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)
Osservatorio di Arcetri
Osservatorio di Arcetri
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
Osservatorio di Arcetri
Beyond Conventional Adaptive Optics, Venezia 7-10 May 2001
Large stroke, no hysteresis
240
mm
P36 prototype
36 act. prototype (=633nm) Astigmatism PtV=12m
Osservatorio di Arcetri
Beyond Conventional Adaptive Optics, Venezia 7-10 May 2001
• 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
Beyond Conventional Adaptive Optics, Venezia 7-10 May 2001
MMT336 ASPHERIC SHELL
Assembled unit
Magnets
642mm diam.2mm thick
(12mm diam)
Cap. sensor armatures (ref.plate)
Osservatorio di Arcetri
Beyond Conventional Adaptive Optics, Venezia 7-10 May 2001
• 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
Beyond Conventional Adaptive Optics, Venezia 7-10 May 2001
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
Osservatorio di Arcetri
Beyond Conventional Adaptive Optics, Venezia 7-10 May 2001
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
++
Osservatorio di Arcetri
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)
Osservatorio di Arcetri
Beyond Conventional Adaptive Optics, Venezia 7-10 May 2001Feed-forward matrix -
modal
(Cap. sensor readings)
Modal excitation:improved SNR
Osservatorio di Arcetri
Beyond Conventional Adaptive Optics, Venezia 7-10 May 2001
The FF matrix fitting
Osservatorio di Arcetri
Beyond Conventional Adaptive Optics, Venezia 7-10 May 2001
Modal stiffness
Act stiffness 0.2N/m
Local ctrl dominates
FF dominates
Osservatorio di Arcetri
Beyond Conventional Adaptive Optics, Venezia 7-10 May 2001
Step response
Only Prop. - Gain=0.2N/m (40m gap) Settling time 1.5ms
CommandPosition
Osservatorio di Arcetri
Beyond Conventional Adaptive Optics, Venezia 7-10 May 2001
Step response
FF Force
Ctrl+FF Force
Osservatorio di Arcetri
Beyond Conventional Adaptive Optics, Venezia 7-10 May 2001
Step response
Osservatorio di Arcetri
Beyond Conventional Adaptive Optics, Venezia 7-10 May 2001
Step response
Osservatorio di Arcetri
Beyond Conventional Adaptive Optics, Venezia 7-10 May 2001
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
Osservatorio di Arcetri
Beyond Conventional Adaptive Optics, Venezia 7-10 May 2001
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
(Cap. sensor readings)
Osservatorio di Arcetri
Beyond Conventional Adaptive Optics, Venezia 7-10 May 2001
Modal analysis
(Cap. sensor readings)
Osservatorio di Arcetri
Beyond Conventional Adaptive Optics, Venezia 7-10 May 2001
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