optics for wide field x-ray imaging dick willingale – axro december 2012 optics for wide field...

Optics for Wide Field X-ray ImagingDick Willingale – AXRO December 2012

Optics for Wide Field X-ray Imaging

Dick WillingaleUniversity of Leicester

AXRO 2012Prague December 11th – 13th 2012


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…

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Soft X-ray large sky area surveys

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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

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Si pore optics - Athena

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• Focal length F=12m• Rib spacing 0.83 mm• Radial width Δr=0.605

mm• Axial length 4.F.Δr/R

Si pore module – Cosine Research

Module layout – Owl design


Wide field Si pore optics

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• 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



• 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 area7



• WFXT Grasp– 0.37 m2 deg2 at 1 keV– 0.075 m2 deg2 at 4 keV– 0.011 m2 deg2 at 6.5 keV

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• 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


Soft X-ray Transient Astronomy

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Line:Lobster module F=300 mm

Red points:Swift BAT short GRBs

Black points:Swift BAT long GRBs

Green points:Swift XRT GRB afterglowsLooking for 1 arc min over 30 degree FOV with collecting area ~ 10 cm2


X-ray Transient ImagingRequire:• 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

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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


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Square Pore MCPs• Glass plate - thickness 1-2 mm – transmission

~67%• Pores d~20 μm, wall~4 μm, L/d~100


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

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• Focal length 500 mm• Channel L/D 50• 1st cross-arm zero 2FD/L=20 mm

• HEW of core ~ 4.8 arc mins• FWHM ~ 3 arc mins• Focusing gain ~ 2700


Lobster eye collecting area• Total collecting area ~22 cm2 at 1

keV• ~ 30% of area in central focused

spot

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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


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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

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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

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• 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

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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

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optics for wide field x-ray imaging dick willingale – axro december 2012 optics for wide field...

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