Adaptive telescope mirror
developments in Arcetri
A. Riccardi
Adaptive secondary concept

Concept: Substitution of conventional M2 telescope mirror with a thin (1.52.0mm) deformable shells controlled in position with large-stroke (~0.1mm)
electromagnetic (voice-coil like) force actuators and using internal capacitive
sensors as position feedback
Electronics boxes
Central membrane
for lateral support
Heat-sink and act. Reference plate
support plate
deformable shell
Control electronics
To the AO Diagnostic communication link
supervisor
400Mbit/s
Gigabit Ethernet Switch
Daisy chain connection
Real time
comm link
2.9 Gbit/s
(MMT160Mb/s)
Communication Board
(1x backplane)
Communication Board
(1x backplane)
Reference Signal
Generator Board (1x backplane)
Communication Board
(1x backplane)
Reference Signal
Generator Board (1x backplane)
Reference Signal
Generator Board (1x backplane)
DSP control Board (14x backplane)
DSP control Board (14x backplane)
DSP control Board (14x backplane)
± 48V, 35 A
Power
DSP control Board (14x backplane)
DSP control Board (14x backplane)
Total computational power:
78 Gmac/s (32bit fp)
Real-time reconstructor on-board
WFS: 30x30 => 34-47ms (z-m)
Slope comm time: 20ms
Communication Board
(1x backplane)
Reference Signal
Generator Board (1x backplane)
DSP control Board (14x backplane)
Communication Board
(1x backplane)
Communication Board
(1x backplane)
Reference Signal
Generator Board (1x backplane)
Reference Signal
Generator Board (1x backplane)
DSP control Board (14x backplane)
DSP control Board (14x backplane)
Liquid cooled crates,
each comprehending 2 backplanes (3x)
Distribution boards
Actuators
Coil
Thin mirror
Gap
Reference signal
3 cooled electronics boxes
2 crates/box
84 custom DSP boards
2 DSP/board - 8 acts/board
32-bit floating-point 470Mmac/s
(MMT: 16-bit integer 40Mmac/s)
Why and adaptive telescope mirror?
Adaptive
Conventional
Secondary
Secondary
Adaptive
Secondary
Less warm
surfaces
WFS
TTM
BS
DM
Coll.
Sci. Camera
K band: 2-2.6
shorter exp.
time
(MLH, PASP)
WFS
Sci. Camera
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-Steward
MG-ADS contract CAAO
LBT Adaptive Secondary
(integration phase, on-sky 2008)
Magellan Adaptive Secondary
(copy of LBT, on-sky ???)
VLT Adaptive Secondary
(design phase, on-sky 2015)
INAF under OPTICON-JRA1
MG-ADS subcontract with ESO
TEC0-TEC1: EU funded under
the ELT-DS project
MG-ADS subcontract INAF
M4-ARU EELT: ESO funded development
MG-ADS proposal for contract
INAF subcontract of MG-ADS
MMT on sky
mV 8.0 (B0V)
Credits:
http://athene.as.arizona.edu/~lclose/AOPRESS/
Existing adaptive mirror in hardware
MMT:
336 act
640mm diam
2.0mm thick
31 mm/act
(Jan 2003)
INAF, Steward Obs, Microgate Srl, ADS Int. Srl
LBT (2 units):
672 act
911mm diam
1.6mm thick
31 mm/act
(in production)
P45proto
Integration of final unit
640mm
911mm
LBT integration progress
Next ASM generation: VLT-DSM
VLT-DSM
VLT-DSM
1.1m
1170 act.
 29 mm pitch
 1 ms response
ESO, Microgate Srl, ADS Int. Srl, INAF
Current technology: a comparison
Current technology: a comparison
Current limitation in BW (or stroke)
Capsens-coil crosstalk
Currently it limits derivative gain
Some level of natural damping is still req.
70um gap: 0.7-0.9ms settling time
100um gap: 1.2-1.7ms settling time
Item to solve in the commissioned studies
especially for TEC0 (larger mass, larger gap,
larger derivative gain required)
CL Actuator transfer
7kHz
function (with deriv gain=0)
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


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50-100mm actuator pitch (2000-500 acts)
200mm PtV stroke (±2as on-sky for 42m telesc.) (300mm goal)
with high efficiency actuator prototype
DM-TEC 1: tweeter corrector, low stroke field stabilizer



corrector25-30mm actuator pitch (7800-5400 acts)
100mm PtV stroke (±1as on-sky for 42m telesc.) (200mm 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:



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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