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 compensationnm
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