Thin glass sheets for innovative mirrors in astronomical applications Como, July 9 th 2010 by...

Thin glass sheets for Thin glass sheets for innovative mirrors in innovative mirrors in

astronomical applicationsastronomical applications

Como, July 9th 2010

by

Rodolfo CanestrariRodolfo Canestrari

INAF-Astronomical Observatory of BreraINAF-Astronomical Observatory of Brera

Supervisors: Dr. Mauro Ghigo

Dr. Giovanni Pareschi

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

E-ELT (2018)Primary mirror: 42m diam, 908 act. segmentsSecondary mirror: 6m diam, monolithic

GMT (2018)Primary mirror: 25m diam, 7act. segmentsSecondary mirror: 3.2m diam, 7 segments

TMT (2018)Primary mirror: 30m diam, 738 act. segmentsSecondary mirror: 3.6m diam, adaptive

Future optical telescopes, ELT class

E-ELT Gregorian design

M2: 4.8 m segmented concave deformable mirror, built-in adaptive optic

E-ELT former optical design

E-ELT site: Cerro Armazones, Chile

GMT site: Cerro Las Campanas, Chile

TMT site: Mauna Kea, Hawaii

Conventional technique for ASM production

Pre-integration of final unit

LBT (2 units): = 911 mm = 1.6 mm

911mm

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

Borofloat glass

mould

2: Hot Slumping

OvenVacuum-tight

muffle

Heater elements Heater elements

3: Metrology by interferometry (astatic support)

1: Borofloat sheet and mould

Hot slumping concept for thin glass shells

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

Ghigo et al. – 6691-0K SPIE 2007Canestrari et al. – 7015-3S SPIE 2008

Ghigo et al. – 7439-0M SPIE 2009

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

Considerations on critical aspects:Thermal characterization of the

oven

Measured and simulated thermal behavior of the small oven

1 2

45

6

1

3

45

Thermocouple sensors disposition inside the small oven

Thermal model of the entire system: oven + muffle + mould

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!

For the slumping of a 0.5 m diameter glass shell the thermal cycle is of about 60 hours.

Considerations on critical aspects:Thermal cycle

Thermal cycle adopted for the slumping experiments.

Six main phases are clearly visible.

Stainless steel AISI 310

Weight = 190 kg

External Diameter = 816 mm

Height = 516 mm

Vacuum seal at about 650 °C

Considerations on critical aspects:The muffle

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:

Considerations on critical aspects:Materials choice and procurement

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

850 °C1200 °C1450 °C1900 °CMax application temperature

Very goodVery goodVery goodVery goodHigh temperature stability (cycles)

NOPossibleNONOVoids, inclusions

NO-WhiteYESNO-BlackNO-WhiteTransparency

NOYESYESYESGlass adhesion (from tests performed)

2.532.213.212.85Density (g/cm3)

1.601.3110224Thermal Conduct. (W/mK)

Very good =0.004*10-6K-1

Probably not very good

Good =0.01*10-6K-1

Good (quantitative data not available)

CTE homogeneity

2.0 *10-6K-10.5 *10-6K-14.0 *10-6K-18.2 *10-6K-1CTE (RT to 1000°C)

62058030001900Knoop Hardness (HK 0.1/20)

837247690Elastic Modulus (Gpa)

Zerodur K20Quartz TechnicalHP SiCHP AluminaProperty

70 K€ 60 K€ 100 K€ 60 K€ Mould Cost ( 0.7m)

Several metersDifficult Sectors brazingSectors brazingScalability to 1.5 m

Only SchottMany producersMany producersMany producersMaterial availability

3D machine + Patch map

Interferometer surface map

3D machine + Patch map

3D machine + Patch mapMould characterization

<10 Å< 5 Å< 5 Å10-20 ÅMicroroughness

Slower than quartz

FastVery slowSlowPolishability / figuring

GoodVery goodIn green bodyIn green bodyMachinability

Temperature gradients must be avoided!

R(T)=R0(1+* T)

CTE mismatch between mould and glass:

CTE homogeneity of the mould:

0.1 m/m K 7 m at @ Tslump

F=-2*(F2/)*T(Tback-Tfront)

Glass sheet face-to-face thermal gradient:


Alumina moulds

Quartz moulds

SiC moulds


From tests performed:

-Alumina, Quartz and SiC show sticking to the glass

-K20 doesn’t stick up to 660°C

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:The optical test bench

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.

Load cell

Actuator

Fixed point

Magnetic strip

Air inlet

Canestrari et al. – SPIE Proc. 7015-3S

Dressing with white coat, hair cap, shoes

cover, face mask, gloves

Deep cleaningand

paint peel-off

Considerations on critical aspects:Set-up of a suitable slumping procedure

Stacking of glass and mould after the paint is

been peeled off

glass sheet

muffle inside the ovenThe slumping Crew


- No dust contamination- Very circular pattern of fringes- Slumped also at the edge- Full copy of the mould

- 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

Some examples and results

Fringes between mould and glass: very circular and without

dust contamination

218 nm rms → /3 rms over 130 mm diam.

Interferometric measurement of a slumped glass shell having radius of curvature of 4000 mm, diameter of 130 mm and 2 mm thickness



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.

Same sample but measured on 80 mm diameter


With longer thermal cycles (longer soaking and cooling times) very few

interference fringes were visible

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


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 m PV 3’’ of figure error from the mould

Interferometric measurement of a slumped glass shell having radius of curvature of 5000 mm, diameter of 500 mm and 1.7 mm thickness

The glass shell went broken during optical tests…

ask Mauro for further details

What’s a pity!


The tuning of parameters for the scaled-up procedure is not an easy job, but the results are very encouraging

254 nm rms → /2.5 rms over 250 mm diam.

Interferometric measurement of a slumped glass shell having radius of curvature of 10000 mm, diameter of 500 mm and 1.7 mm thickness


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

- 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

Outline of the talk – Part II

Collecting area > 100 m2

Angular resolution: some arcmin

Sensibility ~1/100 Crab (2x10-13 ph cm-2 s-1 @ 1 TeV)

Near UV light concentrator (300-600 nm) emitted by air Cherenkov effect from Very High Energy “events” (100GeV-10TeV).

What is a Cherenkov telescope?

Analysis of the shower’s image in the camera:

- /hadron separation;- incoming direction;- energy of primary photon

VERITAS MAGIC

H.E.S.S. CANGAROO

The MAGIC telescopes

The twins MAGIC telescopes – La Palma (Canary Islands) – 2200 m asl

Area: 240 m2

17 m

FoV: 3 deg

Focal length: 17 m

CTA: the Cherenkov Telescope Array

- Low-Energy section -~20m telescopes4 - 6° FoV0.08 - 0.12° pixelsParabolic/Hybrid f/D~1.2

- Core-Energy array -12m telescopes7 - 8° FoV 0.16 - 0.18° pixelsHybrid f/D =1.35

- High-Energy section -4-7 m telescopes8 - 10° FoV0.2 - 0.3° pixelsDC or SO f/D 0.5-1.7

Main features:Main features:

-Enhance the sensibility of a factor Enhance the sensibility of a factor 10 (up to 1 mCrab);10 (up to 1 mCrab);

-Improve the angular resolution;Improve the angular resolution;

-Wider energy coverage (10GeV-Wider energy coverage (10GeV-100TeV);100TeV);

-Flexibility;Flexibility;

-Observatory infrastructureObservatory infrastructure

Positively evaluated, Preparatory Phase funded:

FP7-INFRA-2010-2.2.10: CTA (Cherenkov Telescope Array for Gamma-ray astronomy)

STATE OF THE ART CTA

Collecting area

Mirror-segment/Area

Cost/m2

Weight/m2

Expected life

about 400 m2

0.3 – 1 m2

2 – 3.5 k€

20 – 40 kg/m2

Few years

about 10000 m2

1 – 2 m2

1.5 – 2 k€

10 – 25 kg/m2

10 years

Mirrors requirements for CTA

CTA will need more, larger, cheaper, lighter and long lasting mirrors!

• 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

Cherenkov vs Optical

Cold Glass Slumping technique: MAGIC II-concept-

Front glass sheet

Al honeycomb core

Back glass sheet

PVC coatingAl + SiO2

Cold Glass Slumping technique: MAGIC II

-development-

Aluminum master Typical mirror segment

Points: 392

P-V: 21.5 m

RMS: 4.6 m

Points: 392

P-V: 62.3 m

RMS: 15.3 m

(The color palette is inverted on this surface)

Cold Glass Slumping technique: MAGIC II-results-

Legend:

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


Dedicated optical bench equipped with:

2 laser sources: - alignment - measure

1 CCD camera for image acquisition1 flat folding mirrorPossibility to measure up to 40 m r_curv


PSF 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


Panel

Preparation

Slumping &

Curing

Glass

Gluing


-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


-production-

Media 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)


-production-


-mounting-

White protective foil used for protection of mirror surface, operator’s eyes and for telescope motion


-mounting-


-mounting-

• 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.

CTA will need more, larger, cheaper, lighter and long lasting mirrors!

Cold glass slumping technique: toward CTA-concept-

Master cleaning Foam boards assembly and machining

Sandwich preparation

Vacuum release

Mirror panel coating

Glue curing


Input data:

- Hexagonal shape: 1.2 m face-to-face

- Radius of curvature: about 32 m

- Three support points

Observing condition, loads:

- gravity

- wind up to 50 km/hrs

Acceptance Criteria:

- Maximum slope error: <0.1 mrad

- Maximum weight: 20 kg/m2

- Maximum stress developed: < Yield strength of materials

Survival condition, loads:

- gravity

- wind up to 180 km/hrs

- snow up to 30 cm thick

Cold glass slumping technique: toward CTA

-preliminary design-

Peak to Valley < 15 m

Slope < 0.03 mrad

Weight < 15 kg/m2

Current panel configuration:

glass skin: 1.2 mm

foam core: 60 mm

glass skin: 1.2 mm


3rd principal stress (foam) < 0.75 MPa

3rd principal stress (glass) < 14 MPa


• Density: ~ [0.1 - 0.165] g / cm3

• CTE ~ 9 m · K / m

• Waterproof

• Easily machined

• High compressive strength

• Very cheap for astro applications


-results-

Traction machine

Cores Uretan

Bacon Industries

Hysol

Cores Ocean

Tests were performed

in collaboration with

Politecnico di Milano

HysolBacon

IndustriesCores 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

UV (simulated about 6 months of continuous exposition to UV)

Change in color

No apparent ageing

Strong ageing, possible changing in polymerization

Almost no ageing, very slight change in color

Easiness of application

Medium,

temp curing

Very difficult,

High temp curing

Not good,

room temp curing

Very good,

room temp curing

Costs (for 1 kg) 200 Euro 300 Euro 20 Euro 25 Euro


-results-


-results-

Size: 600 x 600 x 40 mm

D90 = 1.2 mradSize: 600 x 600 x 40 mmRadius of curvature: 35.8 mWeight: 4.5 kg ~ 12 kg/m2


-results-

As manufactured

After cycle #1 After cycle #2

No measurable changes in PSF were observed after few

thermal cycles


-results-

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)

glue

foam core

rear glass sheet

slumped glass sheet

mould

vacuum suction mirror panel release

STEP 2: Sandwiching concept

Vacuum-tight muffle

Heater elements Heater elements

STEP 1: Hot slumping concept of thin glass sheet

Thermal cycle

Derived from ESO E-ELT FP6 R&D program

What about segmented primary mirrors?

STEP 1Ghigo et al. – 6691-0K SPIE 2007

Canestrari et al. – 7015-3S SPIE 2008Ghigo et al. – 7439-0M SPIE 2009

STEP 2Canestrari et al. – 7018-0D SPIE 2008Canestrari et al. – 7437-11 SPIE 2009

Mirror panel during the glue curing

Interferometric setup

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

Dust grain


Foam boards assembly

Curing of the glue

Hot slumped glass on the mould

Mirror panel after the release


Sun image

Focal spot: d ~ 4 mm

Size: 500 x 40 mmRadius of curvature: 9.85 mWeight: 2.5 kg ~ 12 kg/m2

Mirror image of a filament lamp


Figure error of ~1.5


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