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The adaptive secondary mirror for the 6.5 conversion of the MMT

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Presentation on theme: "The adaptive secondary mirror for the 6.5 conversion of the MMT"— Presentation transcript:

1 The adaptive secondary mirror for the 6.5 conversion of the MMT
Presented by A. Riccardi A. 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, Italy 2 – Microgate, Bolzano, Italy 3 – ADS, Lecco, Italy 4 – Media Lario, Lecco, Italy 5 – Steward Observatory, Tucson, AZ, U.S.A.

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

3 Adaptive Secondary Concept
Conventional Secondary Adaptive Secondary Adaptive Secondary Sci. Camera WFS Less warm surfaces WFS BS K band: 2-2.6 shorter exp. time (MLH, PASP) TTM DM Coll. Sci. Camera

4 Large stroke, no hysteresis
P36 prototype 240mm 36 act. prototype (l=633nm) Astigmatism PtV=12mm

5 System Layout guido mirror diameter 642 mm mirror thickness 2 mm
membrane in-plane restraint 336 moving magnet actuators nominal air gap ~ 40 mm 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) guido

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

7 Electronics – control system
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)

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

9 Control loop DSP @625Hz Feed-forward + + - k-th Curr. Driver + coil
Commands from WFC @625Hz + Feed-forward + - k-th Curr. Driver + coil 40kHz local loop Linearization Capsens DM DSP DSP of k-th channel

10 Feed-forward matrix - zonal
FF matrix (Cap. sensor readings)

11 Feed-forward matrix - modal
Modal excitation: improved SNR (Cap. sensor readings)

12 The FF matrix fitting

13 Modal stiffness Act stiffness 0.2N/mm FF dominates
Local ctrl dominates

14 Step response Position Command
Only Prop. - Gain=0.2N/mm (40mm gap) Settling time 1.5ms

15 Step response Ctrl+FF Force FF Force

16 Step response

17 Step response

18 Less Bandwidth wrt P36 2mm 7.5mm P36 MMT336 G=0.8M/mm S.T.=0.7ms
Less damping, but better static control of the edge

19 Turbulence compensation
nm l=550nm, v=6m/s, 8sec nm Input turbulence Residual of first 200 mode Correction (scale X10) 2.3mm rms 140nm rms on acts. Max 0.4N rms (|DT|<1.2 C) (Cap. sensor readings)

20 Modal analysis (Cap. sensor readings)

21 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

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