Adaptive telescope mirror developments in Arcetri A. Riccardi

Adaptive telescope mirror developments in Arcetri

A. Riccardi

Adaptive secondary concept

Reference plateHeat-sink and act.support plate

Electronics boxes

deformable shell

Concept: Substitution of conventional M2 telescope mirror with a thin (1.5-2.0mm) deformable shells controlled in position with large-stroke (~0.1mm) electromagnetic (voice-coil like) force actuators and using internal capacitive sensors as position feedback

Central membranefor lateral support

Control electronics

Communication Board (1x backplane)

DSP control Board (14x backplane)

Reference Signal Generator Board (1x backplane)
















Liquid cooled crates, each comprehending 2 backplanes (3x)

Distribution boards

Actuators

Gap

Thin mirrorReference signal

Power

± 48V, 35 A

Real timecomm link2.9 Gbit/s(MMT160Mb/s)

Daisy chain connection



Coil

3 cooled electronics boxes2 crates/box84 custom DSP boards2 DSP/board - 8 acts/board32-bit floating-point 470Mmac/s(MMT: 16-bit integer 40Mmac/s)

Gigabit Ethernet SwitchDiagnostic communication linkTo the AOsupervisor 400Mbit/s

Total computational power:78 Gmac/s (32bit fp)Real-time reconstructor on-boardWFS: 30x30 => 34-47s (z-m)Slope comm time: 20s

Why and adaptive telescope mirror?

WFS

Sci. Camera

DM

Coll.

TTMBS

ConventionalSecondary

AdaptiveSecondary

Sci. Camera

WFS

Less warmsurfaces

K band: 2-2.6shorter exp.time(MLH, PASP)

AdaptiveSecondary

Advantages

Un solo correttore per tutti i fuochi (es. LBT: 4 fuochi/pup)Maggiore riflettività (5 riflessioni eliminate: 0.985=0.90)Minore emissività IR (1/3-1/4 exp.time K-N bg-limited)Compattezza della parte di sensing (maggiore stabilità)Attuatori elettromagnetici con feedback capacitivo:

• Grande stroke: TTM+DM+chopper+FS in un’unica unità• Unità robusta rispetto malfunzionamento di attuatori

Tecnologia estendibile a specchi adattivi per ELTs• Grande stroke (wind bufferting)• >104att., grande numero attuatori grandi specchi adatt.

Overview of developments

MMT Adaptive Secondary(on sky 2000)

Joint venture OAA-StewardMG-ADS contract CAAO

LBT Adaptive Secondary(integration phase, on-sky 2008)

VLT Adaptive Secondary(design phase, on-sky 2015)INAF under OPTICON-JRA1MG-ADS subcontract with ESO

TEC0-TEC1: EU funded underthe ELT-DS projectMG-ADS subcontract INAF

M4-ARU EELT: ESO funded developmentMG-ADS proposal for contractINAF subcontract of MG-ADS

Magellan Adaptive Secondary(copy of LBT, on-sky ???)

MMT on sky

mV 8.0 (B0V)

Credits:http://athene.as.arizona.edu/~lclose/AOPRESS/

Existing adaptive mirror in hardware

MMT:336 act640mm diam2.0mm thick31 mm/act (Jan 2003)

640mm

P45protoP45proto

LBT (2 units):672 act911mm diam1.6mm thick31 mm/act(in production)

Integration of final unitIntegration of final unit

911mm

INAF, Steward Obs, Microgate Srl, ADS Int. Srl

LBT integration progress

Next ASM generation: VLT-DSM

VLT-DSM1.1m1170 act. 29 mm pitch 1 ms response

VLT-DSMVLT-DSM

ESO, Microgate Srl, ADS Int. Srl, INAF

Current technology: a comparison

Current technology: a comparison

Current limitation in BW (or stroke)

7kHz

Capsens-coil crosstalkCurrently it limits derivative gainSome level of natural damping is still req.70um gap: 0.7-0.9ms settling time100um gap: 1.2-1.7ms settling time

CL Actuator transferfunction (with deriv gain=0)

Item to solve in the commissioned studiesespecially for TEC0 (larger mass, larger gap, larger derivative gain required)

In case of glass, keeping constant g-quilting:Mass per actuator: ~6.5g (LBT,TEC1-30mm) : ~350g (TEC0-100mm) : ~30g (TEC0-50mm)

Noise vs gap (i.e. stroke)

40um gap 70um gap

130um gap

TEC0 and TEC1 target

Feasibility study of 2.5m DM with actuator spacing of: DM-TEC 0: woofer corrector, medium stroke field stabilizer

50-100mm actuator pitch (2000-500 acts) 200m PtV stroke (±2as on-sky for 42m telesc.) (300m goal) with high efficiency actuator prototype

DM-TEC 1: tweeter corrector, low stroke field stabilizer corrector25-30mm actuator pitch (7800-5400 acts) 100m PtV stroke (±1as on-sky for 42m telesc.) (200m goal) with scale down prototype (~100act)

Towards an Adaptive ELT

Part of the technological solutions currently used cannot directly transferred to Adaptive ELT, in particular:

Production of optical flat/concave/convex thin (~2mm) glass shell with diam>1m is not proven. See FP6 studies by SESO and INAF-Brera.See studies with other material like CFRP

Lateral support from central membrain could induce too large stresses (diam) on glass and for membrane buckling. Alternative lateral support shall be studied also to avoid holes in case of segments

Current reference+cold plate scheme is not applicable for large mirrors: More favorable stiffness-to-mass ratio backplate will be studied.

Larger stroke required (70um -> 100-300um): larger dynamical range capacitive sensor shall be studied with reduced noise at large gap. Crosstalk has to be reduced to increase electronic damping (large gap with large BW).

Comparison among alternative materials

CFPR by “Composite Mirror applications”

Tucson, AZ

Documents

Adaptive telescope mirror developments in Arcetri A. Riccardi