7.5mm

P36 MMT336

2mm

Less damping, butbetter static controlof the edge



Turbulence compensationnm nm

Input turbulence Residual of first 200 modeCorrection (scale X10)

r0=15cm @ =550nm, v=6m/s, 8sec

2.3m rms140nm rms on acts.Max 0.4N rms(|T|<1.2 C) WCcondterm

WNeff

/5.3..

/5.0




Modal analysis




Conclusions

• The control gain achieved is high enough to ensure proper tracking and correction of atmospheric turbulence.

• The unit is now on the optical-test tower at the Mirror Lab for optical characterization

• Work is now in progress to optically flatten the mirror and calibrate the capacitive sensors

• According to the current schedule we plan to close the optical loop at the optical-test tower in June

Beyond Conventional Adaptive Optics Venezia 7-10 May 2001 The adaptive secondary mirror for the 6.5...

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