Ramsey, Brian D.

NASA Technical Reports Server

X-Ray Astronomy Group
Optics Requirements for X-ray Astronomy & 
Developments at the Marshall Space Flight Center
Birth of X-Ray Astronomy
• In 1962, Riccardo Giacconi and colleagues at 
AS&E flew sounding rocket to look at x-ray 
fluorescence from the moon
• Lunar signal was overshadowed by very strong 
emission from the Scorpious region
• Discovered the first extra-solar x-ray source, 
Sco X-1, and pervasive x-ray background
• This was the effective birth of x-ray astronomy
https://ntrs.nasa.gov/search.jsp?R=20120015353 2019-08-30T22:17:12+00:00Z
X-Ray Astronomy Group
Altitude 
(km)
The Atmosphere Protects Us
X-Ray Astronomy Group
X-Ray Astronomy
First X-Ray Satellite
The UHURU spacecraft was launched in 1970
It weighed just 140 pounds, not much more than 
the rocket experiment
X-Ray Astronomy Group
X-Ray Astronomy
UHURU
Used a simple collimator 
system, to locate x-ray 
sources in sky
It operated for 3 years and 
discovered 339 sources in the 
whole sky
X-Ray Astronomy Group
X-Ray Astronomy
Early observations
From these early observations a picture emerged 
of a typical x-ray source:
A compact object (neutron star, black hole, white 
dwarf) orbiting around a normal star
Matter streams down on to the compact object 
forming an accretion disk
As the matter spirals down and is compressed it 
gets very hot and emits x rays
X-Ray Astronomy Group
Today .. The Chandra Observatory
X-Ray Astronomy Group
Today .. The Chandra Observatory
• School-bus-size x-ray observatory
• 100,000 times more powerful than 
UHURU
• Uses special mirrors to form highly 
detailed images
• In deep fields, more than 1000 new 
sources per square degree
X-Ray Astronomy Group
Chandra Images : Cas-A Supernova Remnant
X-Ray Astronomy Group
Chandra Images : Center of our Galaxy
X-Ray Astronomy Group
The Crab Nebula and its Pulsar
X-Ray Astronomy Group
X-Ray Optics
Why focus x rays ?
1) Imaging - obvious
2) Background reduction
- Signal from cosmic sources very faint, observed against a large 
background
- Background depends on size of detector and amount of sky viewed
> Concentrate flux from small area of sky on to small detector
enormous increase in sensitivity
First dedicated x-ray astronomy satellite – UHURU 
mapped 340 sources with large area detector (no optics)
Chandra observatory - ~ same collecting area as UHURU
 5 orders of mag more sensitivity --- 1,000 sources / sq degree in deep 
fields
 1 background count / keV year !
X-Ray Optics has revolutionized x-ray astronomy
X-Ray Astronomy Group
X-Ray Optics
• X Rays undergo total external reflection at shallow graze 
angles
- Critical angle (away from absorption edges) 
> ~ c (deg) = 0.93  (nm) ( (g/cm3)
X-ray reflectivity, nickel at 0.5 deg
0
0.1
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Energy (keV)
R
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• Can use this phenomenon in focusing x-ray 
telescopes
Reflect x rays to a common focus
Single parabola gives severe off-axis distortions, … 
Wolter-1 geometry adopted
X-Ray Astronomy Group
Approaches to Fabrication
• X-ray optics are very challenging to fabricate. Because of very short 
wavelength of x-rays the mirror surface must be smooth to ~ 0.5 nm rms. 
• Also, for good angular resolution, the figure must be accurate to < 1 
micron.
X-Ray Astronomy Group
Approaches to Fabrication
Classical Optical grinding and polishing
Chandra, Rosat, Einstein
Advantage: Superb angular resolution
Disadvantage: High cost, large mass, difficult to nest
Electroformed Nickel Replication
XMM, JETX/Swift, SAX
Advantage: High nesting factor, good resolution
Disadvantage: Medium cost,  mass (high density of nickel)
Segmented foil 
ASTRO-E, ASTRO-F, ASCA, BBXRT
Advantage: Light weight, low cost
Disadvantage: Relatively poor angular resolution (arc-minute-level)
X-Ray Astronomy Group
Approaches: Chandra
X-Ray Astronomy Group
Approaches: Chandra
• Fabricated using thick ceramic, which is meticulously polished and 
figured, one shell at a time.
• Obtain superb angular resolution  ------ 0.5 arcsec HPD
• But very costly to fabricate ($500M) and very heavy (1000 kg)
• So, other approaches to x-ray optics have been used that trade 
the superb angular resolution for ease of fabrication and lighter 
weight (and cost)
X-Ray Astronomy Group
Approaches: Foil Optics
• Fabricated using very thin aluminum foils as reflectors. Foils held in 
slots in housing.
• Obtain poor angular resolution – 1-2 arcminute HPD
• But extremely light weight allowing for many individual reflectors, 
and thus large collecting area
X-Ray Astronomy Group
Approaches: Electroformed Nickel Replication (ENR)
• Electroform thin nickel shells from superpolished and figured 
masters (mandrels)
• Obtain intermediate level angular resolution (~ 15 arcsec HPD)
• But considerably less expensive to fabricate and considerably 
lighter
• Electroformed nickel optics are being fabricated at MSFC for 
various programs.
X-Ray Astronomy Group
Challenges for Future Missions
X-Ray Astronomy Group
Mirror Fabrication for (near) Future Missions
X-Ray Astronomy Group
Mirrors for (far) Future Missions – Active Optics
How can we achieve sub-arcsec resolution with thin optics ?
One option is to utilize active control of mirror figure, as is done in optical astronomy
X-Ray Astronomy Group
Mirrors for (far) Future Missions – Active Optics
Technique has been used for 
synchrotron x-ray optics, but in its 
infancy in x-ray astronomy.
• Difficulty is the football-field-size 
areas that must be controlled.
• Algorithms needed that converge
• Power requirements and stability, 
etc
X-Ray Astronomy Group
Mirrors for Future Missions – Differential Deposition
Vacuum deposit a filler material to compensate for figure imperfections
Proof of concept work underway at MSFC
X-Ray Astronomy Group
X-ray testing
Surface profile 
metrology
Develop correction 
profile “Hitmap”
Simulations – translation 
velocity of shell 
Differential deposition
Surface profile 
metrology
X-ray testing
Process sequence - differential deposition
X-Ray Astronomy Group
Correction
stage
Average 
deposition 
amplitude
(nm)
Slit-size
(mm)
Angular 
resolution
(arc secs)
1 300 5 3.61
2 40 2 0.68
3 4 1 0.22
4 1 0.25 0.14
Theoretical performance improvement
-400
-300
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-100
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300
0 100 200 300 400 500 600
H
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Axial position along mirror (mm)
Before
Correction
After Correction
-1.2
-0.7
-0.2
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0.8
0 100 200 300 400 500 600
H
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Axial Position along mirror (mm)
Before Correction
After Correction
Simulations performed on X-ray shell  
profile of 8 arc sec simulated HPD
X-Ray Astronomy Group
• Variation of sputtered beam profile along the length of mirror –
particularly for short focal length mirrors
• Deviation in the simulated sputtered beam profile from actual  profile, 
beam non-uniformities, etc
• Positional inaccuracy of the slit with respect to mirror
• Stress effects
• Metrology uncertainty
Possible practical limitations
X-Ray Astronomy Group
Metrology limitation
•Potential for ~arc-second-level 
resolution – with MSFC’s 
metrology equipment
•Sub-arc sec resolution could be 
possible with the state-of-art 
metrology equipment
Correction 
stage 
Average 
deposition 
amplitude 
(nm) 
Slit-size 
(mm) 
Metrology 
uncertainty 
(nm) 
Angular 
resolution 
(arc secs) 
1 300 5
± 0 3.6
± 10 3.6
± 50 7.3
2 40 2
± 0 0.6
± 1 1
± 5 2
± 10 3.5
3 4 1
± 0 0.2
± 0.5 0.2
± 1 0.5
± 2 0.8
Simulations performed on X-ray shell of 8 arc sec 
simulated HPD
X-Ray Astronomy Group
Mirrors for Future Missions – Differential Deposition
X-Ray Astronomy Group
Material and process selection
Platinum-Xenon Platinum-Argon
power pressure roughness deposition rate power pressure roughness deposition rate
75 15 1.950 0.130 75 15 2.060 0.140
90 15 2.043 0.230 90 15 1.933 0.190
75 30 1.895 0.170 75 30 1.868 0.160
90 30 1.810 0.250 90 30 2.083 0.220
Nickel-Xenon Nickel-Argon
power pressure roughness deposition rate power pressure roughness deposition rate
75 15 1.915 0.290 75 15 1.995 0.180
90 15 2.070 0.360 90 15 1.778 0.240
75 30 3.093 0.240 75 30 2.260 0.220
90 30 3.630 0.310 90 30 2.210 0.290
Tungsten-Xenon Tungsten-Argon
power pressure roughness deposition rate power pressure roughness deposition rate
75 15 1.965 0.300 75 15 1.900 0.120
75 30 1.805 0.290 75 30 2.125 0.290
90 30 1.993 0.370 90 30 - -
75 50 2.075 0.290 75 50 1.998 0.310
90 50 2.423 0.370 90 50 1.868 0.370
Units: power-Watts, pressure-mTorr, roughness- Å rms, deposition rate – Å/sec
X-Ray Astronomy Group
-0.6
-0.4
-0.2
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0 5 10 15 20 25
P
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µ
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Axial position along mirror (mm)
desired profile
profile before coating
profile after coating
Figure error 
improvement from 0.11 
µm to 0.058 µm rms
Slope error improvement 
from 12 arc sec to 7 arc 
sec rms
-0.5
-0.4
-0.3
-0.2
-0.1
0
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P
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h
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µ
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Axial position along mirror (mm)
desired profile
profile before correction
profile after correction
simulated profile
Proof of concept on few-cm-scale medical imaging optics
X-Ray Astronomy Group
Current Status
• Since submitting this RFI response we have been notified of APRA /ARA funding
• This will allow us to build a custom system and demonstrate the technique on 
larger full shell (MSFC) and segmented (GSFC) optics
• We hope to be able to demonstrate < 5 arcsec performance in 3 years
•To go beyond this, (arcsecond level) is very difficult to judge as we have not yet 
discovered the problems. 
• May necessitate in-situ metrology, stress reduction investigations, 
correcting for gravity effects, correcting for temperature effects 
• Some of this will become obvious in early parts of the investigation
• Top-of-head estimate – ~ 5 years total and additional $2-3M
X-Ray Astronomy Group
Long Trace Profiler
•Pencil beam interferometry
•Measure spatial wavelengths  starting 
from 1 mm upto several 100’s of mm
•Laser beam scans point-by- point - slope 
data
•Position of the beam at the detector –
direct measure of the slope
•Accuracies possible <1 urad
•Multiple measurements – 2D topography
Time taken to measure is about 5 
mins for 300 mm sample length
X-Ray Astronomy Group
• Make use of advanced technology 
• Higher resolution 2D detectors
• Stable optical sources 
• Increase the speed & accuracies of measurements
• Higher density data – complete information of mandrel or shell
• Multiple beams – simultaneous measurements?
Increase the speed ?
X-Ray Astronomy Group
Multi-beam LTP
•Number of beams – 5 to 10
•Spatial separation of beams – 1 to 2 mm
•Proof-of-concept
•To study the limitations of the approach
X-Ray Astronomy Group
• Target – decrease the time of measurement – reasonable accuracy
• Existing systems
- linear array detector – 25 µm X 2.5 mm pixel size
- 1 m focal length FT lens 
- Lens and detector → 0.25 µrad
• 2D detector requirements
- multiple beams in one plane and 
beam translation on other plane
- angular range +/- 15 mrad 
- Detector atleast 20 mm X 20 mm area 
- < 8 µm pixel size
Multi-beam LTP Requirements
fl (mm) 500 550 600 700 800
pixel (µm)
4 0.12 0.11 0.10 0.09 0.08
8 0.24 0.22 0.20 0.17 0.15
10 0.30 0.27 0.25 0.21 0.19
14 0.42 0.38 0.35 0.30 0.26
18 0.54 0.49 0.45 0.39 0.34
Resolution (µrad)
X-Ray Astronomy Group
• Detector Procured (1st Vision’s JAI AM-1600GE)
- 36 mm x 24 mm area
- 7.4 x 7.4 µm pixel size
- 3.04 fps
• Custom designed FT lens
- 500 mm focal length
- 50 mm diameter
- Low distortion – minimize the effects of lens on systematic 
errors
2D CCD detector & FT lens
X-Ray Astronomy Group
FT lens design
• Less number of elements – two element system
• Air-spaced doublet lens 
• Anti-reflection coating
Beam splitter FT lens
X-Ray Astronomy Group
Multiple beam generation
• Multiple beams of almost 
equal intensity
• Spatial separation (2.4 mm)
• Angular separation (250 
µrad)
• Wedged etalon approach
• Customized coating on one 
side
• 100% reflection coating on 
the other
• Dimension – 50 x 50 x 3 mm 
• Wedge – 60 µrad
X-Ray Astronomy Group
Coating – wedged etalon
Possible approaches
•Continuous ideal gradient 
coating
•Linear approximation to 
ideal
•Multilayers 
•Discrete coatings – each 
with constant reflectivity
X-Ray Astronomy Group
Multiple beam generation – wedged etalon
• Silver coating with 
step reflectivity 
approach
X-Ray Astronomy Group
Cube beamsplitter
•Purchased 
50mm X 50 mm 
beam splitter -
tested with 
zygo for 
wavefront error
•Analysis of 
test result in 
progress
X-Ray Astronomy Group
Software
• Berkeley National Labs - provided software code
• 3 beams – being adapted to  new detector for multiple beams – Maxima of single 
peak
• Tests are underway to check the speed of readout & processing – 0.5 fps for full 
frame of 4872x3248 – 1.3 fps for partial frame of 4872x800
-2
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P
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Time (Secs)
beam
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beam
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beam
3
Stability check
Laser beam
X-Ray Astronomy Group
Future work
• Immediate work
 Replace detector 
 Etalon test & analysis
 Detector software
• Assemble components
• Test measurements 
• Modular approach


Optics Requirements for X-Ray Astronomy and Developments at the Marshall Space Flight Center

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