1. 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

2. 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…

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

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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

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

14. 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

15. 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

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

17. 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

18. K-B Stack – Schmidt Geometry

19. 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

20. 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

21. 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

22. 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