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

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

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

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

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

LAM - Stress Mirror Polishing technique

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

First application (my Thesis): Large lightweight primary mirrors

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

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

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 ?

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

SMP for lightweight mirrors

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 »

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 =

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

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

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

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

Second application: What about 3D printing ?

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 !

Thank you !