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.

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

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

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

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

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

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

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

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

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

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)

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

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

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

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