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.
Osservatorio di Arcetri People (some of them) Osservatorio di Arcetri
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
Large stroke, no hysteresis P36 prototype 240mm 36 act. prototype (l=633nm) Astigmatism PtV=12mm
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
Assembled unit Magnets guido Cap. sensor armatures (ref.plate) MMT336 ASPHERIC SHELL guido 642mm diam. 2mm thick Magnets (12mm diam)
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)
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
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 -3dB@56kHz Linearization Capsens DM DSP DSP of k-th channel
Feed-forward matrix - zonal FF matrix (Cap. sensor readings)
Feed-forward matrix - modal Modal excitation: improved SNR (Cap. sensor readings)
The FF matrix fitting
Modal stiffness Act stiffness 0.2N/mm FF dominates Local ctrl dominates
Step response Position Command Only Prop. - Gain=0.2N/mm (40mm gap) Settling time 1.5ms
Step response Ctrl+FF Force FF Force
Step response
Step response
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
Turbulence compensation nm r0=15cm @ 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)
Modal analysis (Cap. sensor readings)
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