The adaptive secondary mirror. Current technology for MMT/LBT A. Riccardi 1, G. Brusa 1, C. Del Vecchio 1, P. Salinari 1, R. Biasi 2, M. Andrighettoni 2, D. Gallieni 3 1 – Osservatorio Astrofisico di Arcetri, Firenze, Italy 2 – Microgate, Bolzano, Italy 3 – ADS, Lecco, Italy

Osservatorio di Arcetri ESO-INAF cooperation Bologna 11/07/2002 People (MMT336) Osservatorio di Arcetri INAF-OAA (Italy): Concept development. Testing, calibration and diagnostic software ADS (Italy): mechanical engineering, drawing and assembly Microgate (Italy): electronics and DSP software Mirror Lab-SO (USA): optics Current development: LBT adaptive secondaries

Osservatorio di Arcetri ESO-INAF cooperation Bologna 11/07/2002 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 ESO-INAF cooperation Bologna 11/07/2002 MMT336 ASPHERIC SHELL Assembled unit Magnets 642mm diam. 2mm thick (12mm diam) Cap. sensor armatures (ref.plate)

Osservatorio di Arcetri ESO-INAF cooperation Bologna 11/07/ 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 ESO-INAF cooperation Bologna 11/07/2002 Step response Only Prop. - Gain=0.2N/  m (40  m gap) Settling time 1.5ms Command Position

Osservatorio di Arcetri ESO-INAF cooperation Bologna 11/07/2002 Step response

Osservatorio di Arcetri ESO-INAF cooperation Bologna 11/07/2002 Modal analysis (Cap. sensor readings) r 0 =550nm, v=6m/s, 8sec

Osservatorio di Arcetri ESO-INAF cooperation Bologna 11/07/2002 Closed optical loop Open loop Closed 550Hz MMT336+SH 12x12+WF computer Data from G. Brusa (CAAO-Mirror Lab)

Osservatorio di Arcetri ESO-INAF cooperation Bologna 11/07/2002 MMT336 Flattening The flattening of the shell applying 150 mirror modes: 34nm RMS The central rings are artifacts due to the olographic technique used for testing convex mirrors Data taken at Mirror Lab

Osservatorio di Arcetri ESO-INAF cooperation Bologna 11/07/2002 Tests at MMT telescope

Osservatorio di Arcetri ESO-INAF cooperation Bologna 11/07/2002 Tests at MMT telescope

Osservatorio di Arcetri ESO-INAF cooperation Bologna 11/07/2002 Problems at the telescope

Osservatorio di Arcetri ESO-INAF cooperation Bologna 11/07/2002 Tracking and wind effect Wind speed 8m/s, 60s star tracking at 75deg elevation Wind direction

Osservatorio di Arcetri ESO-INAF cooperation Bologna 11/07/2002 MMT336 Schedule The adaptive secondary mirror will be disassembled for the following fixings: Cooling plate Unit shortening Gap protection from dust contamination Refplate galvanic Au plating Next AO run at MMT telescope in less then 6 months

Osservatorio di Arcetri ESO-INAF cooperation Bologna 11/07/2002 LBT672 summary 672 actuators in 14 rings 31mm act-to-act ave dist 642mm 911mm MMT336 LBT672

Osservatorio di Arcetri ESO-INAF cooperation Bologna 11/07/2002 LBT672 electronics 3x2 crates 14 ctrl board per crate (84 tot) 4 380Mflop/s 32bit-FP DSP per board (ADSP-21160) 2 actuators per DSP (336 tot.) Total computational power: 107 Gflop/s Communication to the adaptive unit: 2.5Gbit/s (true data throughput)

Osservatorio di Arcetri ESO-INAF cooperation Bologna 11/07/2002 Scalability: number of acts Considering LBT adaptive secondary electronics performance: Command vector communication+internal loop computations time delay contribution: 22  s/1000acts 5000acts approx 0.1ms delay (<1/10 of WFS sampling time) Current electronics technology allows 5000act per mirror. Larger number of actuators needs mirror segmentation. Density constrain with current technology: density>28-30mm/act (LBT like) High magnification gives poor sapatial sampling

Osservatorio di Arcetri ESO-INAF cooperation Bologna 11/07/2002 Scalability: thickness/spacing Setting a given gravitational quilting  q rms error g t=1.6mm l=31mm (LBT) t=11mm l=100mm

Osservatorio di Arcetri ESO-INAF cooperation Bologna 11/07/2002 Adaptive sec. location OWL Euro50 Ground layer: 4m M2 (31cm) 10km layer: 1.5m M2 (83cm) Ground layer: 2.35m M6 (106cm) 8km layer: 4.2m M5 (60cm) K band J-H band I-J band H-K band

Osservatorio di Arcetri ESO-INAF cooperation Bologna 11/07/2002 Conclusions Adaptive secondary mirror technology is a proven technology that is available and ready for AO correction for ELTs in the NIR Effective correction in V band using non-primary mirrors requires a step up in the number of actuators with the right actuator density that is not available right now Adaptive secondary concept and technology can be applied to primary mirrors to achieve effective correction in V band Warning: delaying a solution for an effective correction in the V band for ELTs could avoid a future upgrade in this direction. Let’s think about a compatible V solution now!

Osservatorio di Arcetri ESO-INAF cooperation Bologna 11/07/2002 Scalability: Forces Rms Force/act to correct turbulence Mass/act to support Stating a given residual fitting error and a given quilting error