At the X-Ray Astrophysics Laboratory, we pioneered the foil approach in X-ray imaging. For the project Suzaku , 5 telescopes were made at the Mirror Laboratory . Each of these telescopes consists of about 1400 conically shaped mirrors, nested in about 175 layers and each takes the form of a quadrant of a section of a cone. These foils will be positioned in telescope housings, and in a 2-stage configuration which will focus x-ray at a focal length of 4.5 or 4.75 meters. Together with a pre-collimator stage, each telescope has a diameter of about 40 cm, and weighs approximately 20 kg.

The X-ray telescope aperture is an annulus filled with thin foil mirrors. Mirrors are assembled in quadrants. In each quadrant, mirrors are supported at the top and bottom edges by slotted radial bars. The slots in these bars provide the alignment of the mirrors. In the current two stage design, two reflections are required to bring the reflected rays to focus. In such design, in fact, all the conical mirrors in each stage are, in fact, confocal. The angle of incident for an axial ray on the primary (entrance) stage is three times that on the secondary (exit) stage, for each corresponding pair of mirror.

For the Suzaku Telescopes, the focal lengths are 4.75 m for the four XIS system and 4.50 m for the XRS system. The plate scales are 0.72 and 0.76 arc-min/mm, respectively. Suzaku X-Ray Telescope design parameters are listed in the table below. Those of ASCA Telescopes are also given for reference.

Each foil mirror is composed of an alumiumum substrate, a smooth metallic surface which is responsible for the reflection of x-ray at grazing incidence, and a layer of epoxy which serves to bond the reflecting surface to the substrate. The light-weight aluminum substrate is 155 micrometer thick, providing most of the mechanical strength of the mirror as well as the proper shape. The metallic surface for x-ray reflection is a thin layer of gold or platinum. The high (electron) density of these material provides excellent reflection of x-ray at small angle of incidence. The epoxy layer bonding the gold layer and the aluminum foil is approximately 25 micrometer thick.

The fabrication of such mirror consists of the following processes:

• Preparation of flat aluminum foil from sheets.
  Sections of circular arc are cut from flat aluminum sheets. With the present 2-stage design, the inner and outer radii of curvature are the same for each flat foils of a particular stage. Different arc lengths of the same circular annulus (one of each stage) are cut in order to make mirrors of different nesting radii. These foils are treated and cleaned to remove surface contamination as well as boundary distortion.
  
  
• Forming of flat aluminum foil into conical sections.
  Flat foils are rolled machincally into approximate conical shapes, again, different radii for different nesting position. A number of them are stacked together and are placed against a conical mandrel of matching dimension. The stack is sealed and the volume containing the foil is evaluated, thus allowing atmospheric pressure to act on the foils. The foils are then heat-formed, and they subsequently take the precise shape of the parent mandrel.
  
  
• Sputter thin layer of gold onto glass mandrels.
  Films of gold or platinum, of about 2000 Angstroms, are coated on glass mandrels by sputtering. The glass mandrels are cylinders 15 cm in height in order to accomodate the 10 cm width foils. These mandrels are glass which has a very smooth surface with roughness of a few angstroms over scales of mm. These glass mandrels are used for surface replication.
  
  
• Coupling of the aluminum foil with the gold-coated glass mandrel.
  Small amount of liquid epoxy is sprayed onto the conically shaped foils as well as gold-coated replicating glass mandrels. A thin layer of epoxy is desirable in order to reduce mismatch of thermal expansions between the epoxy and the aluminum substrate.
  
  A foil and a mandrel, with wet epoxy on them, are coupled together in vacuum in order to avoid trapping any air. The liquid epoxy also serve as a mild buffer for the mating the glass and the aluminum.
  
  Coupled foil-mandrels are now placed in oven for epoxy curing.
  
  
• Removal of replicated foils from mandrels.
  The foil can be removed from the glass mandrels after the epoxy is cured. The removal is possible due to the non-sticking nature of gold on glass. The foil, now with the gold layer on it, acquires the smoothness of the glass.
  
  

Replicated foils are examined with optical means before further tests are made in the x-ray. Examinations are performed to check the quality of both the mirror configurations as well as surface roughness.

• Visual Examination.
  Visual Examination by naked eyes are the preliminary checks for gross mal-replication.
  
  
• Image of a Stand-alone Foil.
  The focussing characteristics of each individual foil is examined using a uniform parallel beam of white light.
  
  
• Surface Roughness under a microscope.
  Larger scale surface roughness can be discerned under a microscope. Depending on the magnification of the microscope, surface feature of lateral scale of a mircron to a millimeter can be examined.
  
  
• Waviness and surface roughness with a laser scan micrometer.
  A laser scan mircrometer with linear and rotational travel is used to examine both the surface waviness and roughness. The lateral length scale is limited by the range of the travelling stage. In the present set up, the scanning can be done over a range of more than 20 cm, with a resolution of approximately 30 micron, primarily limited by the size of the laser beam. The depth of the scan has a resolution of 0.1 micron. A typical session of a linear scan is shown (left).
  
  
• Surface roughness with an optical interferometric profiler.
  Finer details of surface characteristics can be more efficiently resolved with an optical interferometric profiler. Such profiler provides 2D and 3D display and analysis. The lateral resolution is better than a micron at high magnification, with a depth resolution of about 0.5 Angstrom. Roughness at the sub-nanometer range is important for x-ray reflectivity. A Typical scan of a gold-coated foil is shown.
  
  
• Surface microroughness with a scanning probe microscope.
  Roughness at sub-micron scale is measured with an atomic force microscope.