Optics for Wide Field X-ray Imaging AXRO 2012 Prague December 11th – 13th 2012 Optics for Wide Field X-ray Imaging Dick Willingale University of Leicester

Scientific Motivation Soft X-ray surveys High angular resolution Large sky area Faint sources – AGN, Clusters of Galaxies… Soft X-ray transient astronomy Good angular resolution All sky Short lived phenomena – GRBs, Novae…

Soft X-ray large sky area surveys Looking for 5 arc sec over 1 degree FOV with collecting area ~ 4000 cm2

WFXT approach Proposal to NASA – P.I. S. Murray Optics – INAF/Brera – G. Pareschi Thin shell Wolter I with polynominal figure

Si pore optics - Athena Si pore module – Cosine Research Focal length F=12m Rib spacing 0.83 mm Radial width Δr=0.605 mm Axial length 4.F.Δr/R Module layout – Owl design

Wide field Si pore optics Focal length F=10m Wide rib spacing 3.32 mm Radial width Δr=0.605 mm Fixed axial length 28 mm Avoid small radius modules

Wide field Si pore optics Original pore geometry – grasp 0.19 m2 deg2 at 1 keV Wide field pore geometry – grasp 0.34 m2 deg2 at 1 keV HEW limited by conical approximation Can be improved by including axial curvature Using Ir coating – a C overcoat would increase the low energy area

Wide field Si pore optics Harder response than WFXT 460 cm2 at 6.5 keV – Fe K lines Grasp 0.34 m2 deg2 at 1 keV 0.097 m2 deg2 at 4 keV 0.013 m2 deg2 at 6.5 keV WFXT Grasp 0.37 m2 deg2 at 1 keV 0.075 m2 deg2 at 4 keV 0.011 m2 deg2 at 6.5 keV

Soft X-ray Transient Astronomy Line: Lobster module F=300 mm Red points: Swift BAT short GRBs Black points: Swift BAT long GRBs Green points: Swift XRT GRB afterglows ? Looking for 1 arc min over 30 degree FOV with collecting area ~ 10 cm2

X-ray Transient Imaging Require: FOV ~30 by 30 degrees or larger Continuous coverage ~1000 square degrees or much more Collecting area > few cm2 Sensitivity to transient sources - Δt 1 second - 1 day A true imaging optic to give maximum sensitivity Wolter I: FOV diameter limited to ~twice grazing angle - only ~2o Could use a fly’s-eye of small Wolter Is but very inefficient 2 in-plane reflections - lateral inversion in the image plane Solution: Square pore or Kirkpatrick-Baez geometry

Square pore geometry - Angel Focusing by 2 reflections from adjacent walls of a square pore Reflection planes orthogonal - No lateral inversion in image plane Focusing independent of rotation of pore about pore axis φ Pores on spherical surface radius R Pore axes point to common centre of curvature Image on a spherical surface – radius f=R/2 – NO limit to the FOV

Square Pore MCPs Glass plate - thickness 1-2 mm – transmission ~67% Pores d~20 μm, wall~4 μm, L/d~100 K-B stacks

Pore packing geometries Cartesian packing – Lobster Eye - high aperture utilization but correlated single reflection background Random packing – inefficient use of aperture Waffle packing -redistribute the single reflection background but retain high aperture utilization No centre of symmetry – no preferred axis – optimum for wide field imaging Octagonal packing, φ=45 degrees Radial packing, φ=0 degrees With radial packing require tandem plates to approximate Wolter I imaging Optimum for narrow field imaging

Lobster eye PSF Plate imperfections: Figure errors and surface roughness of the channel walls 4.5 Å rms Misalignment of the channel axes ± 1 arc mins Channel rotation errors ± 1 degree Channel shear ~1/40 of channel pitch HEW of core ~ 4.8 arc mins FWHM ~ 3 arc mins Focusing gain ~ 2700 Focal length 500 mm Channel L/D 50 1st cross-arm zero 2FD/L=20 mm

Lobster eye collecting area Total collecting area ~22 cm2 at 1 keV ~ 30% of area in central focused spot Crab ~93 cts/sec

10 x Lobster modules Total FOV ~9000 sq degrees 6 degree cant angle of modules 240mm square Lobster optics frames, 500mm focal length Angular resolution ~3 arc mins Collecting area ~9 cm2 at 1 keV

K-B Stack – Schmidt Geometry

K-B Si Pore module Module shown is a Wolter I conical approximation prototype Could easily be constructed in the Kirkpatrick Baez geometry square aperture, side length S number of plates Np=S/P P=760 μm T=150 μm D=610 μm open fraction front+rear 64% Wolter I Si pore module – Cosine Research No plate curvature required Plates wedged so point at common centre of curvature All stacks are identical

Packing of K-B stacks into an aperture Si stacks suitable for larger instruments Focal length 5 m (needs to be > ~2.5 m) Collecting area ~100 cm2, angular resolution ~20 arc seconds Sensitivity 5-50 better than Lobster module F=500 mm Narrow field - pointed wide field - survey

Wide field K-B stack Grazing angle 1 degrees slot width 0.605 mm axial slot length 35 mm FOV 20 degrees diameter Collecting area ~110 cm2 at 1 keV (~constant over FOV) HEW ~22 arc seconds (constant over FOV) Grasp 3.46 m2 deg2 Focusing gain ~13300 Area at 13 cm2 at 6.5 keV HEW limited by flat plates – can be improved using axially curved plates Vignetting at 1 keV Area vs. energy

Summary Si pore optics modules with wide rib spacing and fixed axial length can provide grasp same as WFXT better than 10 arc sec imaging over 1 degree FOV can be improved by introducing axial curvature Glass square pore MCPs provide ~3 arc min imaging and focusing gain of ~2700 30 by 30 degree FOV area ~9 cm2 at 1 keV Si pore K-B stacks in Schmidt geometry provide 20 degree diameter FOV – ~300 deg2 with ~100 cm2 at 1 keV and HEW of ~20 arc secs Grasp is 10x WFXT – focusing gain 13300 A new class of instrument deep wide field imaging – faint transient imaging