1. The Adaptive Mirror for the E-ELT
E. Vernet, M. Cayrel, N. Hubin (ESO)
R. Biasi, G. Angerer, M. Andrighettoni, D. Pescoller (Microgate)
D. Gallieni, M. Tintori, M. Mantegazza (ADS)
A. Riccardi, M. Riva, G. Pariani, R. Briguglio, M. Xompero (INAF)
The M4 is currently in a preliminary design update phase to take into account the change of the telescoep diameter but also the additional requirements which have been requested to M4. all these additional requirements are not related to AO correction but more to stability requirement and flatness requirements of the mechanical structure itself.

2. Outline M4 key functional requirements
Positioning system & wavefront correction
AO requirements & additional features
Current design of M4
Demonstration prototype design & objectives
Project timeline

3. An adaptive mirror in the E-ELT
Main goals:
Provide adaptive correction
Cancel part of telescope wind shaking & static aberrations
10 degree FOV
2 Nasmyth focii
In plane centering & tip-tilt system
The M4 Mirror is the deformable mirror slightly elongated with a maximum external optical diameter of 2387mm. The inner optical diameter is 540mm while the M4 must provide a 520mm free central hole for the telescope optical beam propagation. The central hole has a conical shape to allow the 10 degree field of view to go through M4 without any vignetting.
With an orientation angle of 7.75 degree, the M4 Adaptive Subunit requires system to feed the two Nasmyth focii. The baseline design includes a rotator at the level of the interface with the telescope.
Less than 5 minutes shall be needed by M4 to switch between Nasmyth focii. The focus selection mechanism shall have a repeatability of at least 30 mechanical arcsec as measured about any of the three local axes of the M4 Mirror Subunit

4. Positioning system
Decentering capability of +-20 mm (0.5mm accuracy, 0.05 resolution)
Tip-tilt capability of +-2 arcmin (0.5 arcsec accuracy, 0.27 resolution)
Cross-coupling with tilt <54 mas/mm PV lateral displacement
Cross-coupling with lateral displacement 1.85 micron/arcsec PV tilt
0.5mm position stability in xy & 1 arcsec orientation stability (with LUT), 0.5mm position stability in z (without LUT)
The M4 Adaptive Subunit is mounted on a positioning system providing a first stage large stroke low frequency mechanical tilt, a two dimensions rigid body decentering degrees of freedom and a focus selector.
Forty millimeters stroke displacements have been foreseen for realignment of M4 with respect to M1 during observation and initial stroke needed during integration on the telescope.
 The tip-tilt range of four arc-minutes will allow correcting quasi-static perturbations due to thermal loads and gravity effects. All the degree have been specified with stringent cross coupling requirements (limited by the system resolution for small displacements and tip-tilt) and good positioning accuracy (~400 times smaller than the total stroke) to allow a reliable and efficient alignment during preset.
Each leg has been specified with the following requirements to fulfill M4 requirements:
Maximum leg elongation velocity
1 mm/s
Lateral displacement absolute accuracy
23 micron
Lateral displacement resolution
1.1 micron
Maximum tip-tilt velocity
10 arcsec/s
Absolute tip-tilt accuracy
0.5 arcsec
Tip-tilt resolution
0.27 arcsec
Courtesy of Adoptica

5. Wavefront corrector
Correct for both low and high spatial frequencies wavefront errors
Stroke budget includes:
Quasi static term(<1Hz) for misalignment errors due to gravity
Term for wavefront errors due to wind load on the telescope structure, M1, M2 (tip, tilt, focus, coma, astigmatism)
Stroke for atmospheric disturbances (15% of total stroke)
Stroke for manufacturing, gravity and thermal effects (35%)
TOTAL STROKE BUDGET: 140 micron
50%
A quasi static stroke budget needed to correct at low frequency (<1Hz) wavefront errors due to misalignment of the different mirrors with gravity. A budget of 10 micron PtV wavefront has been foreseen to correct for the first 20 Zernike quasi-static aberrations

6. AO specifications Temporal WFE <60nm rms
Best
Median
Bad
Worst
Seeing
0.5arcsec
0.85arcsec
1.1 arcsec
2.5 arcsec Lo
25 m
50 m
100 m
Tau o
0.7msec
2.5msec
1.5msec
Fitting nm rms
120
145
180
0.5’’ FWHM
All OA spec in term of fitting include quasi stratic errors and stroke for wind load as well as cophasing errors
Temporal WFE <60nm rms
-3dB Closed loop bandwidth > 400Hz
Segment cophasing

7. Additional features Thermal control:
optical surface within [-0.5,+1]ºC
Any other external surface within [-1.5,1.5]ºC
Diagnostics
Maintainability
Safety functions:
Earthquake detection

8. Current design characteristics
5190 actuators (4326 in pupil)
6 segment shells ~2mm thick
Light-weighted structural reference body
A “mirror cell” with load spreaders
Hexapod for tip-tilt and decentering
A rotator for Nasmyth selection
Courtesy of Adoptica

9. Shells
Each segment has comparable size as DSM (1 m radial direction, 1.2m on other direction)
Av. thickness tolerance incl wedge :+-15 micron
Local error:10 micron PtV wedge removed
31.5 mm triangular actuator pattern
Residual optical error after fitting 14nm rms WF (goal 8nm rm WF)
24 membranes for lateral restraint

10. Brick concept Self standing Line Replaceable Unit
Modularity: three types of bricks with respectively 15, 28, 36 actuators
Voice coil motors
Mounting structure + cooling plate for actuators & electronics
Electronics:
Capacitive sensor board
Voice coil driver board
Power + logic on a fin identical for all brick types
Tool for alignment and fixation on the reference body structure
Courtesy of Adoptica

11. Actuators and local sensors
Updated voice coil motor more compact: 36mm long and 15 mm diameter screwed on the cold plate
Same contactless technology but enhanced design for more reliable actuators
New capacitive sensor armature design, signal pick-up strategy and contacting to electronics boards to overcome the problems seen on current units (major source of non working actuators)

12. Reference body Triangular structure ~2500 m diameter
Conical central hole for optical beam clearance
Structure overall thickness 300mm with smaller ribs 100mm thick.
~ 25mm thick front face
Two materials currently traded-off: Zerodur and SiC
Reference body holds 180 bricks (about 2 kg each)
Courtesy of Adoptica

13. Demonstration Prototype upgrade
800 mm long, 454mm large
Using same shell as phase B DP
New reference body following M4 design
10 pre-production bricks 15mm thick with 28 actuators (some partially covered by shells)
Liquefied gas used for cooling
Compliant with real time, control, safety and timing interface requirements
with a central drilled hole to pass the wires and more reliable design
Bias magnet at the center to hold the shell when the coil is off.
Courtesy of Adoptica

14. DP objectives Design validation: System performance:
Voice coil actuator design,
Capacitive sensor armature,
Capacitive sensor pick-up,
Shell edge controllability,
Cophasing stability,
Cooling plant efficiency,
Power dissipation,
SW ad HW safety features
Design validation:
Brick concept,
Brick interfaces,
Reference body material,
Control aspects,
Cooling plant design

15. Project timeline
DP Reference body material selection: mid September 13
DP Start of procurement: Oct 13
Sub-systems assembly readiness review: Dec 13
DP Electromechanical Testing: late Spring 14
DP Optical Testing: Summer 14
Preliminary design review: September 14