Proceedings of Micro-lithograph 2003, at Santa Crala 2003.2.24-28
Development of an ultra-high precision X-ray telescope with an
adaptive optics system
Shunji Kitamoto, Haruko Takano, Harue Saitoh, Norimasa Yamamoto, Takayoshi Kohmura,
Kazuharu Suga and Hiroyuki Sekiguchi
Rikkyo University, 3-34-1, Nishi-Ikebukuro, Toshima-ku Tokyo, 171-8501, Japan
ABSTRACT
We are developing an ultra high precision Soft X-ray telescope.
for 13.5nm band using Mo/Si multilayers.
The design of the telescope is a normal incident one
Two ideas are introduced.
to monitor the precision of the optics system.
One is the optical measurement system in order
The other is the adaptive optics system with a deformable mirror.
Using an X ray-optical separation filter, we can always monitor the deformation of the optics by optical light.
With
this information, we can control the deformable mirror to compensate the system deformation as a closed loop system.
We confirmed that the absolute precision of the wave front sensor was less than 3 nm rms.
deformable mirror was roughly 5 nm rms.
effective diameter of 80mm.
The precision of the
The shape of the primary mirror was an off-axis paraboloide with an
This primary mirror was coated by Mo/Si multilayers. The reflectivity of the primary
mirror at 13.5nm was ranging from 30 to 50 %. The X ray-optical separation filter was made from Zr with a thickness
of
~170nm.
Keywords:
The transmission of the filter for low energy X-ray was measured and was roughly 50 % at 13.5nm.
EUV, Telescope, Adaptive Optics, Zr Filter, Multilayers
1.INTRODUCTION
We are developing an ultra high precision X-ray Telescope, named X-ray milli-arc-sec Project (X-mas Project), as a
future space mission.
We will report current status of this project.
The X-ray Astronomical Satellite “Chandra”, launched in 1999, is providing us wonderful X-ray images with an
angular resolution of ~0.5 arc-sec and we are enjoying lots of important scientific results[1].
However, the current
performance of the image quality of X-rays is still far from the theoretical diffraction limit.
Figure 1 shows the
diameter or the base line length of the various telescopes as a function of the wavelength.
telescope is determined by the telescope diameter and the wavelength.
The diffraction limit of the
Lines of diffraction limit for 1 arc-sec, 1
milli-arc-sec and 1 -arc-sec are plotted in the figure.
In the radio wave length, the Very Long Baseline Interferometer
(VLBI) and the space interferometer “Haruca” have
~1 milli-arc-sec resolution.
HST have roughly 0.1 arc-sec resolution.
For optical telescope, Subaru and
These values have been almost achieved.
In the X-ray range, Chandra’s
diffraction limit is less than 1 milli-arc-sec, but the actual performance is only 0.5 arc-sec and is far from the diffraction
limit.
If we have a diffraction limit X-ray telescope with 1 m diameter, the resolution is almost same to or better than
that of VLBI.
Figure 2 shows the typical distance and typical diameter of the various celestial objects.
-arc-sec resolution, we can attack some of the black holes.
This might be difficult with current technique.
milli-arc-sec, cosmic jets can be resolved and we can compare the X-ray image and the radio image.
are believed to relate to the black holes.
If we have 1
With 1
The cosmic jets
Its collimation mechanism is a big astronomical problem.
The essential
problem is still unresolved, whether the jet is composed of proton-electron plasma or positron-electron plasma.
have 1milli-arc-sec resolution in the X-ray range, we can try to solve these problems.
If we
Also we can have an image of
nearby stars, and can measure the various motions of the celestial objects, with 1 milli-arc-sec resolution.
In order to achieve an X-ray telescope
with
1milli-arc-sec resolution, the requirement of the small-scale
roughness of the X-ray optics is roughly several Å. This is rather easy.
error is less than wavelength and is roughly 1nm.
The requirement of the large-scale figure
This value is almost impossible.
Since the X-ray telescope must be
equipped on a satellite, the thermal distortion and the gravitational distortion make a deformation of more than 10nm.
We are now trying to overcome this difficulty by applying two ideas.
light. The other is the adaptive optics system.
One is the monitoring of the optics with optical
In this paper, we report our activity to develop the ultra high precision
X-ray telescope.
Figure 1. The diameter and the wavelength of various
telescopes.
The lines of the diffraction limits of 1 arc-sec, 1
milli-arc-sec and 1 -arc-sec
are plotted.
Figure 2. The typical distance and the diameter of various
astronomical objects.
milli-arc-sec,
plotted.
The lines of the angular size of 1
1 -arc-sec and 1 nano-arc-sec are
Figure 3. The current design of the laboratory test telescope.
2.TELESCOPE DESIGN
To have large effective area, the normal incident configuration is easier than the grazing incident telescope.
For
13.5 nm wave length, Mo/Si multi-layers have more than 70 % reflectivity for the normal incident reflection[2,3].
Also, the possible precision of the measurement of the shape of the mirror is a few nm.
choice for the telescope using current technique.
80mm diameter effective area.
focal length is 2000mm.
Thus 13.5 nm band is the best
Figure 3 is the design of the test telescope.
The primary mirror has
The shape is an off-axis poraboloide with a 100mm off from the parabola axis. The
The optical source, deformable mirror and wave front sensor make it possible to monitor the
shape of the telescope and the adaptive feed-back system.
For the X-ray imaging, the X ray-optical separation filter
and a back-side CCD will be used.
3.TELESCOPE COMPONENTS
We are now in the phase of the component test, using the optical light.
system.
We are also preparing the future X-ray test
We report here the current status of the each components.
2.1. Wave Front Sensor
The wave front sensor is a shack-hartmann sensor made by a micro-lens array and a CCD, purchased from Imagin
Optics [4].
The positions of the images of each micro lens array are analyzed and the wave front shape is calculated.
We are now testing the precision of this wave front sensor.
curtain.
The sensor is installed in a clean booth covered by a black
A spherical wave is made by a pin hole and it is exposed onto the wave front sensor.
The wave front shape
is constructed by positions of the image center of the micro-lens array.
components, the residuals are calculated.
figure 4.
After removing the spherical and tilts
The resultant rms deviations is plotted as a function of signal intensity in
We confirmed the residual rms of less than 3 nm.
Figure 4. Rms-deviation from the spherical wave front as a function of the signal intensity.
(a)
(c)
(b)
(d)
Figure 5. Some examples of the deformed shapes of the deformable mirror.
Figure 6.
The picture of the main mirror.
The shape
Figure 7.
The measured reflectivity of the main mirror.
is the off axis paraboloid and the Mo/Si multilayer is
coated.
2.2. Deformable Mirror
The deformable mirror is constructed by 31 element bimorph piezo-electric plates (BIM31 mad by CILAS)[5].
One
element is at the center, and six elements make a circle around the center and on the seconds and 3rd circle 12 elements
are distributed.
The bimorph piezo-electic plates is a two layer piezos with the opposite polarity.
Thus this plate
makes a curvature of concave or convex shape.
The effective diameter is 55mm. The performance of the deformable
mirror is demonstrated by making a flat plane.
Currently 5.26nm-rms of the flatness has been achieved using the
feedback system with Zygo interferometer.
Some examples of the deformed shape are shown in figures 5; (a) shows
the shape when 100V was applied to the center element, (b) , (c), (d) are the shapes when 100 V was applied to one
element in the 1st, 2nd, and
3rd circle, respectively.
The elements in the 3rd circle are out of the effective area and
they will make the boundary shape of the effective area.
2.3. Primary Mirror
The primary mirror is made by Elide polishing [6].
Its shape is an off-axis paraboloide with an effective diameter of
80 mm and focal length of 2000 mm. Mo/Si multi-layers were coated on the mirror surface and the reflectivity has been
measured at 13 positions.
plotted in the figure 7.
Figure 6 shows a picture of the primary mirror.
The examples of the reflectivity are
The reflectivity is a little worth comparing to the other groups, but is ranging from 30% to
50%. The diffraction limit of the image size of 80mm diameter for the 13.5nm wave length is
~50 milli-arc-sec.
2.4.X-ray Optical Separation Mirror
In order to achieve the optical monitoring system, an X ray-optical separation filter is required.
Zr filter has a good transmission [7].
/100
figure error.
In the 13.5nm range,
The donuts-shape frame is polished to get a few Å surface roughness and λ
The 170nm thickness Zr is equipped on the frame.
Figure 8 shows a picture of the X-ray optical
separation filter.
The X-ray transmission is measured.
The measurement was performed at Beam line 11A of the Photon Factory
in the Institute of Material Structure Science, High Energy Accelerator Research Organization (KEK-PF). In this beam
line, a grazing incident monochrometer is installed with varied-line-spacing plane gratings [8,9,10].
By changing the
grating, this beam line can provide soft X rays from 80 eV to 1900 eV. Four setups were used in our measurement and
the conditions are summarized in table 1. When we measured the transmission in the low energy, we used a Ni
double-mirror system to eliminate the higher order light.
The measurement configuration is schematically shown in figure 9. The beam intensity emerging from the
monochrometer (after the Ni double-mirror system) was monitored by measuring the current of the photo-electrons
from an Au-coated W mesh, which was installed in front of the samples.
a beam size of 2 mm  5 mm.
The beam was restricted by a slit, resulting in
The filter were installed on rotational axles and it could be put in or out of the beam.
A window-less photodiode (AXW-100 TS) was used as a detector and its current was measured
The transmission of the filter was calculated as a ratio of currents measured with a
photodiode with the filter and
one without it after normalizing by the current of the photo-electrons from the Au-coated W mesh.
The calibration of
the absolute photon energy was conducted using a K-edge of the 4 m thick pure Al for the 800-1800 eV setup. For the
other three setups, C-K edge energy of 7 m kapton film was used.
respectively, with an accuracy of ~1 eV.
These were adjusted to 1559 eV and 284 eV,
Derived X-ray transmission is shown in figure 10, where the model
transmission of 170 m thick Zr is also potted.
2.5.Back-side CCD
We have a back-side CCD. The detection efficiency is expected to have roughly 30% for 13.5nm
range. The pixel size is 24 m. Because the expected image size is 0.3 m, we need finer pixel
chip and also we are now developing sub-pixel resolution read-out method[11].
Table 1. Measurement condition of the three setups.
Energy Range
Grating
Incident Angle
Double Mirror System
80-200 eV
300 l/mm
86.9
Ni mirror 14.0 deg
150-450 eV
300 l/mm
86.9
Ni mirror 6.0 deg
450-700 eV
800 l/mm
86.9
Ni mirror 4.0deg
800-1800 eV
800 l/mm
88.1
No
Figure 8. The picture of the X-ray optical
separation filter.
Figure 9. The measurement configuration of the
transmission of the X-ray Optical separation filter.
Figure 10. The X-ray transmission of the X-ray Optical separation filter.
4. CONCLUSION
Now we have the primary mirror, the wave front sensor, the deformable mirror, the X-ray optical separation filter, and
back-side CCD.
We confirmed the good performance of the wave front sensor and the deformable mirror. We are still
testing the X-ray optical separation filter and the primary mirror.
resolution.
The back-side CCD does not have enough position
We need to develop the position determination method of the CCD.
After component testing, we will
integrate and try to measure the X-ray performance.
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