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Thin glass sheets for innovative
mirrors in astronomical
applications
by
Rodolfo Canestrari
INAF-Astronomical Observatory of Brera
Supervisors: Dr. Mauro Ghigo
Dr. Giovanni Pareschi
Como, July 9th 2010
Part I
Thin glass mirror shells for
adaptive optics
Outline of the talk – Part I
- ELT telescopes
- Conventional technique for ASM shells production
- Hot slumping concept
- Considerations on critical aspects for a slumping procedure
- Thermal characterization: oven, thermal cycle and muffle
- Materials choice and procurement
- The optical test bench
- Set-up of a suitable slumping procedure
- Some examples and results
- Final remarks
Future optical telescopes, ELT class
E-ELT (2018)
Primary mirror:
42m diam, 908 act. segments
Secondary mirror:
6m diam, monolithic
GMT (2018)
Primary mirror:
25m diam, 7act. segments
Secondary mirror:
3.2m diam, 7 segments
TMT (2018)
Primary mirror:
30m diam, 738 act. segments
Secondary mirror:
3.6m diam, adaptive
E-ELT former optical design
E-ELT site: Cerro Armazones, Chile
GMT site: Cerro Las Campanas, Chile
TMT site: Mauna Kea, Hawaii
E-ELT Gregorian design
M2: 4.8 m segmented
concave deformable mirror,
built-in adaptive optic
Conventional technique for ASM production
1) Grind mating surfaces
to matching curvature
2) Temporary bond upper
meniscus to lower blocking body
3) Grind meniscus to 1.6 mm
thickness and polish
LBT (2 units):
 = 911 mm
t = 1.6 mm
911mm
Pre-integration of final unit
Hot slumping concept for thin glass shells
Heater elements
Heater elements
1: Borofloat sheet and mould
2: Hot
Slumping
Borofloat glass
Oven
mould
3:
Vacuum-tight
muffle
Metrology by interferometry (astatic support)
Ghigo et al. – 6691-0K SPIE 2007
Canestrari et al. – 7015-3S SPIE 2008
Ghigo et al. – 7439-0M SPIE 2009
This study has been performed in
INAF-OAB (Italy) for the
manufacturing of thin shells for
adaptive optics
ESO E-ELT FP6 R&D program
Considerations on critical aspects
 Thermal characterization of the oven, thermal cycle and muffle
 Material choice and procurement
 The optical test bench
 Set-up of a suitable slumping procedure
Considerations on critical aspects:
Thermal characterization of the oven
Thermocouple sensors disposition inside the small oven
3
6
1
1
4
4
2
5
5
Measured and simulated thermal
behavior of the small oven
Thermal model of the entire system:
oven + muffle + mould
Considerations on critical aspects:
Thermal cycle
The Graphic User Interface of the
software for the remote control of the
oven.
Possibility to control and check the
oven’s status through the web.
Alerts on errors sent by mail, Skype and
SMS.
It performs better than Swift! 
Thermal cycle adopted for the
slumping experiments.
Six main phases are clearly
visible.
For the slumping of a 0.5 m diameter glass shell the thermal cycle is of about 60 hours.
Considerations on critical aspects:
The muffle
Stainless steel AISI 310
Weight = 190 kg
External Diameter = 816 mm
Height = 516 mm
Vacuum seal at about 650 °C
Considerations on critical aspects:
Materials choice and procurement
The choice of the materials (mould + glass) must take into account a large
number of properties that shall be properly weighted in a merit function to
reach an acceptable trade-off:
 Mechanical: Young’s modulus, hardness
 Physical: CTE and CTE homogeneity, thermal conductivity, density,
glass adhesion, transparency
 Structural: voids-inclusions, high temperature stability
 Fabrication: machinability, polishability, optical microroughness,
characterization
 General: availability, scalability, costs
Property
Elastic Modulus (Gpa)
Knoop Hardness (HK 0.1/20)
CTE (RT to 1000°C)
CTE homogeneity
HP Alumina
HP SiC
Quartz Technical
Zerodur K20
90
476
72
83
1900
3000
580
620
8.2 *10-6K-1
4.0 *10-6K-1
0.5 *10-6K-1
2.0 *10-6K-1
Good
Good
Probably not
very good
Very good
=0.004*10-6K-1
(quantitative data not available)
Thermal Conduct. (W/mK)
=0.01*10-6K-1
24
102
1.31
1.60
Density (g/cm3)
2.85
3.21
2.21
2.53
Glass adhesion (from tests
performed)
YES
YES
YES
NO
NO-White
NO-Black
YES
NO-White
NO
NO
Possible
NO
Very good
Very good
Very good
Very good
1900 °C
1450 °C
1200 °C
850 °C
In green body
In green body
Very good
Good
Slow
Very slow
Fast
Slower than
quartz
Microroughness
10-20 Å
<5Å
<5Å
<10 Å
Mould
characterization
3D machine + Patch map
3D machine +
Patch map
Interferometer
surface map
3D machine +
Patch map
Transparency
Voids, inclusions
High temperature stability
(cycles)
Max application temperature
Machinability
Polishability / figuring
Material availability
Many producers
Many producers Many producers
Scalability to 1.5 m
Sectors brazing
Sectors brazing
Difficult
 60 K€
 100 K€
 60 K€
Mould Cost ( 0.7m)
Only Schott
Several meters
 70 K€
Considerations on critical aspects:
Materials choice and procurement
CTE mismatch between mould and glass:
R(T)=R0(1+a* T)
CTE homogeneity of the mould:
0.1 mm/m K  7 mm at @ Tslump
Glass sheet face-to-face thermal gradient:
dF=-2*(F2/t)*aT(Tback-Tfront)
Temperature gradients must be avoided!
Considerations on critical aspects:
Materials choice and procurement
From tests performed:
-Alumina, Quartz and SiC show
sticking to the glass
Alumina moulds
-K20 doesn’t stick up to 660°C
Quartz moulds
SiC moulds
Considerations on critical aspects:
Materials choice and procurement
A good trade-off is the Zerodur K20
for the mould coupled with the
Borofloat 33 glass, both from Schott
(Germany)
It offers:
 CTE near to that of the Borofloat 33
 A very high CTE homogeneity, a very important parameter
 No sticking attitude to the glass. Doesn’t need antisticking layer for the
temperatures used up to now in the experiments
 The scalability is not a problem
 The characterization of a non transparent mould using a 3D machine + a spherical
master is a well known and trusted technique
 The cost of the finished mould is not far from those made in the other materials
(except for the Silicon Carbide)
Considerations on critical aspects:
Materials choice and procurement
Considerations on critical aspects:
The optical test bench
Canestrari et al. – SPIE Proc. 7015-3S
This support use an air cushion to sustain the shell weight
during the measurement, a number of load cells provide the
fixed points. The gap between the edge of the glass and the
wall of the support is sealed with a ferrofluid, it’s
maintained in the proper position by a magnetic strip. The
air is injected in the bottom cavity until the readout from the
load cells reach predetermined values. Then the glass shape
is interferometrically measured.
Magnetic
strip
Air inlet
Load cell
Actuator
Fixed point
Considerations on critical aspects:
Set-up of a suitable slumping procedure
Deep cleaning
and
paint peel-off
Dressing with white
coat, hair cap, shoes
cover, face mask, gloves
Considerations on critical aspects:
Set-up of a suitable slumping procedure
glass sheet
Stacking of glass and
mould after the paint is
been peeled off
The slumping Crew
muffle inside the oven
Considerations on critical aspects:
Set-up of a suitable slumping procedure
Some examples and results
- Without vacuum
- Without deep cleaning
- Without pressure
- Presence of dust contamination
- Very irregular pattern of fringes
- Not slumped at the edge
- With vacuum
- With deep cleaning
- With pressure
- No dust contamination
- Very circular pattern of fringes
- Slumped also at the edge
- Full copy of the mould
Some examples and results
Interferometric measurement of a slumped glass shell having
radius of curvature of 4000 mm, diameter of 130 mm and 2 mm thickness
Fringes between mould and glass:
very circular and
without dust contamination
218 nm rms → l/3 rms over 130 mm diam.
Some examples and results
Same sample but measured on 80 mm diameter
In all the segments till now
slumped it is visible a pattern of
features that repeat itself with a
good approximation indicating
that the opposite of this pattern
is very likely also present on
the small mould used for these
tests.
The process is able to deliver
good copies of the mould and
the results till here obtained are
limited from the quality of the
mould optical surface and not
from the slumped segments,
that limit themselves to copy its
surface.
57 nm rms → l/11 rms over 80 mm diam.
Some examples and results
Scaling up to diameters of 500 mm a number of preliminary slumping experiments
has been done in order to optimize the thermal cycle.
The use of a shorter thermal cycle,
with a faster cooling provided a glass
shell having stresses
With longer thermal cycles (longer
soaking and cooling times) very few
interference fringes were visible
Some examples and results
Interferometric measurement of a slumped glass shell having
radius of curvature of 5000 mm, diameter of 500 mm and 1.7 mm thickness
The tuning of parameters for the scaled-up procedure is not an easy job, but the results are very encouraging
Glass shell on the mould and
under sodium light.
5 fringes  1.5 mm PV  3’’ of
figure error from the mould
The glass shell went broken
during optical tests…
ask Mauro for further details 
What’s a pity!
Some examples and results
Interferometric measurement of a slumped glass shell having
radius of curvature of 10000 mm, diameter of 500 mm and 1.7 mm thickness
The tuning of parameters for the scaled-up procedure is not an easy job, but the results are very encouraging
254 nm rms → l/2.5 rms over 250 mm diam.
Final remarks
- We have proposed and developed a technique based on the
concept of the replication of a master.
- This technique is based on the hot slumping of thin glass sheets.
- This technique is able to deliver low cost and large deformable
mirrors with a fast production time.
- This technique is able to deliver copies of the master within
optical quality, right now mostly limited by the master’s quality!
- More developments are needed and better results in terms of
fidelity (of the copy) can be achieved.
Part II
Composite glass mirror panels
for making IACT reflectors
Outline of the talk – Part II
- What is a Cherenkov telescopes?
- The MAGIC Telescopes
- CTA: the Cherenkov Telescopes Array
- Mirrors requirements
- The cold glass slumping technique: MAGIC II mirror panels:
- concept, developments, results and production
- The cold glass slumping technique: toward CTA:
- concept, developments and preliminary results
- Final remarks
What is a Cherenkov telescope?
Near UV light concentrator
(300-600 nm) emitted by
air Cherenkov effect from
Very High Energy “events”
(100GeV-10TeV).
Analysis of the shower’s
image in the camera:
- g/hadron separation;
- incoming direction;
- energy of primary photon
Collecting area > 100 m2
Angular resolution: some arcmin
Sensibility ~1/100 Crab
(2x10-13 ph cm-2 s-1 @ 1 TeV)
VERITAS
H.E.S.S.
MAGIC
CANGAROO
The MAGIC telescopes
The twins MAGIC telescopes – La Palma (Canary Islands) – 2200 m asl
FoV: 3 deg
Area: 240 m2
: 17 m
Focal length: 17 m
CTA: the Cherenkov Telescope Array
- Low-Energy section ~20m telescopes
4 - 6° FoV
0.08 - 0.12° pixels
Parabolic/Hybrid f/D~1.2
- Core-Energy array 12m telescopes
7 - 8° FoV
0.16 - 0.18° pixels
Hybrid f/D =1.35
- High-Energy section 4-7 m telescopes
8 - 10° FoV
0.2 - 0.3° pixels
DC or SO f/D 0.5-1.7
Main features:
-Enhance the sensibility of a factor
10 (up to 1 mCrab);
-Improve the angular resolution;
Positively evaluated, Preparatory Phase funded:
-Wider energy coverage (10GeV100TeV);
-Flexibility;
FP7-INFRA-2010-2.2.10: CTA (Cherenkov
Telescope Array for Gamma-ray astronomy)
-Observatory infrastructure
Mirrors requirements for CTA
STATE OF THE ART
CTA
Collecting area
about 400 m2
about 10000 m2
Mirror-segment/Area
0.3 – 1 m2
1 – 2 m2
Cost/m2
2 – 3.5 k€
1.5 – 2 k€
Weight/m2
20 – 40 kg/m2
10 – 25 kg/m2
Expected life
Few years
10 years
CTA will need more, larger, cheaper, lighter and
long lasting mirrors!
Cherenkov vs Optical
• Primary mirror diameter: 42 m
• Number of panels: 900
• Mass-to-Area: 70 kg / m2
• Cost-to-Area: ~ 100-300 k€ / m2
• Panel angular resolution: 0.1 arcsec
• Production time: 90 m2 / year
Cold Glass Slumping technique: MAGIC II
-concept-
Front glass sheet
Back glass sheet
Al honeycomb core
PVC coating
Al + SiO2
Cold Glass Slumping technique: MAGIC II
-development-
Cold Glass Slumping technique: MAGIC II
-resultsAluminum master
Typical mirror segment
(The color palette is inverted on this surface)
Points: 392
Points: 392
P-V: 21.5 mm
P-V: 62.3 mm
RMS: 4.6 mm
RMS: 15.3 mm
Cold Glass Slumping technique: MAGIC II
-resultsLegend:
 Horizontal lines: intrinsic of the
float glass sheet
 Vertical lines: deriving from the
honeycomb structure
 Dots: from dust specks trapped
between glass and honeycomb
 Shadows: deriving from the copy
of defects of the master shape
Cold Glass Slumping technique: MAGIC II
-results-
Dedicated optical bench equipped with:
2 laser sources: - alignment
- measure
1 CCD camera for image acquisition
1 flat folding mirror
Possibility to measure up to 40 m r_curv
Cold Glass Slumping technique: MAGIC II
-resultsPSF measurement of a typical glass mirror panel (~1 m2) performed in La Palma.
Point-like light source at the radius of curvature ~34 m
D80 ~ 15 mm = 0.44 mrad = 1.5 arcmin
Cold Glass Slumping technique: MAGIC II
-production-
Glass
Gluing
Panel
Preparation
Slumping &
Curing
Cold Glass Slumping technique: MAGIC II
-production-
Aluminum master 1040 x 1040 mm
Front and rear of a produced segment
Size = 985 x 985 mm Weight = 9.5 Kg.
Nominal radius= 35 m
Vernani et al. – SPIE Proc. 7018-0V
Pareschi et al. – SPIE Proc. 7018-0W
Cold Glass Slumping technique: MAGIC II
-productionMedia Lario Technologies (Italy) has produced 112 glass mirror panels currently
integrated on the MAGIC II telescope
With a rate of 2 panels per day the project has been successfully completed in
3 months (from March till mid June 2008)
Cold Glass Slumping technique: MAGIC II
-mounting-
Cold Glass Slumping technique: MAGIC II
-mounting-
White protective foil used for protection
of mirror surface, operator’s eyes and for
telescope motion
Cold Glass Slumping technique: MAGIC II
-mounting-
Cold glass slumping technique: toward CTA
-conceptCTA will need more, larger, cheaper, lighter and
long lasting mirrors!
• Use of thinner glass sheets  more flexibility of the front skin  better copy of the
mould (especially in medium frequencies regime);
• Use of a stiffer core structure: honeycomb  glass foam board
• pre-machined spherical shape;
• reduced spring back;
• better CTE match
• Use of cheaper materials:
• honeycomb  glass foam;
• new epoxy glue;
• Reduce the production’s steps where possible, especially if they are critical and/or
manpower consuming such as the sealing of the borders of the panels.
Cold glass slumping technique: toward CTA
-conceptFoam boards assembly
and machining
Master cleaning
Sandwich preparation
Glue curing
Vacuum release
Mirror panel coating
Cold glass slumping technique: toward CTA
-preliminary designInput data:
- Hexagonal shape: 1.2 m face-to-face
- Radius of curvature: about 32 m
- Three support points
Observing condition, loads:
Survival condition, loads:
- gravity
-
gravity
- wind up to 50 km/hrs
-
wind up to 180 km/hrs
-
snow up to 30 cm thick
Acceptance Criteria:
- Maximum slope error: <0.1 mrad
- Maximum weight: 20 kg/m2
- Maximum stress developed: < Yield strength of materials
Cold glass slumping technique: toward CTA
-conceptCurrent panel configuration:
glass skin: 1.2 mm
foam core: 60 mm
glass skin: 1.2 mm
Slope < 0.03 mrad
Peak to Valley < 15 mm
Weight < 15 kg/m2
Cold glass slumping technique: toward CTA
-concept-
3rd principal stress (glass) < 14 MPa
3rd principal stress (foam) < 0.75 MPa
Cold glass slumping technique: toward CTA
-results• Density: r ~ [0.1 - 0.165] g / cm3
• CTE ~ 9 mm · K / m
• Waterproof
• Easily machined
• High compressive strength
• Very cheap for astro applications
Cold glass slumping technique: toward CTA
-resultsTraction machine
Hysol
Cores Uretan
Bacon Industries
Cores Ocean
Hysol
Bacon
Industries
Cores Uretan
Cores Ocean
Adhesion
glass/honeycomb
>3.5 MPa
1.7 MPa
0.22 MPa
2.8 MPa
Adhesion
glass/Foamglas
0.7 MPa
0.45 MPa
0.37 MPa
0.65 MPa
Change in
color
No apparent
ageing
Strong ageing,
possible
changing in
polymerization
Almost no ageing,
very slight change
in color
Medium,
temp curing
Very difficult,
High temp
curing
Not good,
room temp
curing
Very good,
room temp curing
200 Euro
300 Euro
20 Euro
25 Euro
UV (simulated
about 6 months of
continuous
exposition to UV)
Easiness of
application
Costs (for 1 kg)
Tests were performed
in collaboration with
Politecnico di Milano
Cold glass slumping technique: toward CTA
-results-
Cold glass slumping technique: toward CTA
-resultsD90 = 1.2 mrad
Size: 600 x 600 x 40 mm
Radius of curvature: 35.8 m
Weight: 4.5 kg  ~ 12 kg/m2
Size: 600 x 600 x 40 mm
Cold glass slumping technique: toward CTA
-results-
As manufactured
No measurable
changes in PSF were
observed after few
thermal cycles
After cycle #1
After cycle #2
Final remarks
- We have proposed and developed a technique based on the
concept of the replication of a master.
- This technique is based on the cold slumping of thin glass sheets.
- This technique is able to deliver low cost and large stiff mirrors
with a fast production time.
- This technique is able to deliver copies of the master with very
good precision, typically with a factor 3 in shape accuracy.
Final remarks
- More developments are needed toward CTA: increase the
performances and lower the costs
- better PSF
- radius of curvature
- temperature stability
- Investigation of new materials started and in progress:
- glass foam and low cost glues
- more tests have been just scheduled
- In about a year, the first CTA telescope prototype will be equipped
with these mirrors
Part III
Conclusions of the
conclusions
(or the start of a new one)
What about segmented primary mirrors?
STEP 1:
Hot slumping concept of thin glass sheet
Heater elements
Heater elements
Thermal cycle
Vacuum-tight
muffle
Derived from ESO E-ELT FP6 R&D program
STEP 2:
Sandwiching concept
vacuum suction
STEP 1
Ghigo et al. – 6691-0K SPIE 2007
Canestrari et al. – 7015-3S SPIE 2008
Ghigo et al. – 7439-0M SPIE 2009
STEP 2
Canestrari et al. – 7018-0D SPIE 2008
Canestrari et al. – 7437-11 SPIE 2009
rear glass sheet
glue
mirror panel release
slumped glass sheet
foam core
mould
Stiffening of slumped thin glass shells
Interferometric
setup
Mirror panel during the glue curing
Close view of the mirror panel
Stiffening of slumped thin glass shells
Using an air suction it is possible to remove any
difference in radius of curvature (between slumped
shell and mould) due to CTE mismatch at the
slumping temperature
310 nm rms → l/2 rms over 110 mm diam.
Stiffening of slumped thin glass shells
Hot slumped glass on the mould
Foam boards assembly
Curing of the glue
Mirror panel after the release
Stiffening of slumped thin glass shells
Sun image
Size: 500 x 40 mm
Radius of curvature: 9.85 m
Weight: 2.5 kg  ~ 12 kg/m2
Focal spot: d ~ 4 mm
Mirror image of a filament lamp
Stiffening of slumped thin glass shells
Figure error of ~1.5 l