SPIE Optics for EUV, X-Ray, and Gamma-Ray Astronomy VI Montreal 2014 June 27th – 29th Science requirements and optimization of the silicon pore optics design for the Athena mirror Dick Willingale University of Leicester G. Pareschi, F. Christensen, J-W. den Herder, D. Ferreira, A. Jakobsen, M. Ackermann, M. Collon, M. Bavdaz

Specification – Area – Angular Resolution Area of 2 m2 at 1 keV ~5 times larger than the 3 XMM telescopes sensitivity ~10-17 ergs cm-2 s-1 0.5-2 keV detect AGN z>6, WHIM… spectroscopy ΔE~2.5 eV Angular resolution 5 arc sec HEW ~3 times better than XMM telescopes avoid faint point source confusion resolve structures like clusters of galaxies spectral imaging of SNRs, clusters…

Specification – Field of View Vignetting and field of view FOV >40 arc minutes diameter minimize vignetting minimize degradation of HEW off-axis target grasp (collecting area x FOV product) 0.5 m2 deg2 at 1 keV deep search for faint sources, AGN…

Si pore optics – XEUS to Athena Si pore module – Cosine Research ~3 m aperture - focal length F=12m

Manufacture of SPO modules Rib spacing Axial profile Kink angle≈R/2F ESA - Cosine Research Axial length L=4Fd/R Short - 20 to 102 mm

Area of 2 m2 at 1 keV 6 sectors – radial arms width 20 mm Module apertures ~100x50 mm Radial gaps 10 mm Azimuthal gaps 14 mm Inner radius of aperture 258 mm Outer radius of aperture 1464 mm Total 972 modules Inner ring R=285 mm, L=101.9 mm Outer ring R=1437 mm, L=20.3 F=12 m

Area of 2 m2 at 1 keV Lowest curve outer modules Highest curve inner modules Black - Ir+B4C overcoat Red - Ir

Angular resolution 5 arc sec HEW Conical approximation with F=12 m –> 5.2 arc sec HEW Radial pore width d=0.605 mm Si wafer thickness 0.76 mm – membrane 0.15 mm Axial curvature required – mandrel specification equal on both surfaces d2r/dx2 = -R/16F2 = -θg2/R sagittal Δr=0.16 microns inner module, 0.03 microns outer module In-plane figure errors – 1 arc sec rms Out-of-plane figure errors – 1 arc sec rms but unimportant Focal length errors (kink angle error) – 1 mm rms (F=12 m) Module alignment Rotation about optical axis 2 arc sec rms Shift in aperture plane 0.05 mm rms Tilt errors 1 arc minute rms

Vignetting – off-axis area In-plane vignetting Radial pore size d (fixed) Length of pores L Drops to zero at θ≈d/2L Out-of-plane vignetting Rib spacing drib Length of pores L Drops to zero at θ≈drib/2L Ray tracing of a single module

Field of view vs. drib for Athena Ray tracing full aperture drib 1, 2, 3, 4, 5 mm FOV at half on-axis area 1 keV

FOV, Photon Energy, Grasp – drib 3mm Grasp 0.5 m2 deg2 at 1 keV for FOV 40 arc mins

Angular resolution off-axis Wolter I - principal plane (join plane) flat Wolter-Schwarzschild (W-S) Obeys Abbé sine condition principal plane spherical radius curvature F To implement W-S is simple Change θg - kink angle between 1st and 2nd surfaces in module Mount modules on the correct spherical surface

Using W-S Principal Plane Black – Wolter/conical Green – W-S/conical Blue – Wolter Red – W-S Dotted – W-S limit no errors Error budget – 5 arc sec HEW

Angular resolution off-axis Split curvature between 1st and 2nd surfaces Keep curvature sum constant – on-axis HEW remains constant - θg2/R 1st and 2nd 1st -2 θg2/R, 2nd conical

Angular resolution off-axis Split curvature between 1st and 2nd surfaces Solid black conical-conical Solid red Wolter Solid green curve close to optimum conical 1st surface -2 θg2/R 2nd surface Average HEW 5.6 arc sec 40 arc min FOV 5.9 arc sec 50 arc min FOV Use W-S principal plane

Conclusions Area 2 m2 at 1 keV 3 m aperture F=12m ~970 SPO modules Ir coating Angular resolution 5 arc seconds HEW on-axis axial curvature -2 θg2/R sum over 1st and 2nd surface tight but achievable error budget Minimize vignetting rib spacing 3 mm grasp 0.5 m2 deg2 at 1 keV 40 arc min FOV Minimize degradation of angular resolution off-axis W-S principal plane – change kink angle within modules Curvature 1st conical 2nd -2 θg2/R Mean HEW 5.6 arc sec for 40 arc min FOV

The End