New design and new challenge for space large
ultra-lightweight and stable Zerodur mirror for
future high resolution observation instruments
paper nº66418
83230918-DOC-TAS-FR-001
ICSO-2014
07/04/2015
Ref.:
Summary
2
Context of future High Resolution Instrument development
Lightweight primary mirror new concept design driver
Lightweight design of Assembled Zerodur Mirror &
breadboard results
Lightweight design of Hybrid Sandwich Zerodur Mirror &
breadboard results
Conclusion
07/04/2015
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Context of future High Resolution Instrument development
Future trend for earth or astronomic observation are toward higher spatial
3
resolution and survey areas:
Larger M1 mirror with a M2 far from M1
Mass of space telescopes/ price competitive launcher : strong design
driver
Satellite more agile leading to quick thermal environment change.
Compaction of telescope
a faster telescope inducing a more sensitive
optical scheme v.s mirror position stability
Mirror mass reduction
more sensitive mirror to environment WFE impact
The ideal solution : Space telescope with adaptive optics to reduce mirror WFE
and positioning stability requirements, both correctable thanks to mechanisms and
sensors.
Space adaptive optics architecture & problematic are studied since
years by TAS to specify the different telescope elements
07/04/2015
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Primary mirror requirements for an adaptive high resolution
space instrument
4
Adaptive optics relax optical requirements of M1 & deformable mirror in
intermediate pupil correct M1 WFE :
M1 WFE in flight relaxed but limited :
DM performances (stroke, Zernike coefficient corrected, residual of the
correction, uncertainties of the measurement, period of correction )
High frequencies terms shall remain low
In flight stability shall remain very high between correction period
M1 WFE on ground relaxed to limit of demonstration of the performance to
be achieved ( MTF on ground measurement vs MTF in flight engagement )
Mirror shall still sustain mechanical loads, requested eigenfrequencies
and space environment ;
To optimize such a mirror it is necessary to :
take into account actual performance of deformable mirror
be very stable between two corrections
07/04/2015
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Primary mirror requirements for an adaptive high resolution
space instrument
5
TAS with TSESO & LAM develop in parallel a space deformable mirror
(SDM) ensuring reliability, stability between period of correction and
sustaining launch loads
M1 mirror shall be designed taken into account the demonstrated
performance of such SDM
SDM Madras zernike modes
M1 mirror design optimization is directly coupled to SDM achievable
measured correction performances
and reliable validated performances
07/04/2015
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Primary mirror requirements for an adaptive high resolution
space instrument
6
Earth observation satellites are very agile, pointing sceneries of interest
and anti sun direction to recharge battery
M1 is exposed to very high thermal flux variation along and between orbits
In orbit calibration requires images of good quality and enough spatial
frame, WFE adjustment shall be robust and guaranty full operational with
availability to take operational images at any time
M1 WFE & focus stability in orbit remain a key issue even for
adaptive telescope
07/04/2015
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ni divulgué à un tiers sans l'accord préalable et écrit de Thales Alenia Space - © 2012, Thales Alenia Space
LIGHTWEIGHT PRIMARY MIRROR NEW CONCEPTS
7
On highly lightweight large mirrors, thin section limit thermal conductivity
heat transfer is mostly radiative in orbit and quick changing ensuring no
conductive thermal equilibrium
CTE dominates, thermal conductivity offers low influence on mirror
stability.
Zerodur is far more stable for mirrors than others :
CTE of selected procurement of Zerodur® (≈0,08 10-8 ) is 314 times
lower than SiC
No need of costly and added mass thermal control
No correction in orbit due to thermo-elastic instabilities
Zerodur strength and safe sizing factor recently tested and reworked
ensuring optimised dimensionning of Zerodur
Zerodur mirror mass can now be strongly reduced
07/04/2015
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ni divulgué à un tiers sans l'accord préalable et écrit de Thales Alenia Space - © 2012, Thales Alenia Space
LIGHTWEIGHT PRIMARY MIRROR NEW CONCEPTS
Adaptive optics allows low spatial frequencies terms and long term instabilities8 :
Zerodur with new assemblies concept reinforcing its specific stiffness is therefore the
ideal solution :
No polishing quilting ( HF terms avoided ) ( polishing pressure 3 time less than Sic
and optimised coupled polishing lightweighting sequences )
No short term instabilities
Assembled Z Mirror 25 Kg/m2
from 1,5 to 2 meters
Hybrid sandwich Z Mirror 17 Kg/m2
from 2 to 4 meters
TAS&TSESO develop 2 new generation designs ensuring a strong
breakthrough in size and surfacic mass of space Zerodur mirrors
07/04/2015
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ni divulgué à un tiers sans l'accord préalable et écrit de Thales Alenia Space - © 2012, Thales Alenia Space
Assembled Zerodur Mirror design
ISM fixed at 2/3 of diameter
optimization in
strength and stiffness
Optimized skin thickness to avoid HF terms (
polishing, coating )
Zerodur rear plate linking the 3 ISM I/F
optimization in stiffness
Allow to reduce stiffeners
9
Rear Plate
Reduce machining time and risk
Ensure mass saving
Secondary ribs with evolutive shape
stiffness and low mass
Local reinforcement for ISM bonding
ensure strength
ensure
Evolutive height
of stiffeners
ISM I/F
reinforcement
07/04/2015
Ref.:
Equipped Mirror of 1,5 m weight less than 45 Kg ( 25 Kg/m2)
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Assembled Zerodur Mirror design & performances
Assembled Zerodur Mirror made of 3 parts :
10
Sizing & Optimizing performed thank to very detailed FEM models modelizing in 3 D
bonding elements and all radius between skin and ribs (1 million nodes)
Mirror computed under 15 QSL, sinus up to 125 Hz & acoustic, shows
positive margins & modes above 113 Hz
07/04/2015
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Assembled Zerodur Mirror design & performances
11
Mirror WFE computed thanks to detailed FEM :
SFE map under gravity : 50
nm Rms , on ground
corrected by Madras : allow a
safe characterization of MTF
SFE map due to coating :
effect now anticipated by FEM
prediction & correction during
polishing : residual terms are
corrected by Madras
SFE maps under deformation loads
lead up to 16 nm but only 4 nm after
correction by Madras SDM
Thermo elastic behaviour computed : leading to only 2 to 3 nm WFE
without need of correction :
Zerodur Mirror polishing behavior and environment impact on WFE
allow to ensure a coupled M1/MADRAS WFE of less than 25 nm Rms
in orbit including HF terms
07/04/2015
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Assembled Zerodur Mirror BREADBOARD RESULTS :
400mm mirror breadboard representative of M1 technological
key elements manufactured, polished & tested.
• Skin bonding qualified ( shear strength and tensile test after
ageing cycling
• Mirror tested under severe environments to check
regularly WFE stability :
10 thermal ageing cycles under humidity [-5°C, +40°C]
1 cycle under vacuum
1 cycle [+20°C,+80°C] to simulate cleaning process
12
Between and after all the tests no WFE evolution has been measured
showing the great stability of this assembled mirror design
07/04/2015
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HYBRID SANDWICH ZERODUR® MIRROR
13
Hybrid sandwich Zerodur mirror combine very high thermo-elastic
stability of Zerodur® and very high stiffness/ mass ratio for very large
mirror (2 to 4 meters) ,
Sandwich mirror is made of 2 Zerodur® shaped thin skins, with an
optimized CFRP honeycomb (high Young modulus and conductive
Fibbers and cyanate resin).
ISM are fixed at 2/3 of diameter on reinforcement
Detailed FEA has allowed mirror optimization and sizing vs launch loads
(QSL 15G, sine & acoustic)
2 meter diameter of such equipped
mirror has a surface mass of only
17 Kg/m2
07/04/2015
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HYBRID SANDWICH ZERODUR® MIRROR
14
FEM model takes into account bonding between core and mirror Skin,
and core equivalent orthotropic properties thank to reduction model
homogenizing methods
CTE, CME of sandwich core & bonding have been computed through
dedicated models and such properties introduced in the large model
Bonding meniscus has been particularly studied and 3 D modelized by
FEM to be taken into account in mirror computed performances
07/04/2015
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Hybrid Sandwich ZERODUR® 2m Mirror Performances
15
On large mirror, WFE gravity on ground is rather large (near 200 nm Rms),
but with low frequency(LF) terms, easily compensable by Madras on
ground
Allow a safe telescope and focal plane integration & alignment.
ISM interface deformation impact on WFE is limited : 9nm Rms before & 3
nm after correction by Madras
Coating impact is limited and homogeneous, WFE after anticipation &
correction by Madras is only 4nm
Thermoelastic behavior in orbit show the great WFE stability, only focus
shall be corrected, one to twice per orbit , this correction being in open
loop mode
07/04/2015
Ref.:
SFE map under gravity
SFE map due to coating
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Hybrid Sandwich ZERODUR® Mirror Performances
16
Moisture release impact WFE : 150 nm Rms, even corrected by Madras
residual is high: 60 nm Rms
WFE measurement under vacuum between
a polishing step allow to determine & subtract moisture WFE map
SFE map of moisture release impact
SFE HF map of moisture release
Hybrid Zerodur® sandwich mirror meet the 25 nm WFE performance
goal for a 2 m optical mirror during all its life in orbit when measuring &
correcting moisture impact during polishing
07/04/2015
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ni divulgué à un tiers sans l'accord préalable et écrit de Thales Alenia Space - © 2012, Thales Alenia Space
Hybrid Sandwich ZERODUR® Mirror breadboard
17
300 mm sandwich Zerodur® mirror with CFRP core has been
manufactured polished & tested
Sandwich samples mechanically tested to valid the strength and CTE
of such assembly
Mirror breadboard polished down to 60 nm Rms showing the
convergence of the polishing
07/04/2015
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Hybrid Sandwich ZERODUR® Mirror breadboard
18
Mirror tested under evolutive moisture condition.
WFE evolution measured with a good return to 60 nm after the test, showing
the long term stability.
As predicted, important WFE due to moisture has been measured :
evolution from 60 nm to 130 nm Rms.
Good correlation between measures and prediction is observed
07/04/2015
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Conclusion
19
New extremely lightweight Zerodur® mirror designs with adaptive optics
meet all future stringent requirements,
Ultra-high thermo-elastic stability Zerodur® mirrors allow a safely way to
implement adaptive optics for high to very high angular resolution
telescopes,
Assembled Zerodur Mirror allows to reduce strongly manufacturing time
and secure the development
ThalesAleniaSpace/ ThalesSESO Team under CNES
manufacture today a 1,5 m assembled Zerodur® mirror
contract
TAS and its partners pave the way to a new generation of very
compact and lightweight telescopes. Unequalled developments in
terms of resolution, image quality/ cost ratio in orbit are in prospect.
07/04/2015
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20
TAS & TSESO thanks CNES for their strong support to the
development of these new mirror generations
Thank you for your attention
07/04/2015
Ref.:
Ce document ne peut être reproduit, modifié, adapté, publié, traduit d'une quelconque façon en tout ou partie,
ni divulgué à un tiers sans l'accord préalable et écrit de Thales Alenia Space - © 2012, Thales Alenia Space