Thin glass sheets for innovative mirrors in astronomical applications

1. 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

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. 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

5. E-ELT former optical design E-ELT site: Cerro Armazones, Chile GMT site: Cerro Las Campanas, Chile TMT site: Mauna Kea, Hawaii M2: 4.8 m segmented concave deformable mirror, built-in adaptive optic E-ELT Gregorian design

6. 911mm 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 Pre-integration of final unit

7. Heater elements Heater elements Borofloat glass 2: Hot Slumping mould Oven Vacuum-tight muffle 3: Metrology by interferometry (astatic support) Hot slumping concept for thin glass shells 1: Borofloat sheet and mould 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

8. 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

9. 1 2 4 5 6 1 3 4 5 Considerations on critical aspects: Thermal characterization of the oven Thermocouple sensors disposition inside the small oven Measured and simulated thermal behavior of the small oven Thermal model of the entire system: oven + muffle + mould

10. 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.

11. 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

12. 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

13. Property HP Alumina HP SiC Quartz Technical Zerodur K20 Elastic Modulus (Gpa) 90 476 72 83 Knoop Hardness (HK 0.1/20) 1900 3000 580 620 CTE (RT to 1000°C) 8.2 *10-6K-1 4.0 *10-6K-1 0.5 *10-6K-1 2.0 *10-6K-1 CTE homogeneity Good (quantitative data not available) Good =0.01*10-6K-1 Probably not very good Very good =0.004*10-6K-1 Thermal Conduct. (W/mK) 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 Transparency NO-White NO-Black YES NO-White Voids, inclusions NO NO Possible NO High temperature stability (cycles) Very good Very good Very good Very good Max application temperature 1900 °C 1450 °C 1200 °C 850 °C Machinability In green body In green body Very good Good Polishability / figuring Slow Very slow Fast Slower than quartz Microroughness 10-20 Å < 5 Å < 5 Å <10 Å Mould characterization 3D machine + Patch map 3D machine + Patch map Interferometer surface map 3D machine + Patch map Many producers Many producers Material availability Many producers Only Schott Scalability to 1.5 m Sectors brazing Sectors brazing Difficult Several meters  100 K€ Mould Cost ( 0.7m)  60 K€  60 K€  70 K€

14. Considerations on critical aspects: Materials choice and procurement CTE mismatch between mould and glass: R(DT)=R0(1+a* DT) 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!

15. 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 Alumina moulds Quartz moulds SiC moulds

16. 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)

17. Considerations on critical aspects: Materials choice and procurement

18. Magnetic strip Load cell Air inlet Actuator Fixed point 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.

19. 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

20. 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 muffle inside the oven The slumping Crew

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

22. 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

23. 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.

24. 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.

25. 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

26. 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!

27. 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.

28. 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.

29. Part II Composite glass mirror panels for making IACT reflectors

30. 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

31. 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)

32. VERITAS MAGIC H.E.S.S. CANGAROO

33. The MAGIC telescopes The twins MAGIC telescopes – La Palma (Canary Islands) – 2200 m asl FoV: 3 deg Area: 240 m2 f: 17 m Focal length: 17 m

34. CTA: the Cherenkov Telescope Array - High-Energy section - 4-7 m telescopes 8 - 10° FoV 0.2 - 0.3° pixels DC or SO f/D 0.5-1.7 - 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 • 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-2010-2.2.10: CTA (Cherenkov Telescope Array for Gamma-ray astronomy)

35. Mirrors requirements for CTA CTA STATE OF THE ART about 10000 m2 1 – 2 m2 1.5 – 2 k€ 10 – 25 kg/m2 10 years about 400 m2 0.3 – 1 m2 2 – 3.5 k€ 20 – 40 kg/m2 Few years Collecting area Mirror-segment/Area Cost/m2 Weight/m2 Expected life CTA will need more, larger, cheaper, lighter and long lasting mirrors!

36. 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

37. Cold Glass Slumping technique: MAGIC II -concept- Back glass sheet Front glass sheet Al honeycomb core PVC coating Al + SiO2

38. Cold Glass Slumping technique: MAGIC II -development-

39. Cold Glass Slumping technique: MAGIC II -results- Aluminum master Typical mirror segment (The color palette is inverted on this surface) Points: 392 P-V: 21.5 mm RMS: 4.6 mm Points: 392 P-V: 62.3 mm RMS: 15.3 mm

40. 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

41. 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

42. Cold Glass Slumping technique: MAGIC II -results- 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

43. Cold Glass Slumping technique: MAGIC II -production-

44. 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

45. Cold Glass Slumping technique: MAGIC II -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)

46. Cold Glass Slumping technique: MAGIC II -mounting-

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

48. Cold Glass Slumping technique: MAGIC II -mounting-

49. Cold glass slumping technique: toward CTA -concept- CTA 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.

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