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Thin glass sheets for innovative mirrors in astronomical applications Como, July 9 th 2010 by Rodolfo Canestrari INAF-Astronomical Observatory of Brera.

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Presentation on theme: "Thin glass sheets for innovative mirrors in astronomical applications Como, July 9 th 2010 by Rodolfo Canestrari INAF-Astronomical Observatory of Brera."— Presentation transcript:

1 Thin glass sheets for innovative mirrors in astronomical applications Como, July 9 th 2010 by Rodolfo Canestrari INAF-Astronomical Observatory of Brera Supervisors: Dr. Mauro Ghigo Dr. Giovanni Pareschi

2 Part I Thin glass mirror shells for adaptive optics

3 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

4 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 Future optical telescopes, ELT class

5 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

6 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

7 Borofloat glass mould 2: Hot Slumping Oven Vacuum-tight muffle 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. – K SPIE 2007 Canestrari et al. – S SPIE 2008 Ghigo et al. – M SPIE 2009

8  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

9 Considerations on critical aspects: Thermal characterization of the oven Measured and simulated thermal behavior of the small oven Thermocouple sensors disposition inside the small oven Thermal model of the entire system: oven + muffle + mould

10 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.

11 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

12 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

13 850 °C1200 °C1450 °C1900 °C Max application temperature Very good High temperature stability (cycles) NOPossibleNO Voids, inclusions NO-WhiteYESNO-BlackNO-White Transparency NOYES Glass adhesion (from tests performed) Density (g/cm 3 ) Thermal Conduct. (W/mK) Very good  =0.004*10 -6 K -1 Probably not very good Good  =0.01*10 -6 K -1 Good (quantitative data not available) CTE homogeneity 2.0 *10 -6 K *10 -6 K *10 -6 K *10 -6 K -1 CTE (RT to 1000°C) Knoop Hardness (HK 0.1/20) Elastic Modulus (Gpa) Zerodur K20 Quartz Technical HP SiCHP AluminaProperty  70 K€  60 K€  100 K€  60 K€ Mould Cost (  0.7m) Several metersDifficultSectors brazing Scalability to 1.5 m Only Schott Many producers Material availability 3D machine + Patch map Interferometer surface map 3D machine + Patch map 3D machine + Patch map Mould characterization <10 Å< 5 Å Å Microroughness Slower than quartz FastVery slowSlow Polishability / figuring GoodVery goodIn green body Machinability

14 Temperature gradients must be avoided! R(  T)=R 0 (1+  *  T) CTE mismatch between mould and glass: CTE homogeneity of the mould: 0.1  m/m K  7  m T slump  F=-2*(F 2 /  )*  T (T back -T front ) Glass sheet face-to-face thermal gradient: Considerations on critical aspects: Materials choice and procurement

15 Alumina moulds Quartz moulds SiC moulds Considerations on critical aspects: Materials choice and procurement From tests performed: -Alumina, Quartz and SiC show sticking to the glass -K20 doesn’t stick up to 660°C

16 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

17 Considerations on critical aspects: Materials choice and procurement

18 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 S

19 Dressing with white coat, hair cap, shoes cover, face mask, gloves Deep cleaning and paint peel-off Considerations on critical aspects: Set-up of a suitable slumping procedure

20 Stacking of glass and mould after the paint is been peeled off glass sheet muffle inside the oven The slumping Crew Considerations on critical aspects: Set-up of a suitable slumping procedure

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

22 - 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

23 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 Some examples and results

24 57 nm rms → /11 rms over 80 mm diam. 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 Some examples and results

25 Spatial wavelength range [mm ] TEST 1TEST 2TEST 3TEST / 13 / 11 / 9.7 / / 9 / 6 / 6.4 / / 4.8 / 3 / 4 / / 2.8 / 1.8 / 2.7 / / 2.4 / 1.7 / 2.2 / 2.9 The radius of curvature of the mould is 395 cm 1° TestR=398 cmR= 3 cm 2° TestR=400 cmR= 5 cm 3° TestR=390 cmR=-5 cm 4° TestR=394 cm R=-1 cm The shape of the small mould used in these tests has not been characterized (convex shape) and hence only a comparison in repeatability among slumped segments is possible. Some examples and results

26 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 Some examples and results

27 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! Some examples and results

28 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 mm, diameter of 500 mm and 1.7 mm thickness Some examples and results

29 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.

30 Part II Composite glass mirror panels for making IACT reflectors

31 - 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

32 Collecting area > 100 m 2 Angular resolution: some arcmin Sensibility ~1/100 Crab (2x ph cm -2 s 1 TeV) Near UV light concentrator ( 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

33 VERITAS MAGIC H.E.S.S. CANGAROO

34 The MAGIC telescopes The twins MAGIC telescopes – La Palma (Canary Islands) – 2200 m asl Area: 240 m 2  17 m FoV: 3 deg Focal length: 17 m

35 CTA: the Cherenkov Telescope Array - Low-Energy section - ~20m telescopes 4 - 6° FoV ° pixels Parabolic/Hybrid f/D~1.2 - Core-Energy array - 12m telescopes 7 - 8° FoV ° pixels Hybrid f/D = High-Energy section m telescopes ° FoV ° pixels DC or SO f/D Main features: -Enhance the sensibility of a factor 10 (up to 1 mCrab); -Improve the angular resolution; -Wider energy coverage (10GeV- 100TeV); -Flexibility; -Observatory infrastructure Positively evaluated, Preparatory Phase funded: FP7-INFRA : CTA (Cherenkov Telescope Array for Gamma-ray astronomy)

36 AGIS: Advanced Gamma-ray Imaging System Advanced Gamma-ray Imaging System 36 telescopes array 12 m aperture S-C optical design double reflection telescopes very aspherical surfaces Vassiliev et al. 2007, Astroparticle Physics

37 STATE OF THE ART CTA Collecting area Mirror-segment/Area Cost/m 2 Weight/m 2 Expected life about 400 m – 1 m 2 2 – 3.5 k€ 20 – 40 kg/m 2 Few years about m 2 1 – 2 m – 2 k€ 10 – 25 kg/m 2 10 years Mirrors requirements for CTA CTA will need more, larger, cheaper, lighter and long lasting mirrors!

38 Primary mirror diameter: 42 m Number of panels: 900 Mass-to-Area: 70 kg / m 2 Cost-to-Area: ~ k€ / m 2 Panel angular resolution: 0.1 arcsec Production time: 90 m 2 / year Cherenkov vs Optical

39 Cold Glass Slumping technique: MAGIC II -concept- Front glass sheet Al honeycomb core Back glass sheet PVC coating Al + SiO 2

40 Cold Glass Slumping technique: MAGIC II -development-

41 Aluminum masterTypical 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-

42 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 Cold Glass Slumping technique: MAGIC II -results-

43 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 -results-

44 PSF measurement of a typical glass mirror panel (~1 m 2 ) 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 -results-

45 Panel Preparation Slumping & Curing Glass Gluing Cold Glass Slumping technique: MAGIC II -production-

46 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 V Pareschi et al. – SPIE Proc W Cold Glass Slumping technique: MAGIC II -production-

47 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) Cold Glass Slumping technique: MAGIC II -production-

48 Cold Glass Slumping technique: MAGIC II -mounting-

49 White protective foil used for protection of mirror surface, operator’s eyes and for telescope motion Cold Glass Slumping technique: MAGIC II -mounting-

50 Cold Glass Slumping technique: MAGIC II -mounting-

51 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-

52 Master cleaning Foam boards assembly and machining Sandwich preparation Vacuum release Mirror panel coating Glue curing Cold glass slumping technique: toward CTA -concept-

53 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/m 2 -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-

54 Peak to Valley < 15  m Slope < 0.03 mrad Weight < 15 kg/m 2 Current panel configuration: glass skin: 1.2 mm foam core: 60 mm glass skin: 1.2 mm Cold glass slumping technique: toward CTA -concept-

55 3rd principal stress (foam) < 0.75 MPa 3rd principal stress (glass) < 14 MPa Cold glass slumping technique: toward CTA -concept-

56 Density:  ~ [ ] g / cm 3 CTE ~ 9  m · K / m Waterproof Easily machined High compressive strength Very cheap for astro applications Cold glass slumping technique: toward CTA -results-

57 Traction machine Cores Uretan Bacon Industries Hysol Cores Ocean Tests were performed in collaboration with Politecnico di Milano Hysol Bacon Industries Cores UretanCores Ocean Adhesion glass/honeycomb >3.5 MPa1.7 MPa0.22 MPa2.8 MPa Adhesion glass/Foamglas 0.7 MPa0.45 MPa0.37 MPa0.65 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 Euro300 Euro 20 Euro 25 Euro Cold glass slumping technique: toward CTA -results-

58 Cold glass slumping technique: toward CTA -results-

59 Size: 600 x 600 x 40 mm D90 = 1.2 mrad Size: 600 x 600 x 40 mm Radius of curvature: 35.8 m Weight: 4.5 kg  ~ 12 kg/m 2 Cold glass slumping technique: toward CTA -results-

60 As manufactured After cycle #1 After cycle #2 No measurable changes in PSF were observed after few thermal cycles Cold glass slumping technique: toward CTA -results-

61 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.

62 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

63 Part III Conclusions of the conclusions (or the start of a new one)

64 glue foam core rear glass sheet slumped glass sheet mould vacuum suction mirror panel release STEP 2: Sandwiching concept Vacuum-tight muffle 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 1 Ghigo et al. – K SPIE 2007 Canestrari et al. – S SPIE 2008 Ghigo et al. – M SPIE 2009 STEP 2 Canestrari et al. – D SPIE 2008 Canestrari et al. – SPIE 2009

65 Mirror panel during the glue curing Interferometric setup Close view of the mirror panel Stiffening of slumped thin glass shells

66 310 nm rms → /2 rms over 110 mm diam. 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 Stiffening of slumped thin glass shells

67 Foam boards assembly Curing of the glue Hot slumped glass on the mould Mirror panel after the release Stiffening of slumped thin glass shells

68 Sun image Focal spot: d ~ 4 mm Size:  500 x 40 mm Radius of curvature: 9.85 m Weight: 2.5 kg  ~ 12 kg/m 2 Mirror image of a filament lamp Stiffening of slumped thin glass shells

69 Figure error of ~1.5 Stiffening of slumped thin glass shells


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