1. STRESS MIRROR POLISHING FOR SPACE AND EARTH OBSERVATIONS
PhDs Day – February 7th, 2019
Laboratoire d’Astrophysique de Marseille
STRESS MIRROR POLISHING FOR SPACE AND EARTH OBSERVATIONS
Sabri LEMARED – 2nd year PhD student
PhD supervisor (LAM): Marc FERRARI
PhD co-supervisors (THALES-SESO): Nathalie SOULIER, Thibault DUFOUR, Christian DU JEU
Mélanie ROULET – 2nd year PhD student
PhD supervisor (LAM): Emmanuel HUGOT
PhD co-supervisors (UK-ATC): Caroline ATKINS, Hermine SCHNETLER

2. Stress Mirror Polishing (SMP) for Space and Earth observations
PhD thesis context
Two applications/ two thesis
On-axis parabola: High performance large lightweight primary mirrors
Off-axis parabola: WFIRST coronograph instrument
Perspectives and challenges

3. LAM - Stress Mirror Polishing technique
Principle of the dioptric elasticity method of stress mirror polishing for the production of Schmidt plates (Lemaitre 1974).
Interests:
High quality of optical surfaces  NO high spatial frequency errors!!
Fabrication of aspherical optics using fast spherical polishing with full size tools

4. SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch)
Off-axis mirror:
Generation of 3rd-order Astigmatism
Toric Mirror
Boundary conditions
Two pairs of opposite forces
Center attachment
Material : Zerodur
E= 90600MPa

5. SPHERE instrument VLT SPHERE toric mirrors Delivered to SPHERE in 2011
TM3 to TM1 : Ø400, 40 and 160mm
VLT SPHERE toric mirrors
Delivered to SPHERE in 2011
+ one spare in 2013
Hugot, Ferrari et al Applied Optics 2009 A&A 2012

6. LAM - Stress Mirror Polishing technique

7. First application (my Thesis): Large lightweight primary mirrors
A larger diameter primary mirror provides :
a better angular resolution Θ ≅ 𝝀 𝑫
a better sensitivity ~ D²
Lightweight mirrors allow :
Reducing mass for space telescopes
Maintain a good stiffness (e.g with honeycomb core)
A primary mirror segment of the James Webb Space Telescope

8. First application (my Thesis): Large lightweight primary mirrors

9. First application (my Thesis): Large lightweight primary mirrors
Hooker telescope
(1917)
1 segment
~ 2.5 m
Increasing size
ELT (2024) ~39 m
798 segments ~ 1.45 m each

10. Primary mirror : On-axis parabola
Spherical mirror
Parabolic mirror
y=𝑎 𝑥 2
On-axis parabola:
Generation of 3rd-order spherical aberration (SA3)
On axis parabola

11. SMP for lightweight mirrors A real challenge !
Stress Mirror Polishing (SMP)
Lightweight Mirrors
Need flexible substrate
Rigid mirror for launch
NO high spatial frequency errors
High spatial frequency errors (Quilting)
How to make these two methods compatible ?

12. SMP for lightweight mirrors
Mirror’s cutting view:
Polishing pressure T0
Over-thickness
« Tulip-cutting »
Central force

13. SMP for lightweight mirrors

14. SMP for lightweight mirrors
Focus = -7 nm RMS
SA3 = 18 µm RMS
SA5 = 44 nm RMS
SA7 = -804 nm RMS
SA9 = 47 nm RMS
SA11 = -262 nm RMS
3rd-order spherical aberration
High-order spherical aberrations
«  To produce »
«  To remove »

15. Second application (Melanie PhD’s thesis): Off-Axis Parabola (OAP)
y= 𝑎𝑥 2
light
Off-axis parabola
On axis parabola
Off-axis parabola:
Generation of Astigmatism + Coma =

16. Second application: WFIRST (Wide-Field InfraRed Survey Telescope) coronograph instrument
Courtesy: Zhao Feng, NASA/JPL

17. Second application: WFIRST (Wide-Field InfraRed Survey Telescope) coronograph instrument
8 off-axis parabola
Courtesy: Zhao Feng, NASA/JPL

18. Second application: how to obtain an off-axis parabola ?

19. Second application: What about 3D printing ?
Allows complex structures
Allows new materials
Ceramics
Plastics
In terms of cost and time
Particularly in the prototyping phase

20. Second application: What about 3D printing ?

21. Perspectives and challenges
My PhD’s thesis
Mélanie PhD’s thesis
Reach the best mirror’shape with the right paramaters combination to obtain the targeted parabolic mirror
Design tools in order to deform, polish and test the mirror properly
Make the first lightweight primary mirror prototype using stress mirror polishing
Polish OAP7 for delivering to NASA at the end of March
Optimisation deformation harnesses for others OAPs + freeforms eventually
Testing 3D-printing prototypes
Combining the two processes to make large lightweight, off-axis, parabolic segments for the future extremely large gound-based and space observatories !

22. Thank you !