Beyond Suzaku X-Ray Telescopes
Beyond Suzaku
The Suzaku X-Ray Telescopes, using thin foil mirrors, provide a
light-weighed X-ray imaging system with large collecting area over broad
energy range. By replication of gold surfaces over smooth glass, the
angular resolution of these mirrors are much improved from their
predecessors. Continual effort is being made to further improve the
resolution to keep up with the more demanding needs in futher
astronomical observations. On the other front, major
development has been made recently to extend the imaging capability to
even higher energy, to tens of keV. This is accomplished by coating multi-layers
of materials on thin foil mirrors.
Multi-layer For Foil X-Ray Mirrors
The energy response of X-ray reflection at grazing incidence is limited at
higher energy when single material reflecting surfaces are used.
Efficient reflection at energy of 20 keV and above requires extremely small
grazing angle of incidence with respect to a reflecting surface, even when
metallic surface of dense metal such as gold is used. Such small
angle reflections put more constraints on an X-ray mirror system in
terms of throughput, focal length and accuracy in alignment. Moreover, the requirements
on surface smoothness become more stringent as the photon energy increases.
Multilayers, used also at grazing incidence, however, offer an opportunity for extending
astronomical X-ray imaging to tens of keV.
The basic idea behind multilayer reflection is to build up successive
coherent reflections from thin layers of alternate materials. With a proper choice
of spacings between layers, a constructive coherent wave field can be set up within
the layers, by the superposition of incident and reflected radiation.
Materials with different indices of refraction are needed to give the
needed spacing for the interfaces. Absorption is also a criterion in material
selection, which has be such that there will be gain in the resulting radiation
before it is seriously absorbed.
The thickness of each bi-layer is critical in setting up a
proper spacing for a particular energy band of radiation.
We have set up a facility for research of mulitilayer coating
on thin foil mirrors here
at the Mirror Laboratory of the X-Ray Astrophysics Laboratory.
Key issues addressed and being resolved include uniform
deposition of a multilayer, elimination or reduction of damage to the epoxy
based surface due to overheating in the deposition process, prevention of
buildup of interfacial roughness, and stress induced distortions to the
reflector configuration, etc.
References: "The Multilayer Option for Conical Foil X-Ray Mirrors",
by P.J.Serlemitsos, Y.Ogasaka, Y.Soong, K.W.Chan. in Proceedings of SPIE,
vol. 3113, 244-252, 1997.
The Constellation X-Ray Mission
The X-ray astrophysics mission
Constellation-X , carrying high throughput telescopes with better
angular resolution, will be the next generation mission for very high
sensitivity as well as enhanced imaging in both the soft and hard x-ray band.
In the soft x-ray band (<10 keV), Constellation-X Spectroscopy X-ray Telescope
is to use large effective-area, grazing incidence focusing optics together with
high energy resolution calorimeter to provide high throughput with excellent
spectral resolution. The mission achieves its large collecting area with multiple
identical satellites, carrying multiple aligned telescopes (the Spectroscopy X-ray
Telescopes and the Hard X-ray Telescopes) covering the energy band from 0.25-40 keV.
These sensitivites, over broad energy bands, shall represent a substantial
improvement over current and developing telescopes. At around 1 keV, Constellation-X
aims to provide nearly 100 times larger in effective area
compared to past and current telescopes.
The Constellation-X Spectroscopy X-ray Telescope uses a Wolter Type I design.
When mirrors are fabricated to design specification, they will have much
superior imaging performance than the Suzaku telescopes. The targetted
angular resolutions are 1/4 minute-of-arc on the Spectroscopy X-ray Telescope (SXT)
and 1 minute-of-arc on the Hard X-ray Telescope (HXT).
These better angular resolution is to be achieved with the similar replication
techniques that give smooth surface of about 3 Angstrom root-mean-square micro-roughness
for Suzaku. In order to achieve the 15 arc-second angular
resolution for the SXT, the margin for figure errors, both for the replication mandrels
and the foil substrate will have to be substantially reduced compared to
those of Suzaku.
It should also be noted that these mirror systems are large---they will have
a diamater of up to 160 cm, compared to the 40 cm of that of the Suzaku telescopes.
A segmented approach like that used in Suzaku is still the practical choice
for fabrication. To achieve the large collecting area, the length of each
segment is twice that of Suzaku's, and for higher number of nesting.
The mission also uses improved technology of x-ray micro-calorimeter
x-ray micro-calorimeter
with a superb 2 eV energy resolution at 6 keV. This represents
about a factor of 5 improvement
in spectral resolution over the current state of the art, which
has been demonstrated in the very early phase of Suzaku XRS.
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