X-ray optics at NASA Marshall Space Flight Center
Summary (4 min read)
Introduction
- Assembly, and testing of grazing-incidence optics for x-ray telescope systems.
- Launched in 1999, the Chandra X-ray Observatory 1,2 continues as NASA’s flagship mission for x-ray astronomy.
- Currently, MSFC uses this technology to produce (§3) x-ray telescopes for sub-orbital (balloon and rocket) and in-space missions (§3.1), as well as grazing-incidence optics for (ground-based) laboratory applications (§3.2).
- A disadvantage of replication is that the figure of the replica—especially if the mirror is very thin—does not perfectly match the shape of the precisely figured mandrel.
- The production of replicated x-ray mirrors comprises two top-level procedures—mandrel fabrication (§2.1) and shell fabrication (§2.2).
2.1. Mandrel fabrication
- Figure 2 outlines the basic steps in fabricating a precision mandrel for electroformed-nickel replication (ENR) of a fullshell grazing-incidence mirror.
- The mandrel typically incorporates primary (P) and secondary (S) conic frusta monolithically so that the P and S surfaces of a full-shell replica are consequentially coaxial.
- This feature and the mirror being a full shell greatly simplify alignment of mirror shells into a mirror assembly.
- While MSFC usually employs single-point diamond turning of the electroless nickel to figure the mandrel’s P and S surfaces to the required optical prescriptions, other figuring processes—e.g., precision grinding—are possible and occasionally used.
- Final figuring, smoothing, and superpolishing of the mandrel’s electroless-nickel surface utilize conventional lapping and polishing methods using a custom precision lathe.
2.2. Shell fabrication
- Figure 3 outlines the basic steps in electroforming a nickel replica full-shell grazing-incidence mirror off a precision electroless-nickel-plated aluminum mandrel (§2.1).
- Over the past two decades, MSFC has developed several process refinements that now enable the successful fabrication of very thin (≈ 100-m) ENR mirror shells, although applications typically utilize thicker shells to reduce mount-induced distortions during alignment and assembly.
- One area of ENR process improvement is surface passivation—control of adhesion of the shell onto the mandrel.
- 22 Such a film is hard, negligibly increases the surface roughness, and enables deposition onto a mandrel of optical coatings (including multilayers) that adhere to and release with the shell.
- In extreme instances, the deposition stress may be so tensile that the shell will not release upon cooling.
3.1. X-ray telescopes
- MSFC has produced numerous flight x-ray mirror modules for several balloon, rocket, and satellite missions.
- In most cases, the telescope system comprises multiple ENR mirror assemblies and corresponding detectors, to benefit from the substantial cost savings in optics fabrication afforded by replication.
- Furthermore, for a given mass allocation, multiple mirror assemblies allow relatively stiffer mirrors than does a single mirror assembly with the same collecting area.
3.1.1. High-Energy Replicated Optics to Explore the Sun (HEROES)
- HEROES is a joint balloon mission of MSFC and Goddard Space Flight Center (GSFC), designed to perform hard-xray (25–75 keV) imaging of the sun (during the day) and of cosmic sources (during the night).
- Flown in 2013 September, the HEROES payload includes 8 mirror assemblies with 6-m focal length.
- Each mirror assembly contains 14 coaxially nested iridium-coated ENR shells of 610-mm total (P+S) length and ranging in diameter from 50 mm to 94 mm.
- 24 The HEROES payload is an enhanced version of MSFC’s High-Energy Replicated Optics (HERO) payload, modified by GSFC to allow hard-x-ray observations of the sun.
- 25,26 † Release of the shell from the mandrel upon cooling relies upon the shell material (nickel or a nickel alloy) having a substantially lower coefficient of thermal expansion (CTE) than the predominant mandrel material .
3.1.2. Focusing Optics X-ray Solar Imager (FOXSI)
- FOXSI is a sub-orbital rocket mission led by the University of California at Berkeley, using x-ray optics from MSFC and detectors from JAXA’s Institute of Space and Astronautical Science.
- 27 Designed to perform high-dynamic-range medium-energy x-ray (5–15 keV) imaging of the sun for the study of x-ray microflares, FOXSI has thus far had two successful flights—FOXSI-1 in 2012 November 28 and FOXSI-2 in 2014 December.
- The FOXSI telescope includes 7 mirror assemblies with 2-m focal length.
- The mirror assemblies exhibit an imaging performance with HEW ≈ 25 arcsec and FWHM ≈ 5 arcsec, the later metric being more important for this application.
3.1.3. Micro-X
- Micro-X is a sub-orbital rocket mission led by the Massachusetts Institute of Technology (MIT), designed to obtain soft-x-ray (0.2–3 keV) non-dispersive high-spectral-resolution imaging of supernova remnants.
- 29 Built in collaboration with Goddard Space Flight Center (GSFC) and the National Institute of Standards and Technology (NIST), the detector is a micro-calorimeter pixilated array.
- While the initial Micro-X flight(s) will employ aluminum-foil segmented x-ray mirrors from GSFC, 30 MIT plans to utilize ENR full-shell x-ray optics from MSFC on a later flight (2017 or later).
- The ENR telescope comprises a single mirror assembly with 2.5-m focal length.
- The Micro-X ENR mirror assembly will contain 5 coaxially nested rhodium-coated ENR shells of 600-mm P+S length and ranging in diameter from 383 mm to 444 mm, with a required angular resolution HEW < 30 arcsec.
3.1.4. Astronomical Röntgen Telescope (ART) on Spectrum-Röntgen-Gamma (SRG)
- SRG will conduct an x-ray all-sky survey during its first 4 years, using two complementary arrays of x-ray telescopes.
- The primary telescope array is the Extended RÖntgen Survey with an Imaging Telescope Array , 32 led by the Max-Planck-Institut für extraterrestrische Physik (MPE).
- EROSITA is a soft-x-ray (0.3–10 keV) imaging system comprising 7 mirror assemblies with 1.6-m focal length, each with a pn-CCD detector.
- 33 Each eROSITA mirror assembly comprises 54 coaxially nested gold-coated ENR full-shell mirrors, produced by Media Lario .
- MSFC has completed the ground calibration of the flight mirror assemblies 38 and delivered them to IKI.
3.2. Ground-based applications
- The technology and supporting infrastructure are relevant to several ground-based applications.
- Space applications are usually more demanding than laboratory applications, for which mass constraints and collecting-area requirements are not an issue.
- Thus, grazing-incidence optics for laboratory applications are comparatively smaller with thicker walls, making them less susceptible to induced distortions.
- MSFC is exploring collaborations to utilize ENR grazing-incidence optics for laboratory applications.
- Two such applications are small-animal radionuclide imaging (§3.2.1) and cold-neutron imaging (§3.2.2).
3.2.1. Small-animal radionuclide imaging
- Originally funded by the US National Institutes of Health, MSFC has collaborated with Lawrence Livermore National Laboratory, Smithsonian Astrophysical Observatory (SAO), and University of California in San Francisco to explore the use of ENR grazing-incidence mirrors for radionuclide imaging of small animals.
- 39 About a tenth the size of ENR mirror shells used for x-ray telescopes, the microscope mirror shells have a 480-mm focal length, 60-mm total P+S length, 60–70-mm diameters, and multilayer coating (by SAO).
- The demonstration design employed 4 coaxially nested shells configured to provide 4X magnification radionuclide imaging using iodine 125 I (27 keV) or technetium 99m Tc (18 keV).
- The imaging performance for this design is currently FWHM ≈ 100 m.
3.2.2. Cold-neutron imaging
- Cold neutrons (of order 10 K or 1 meV) have deBroglie wavelengths comparable to x-ray (around 1 keV) wavelengths and reflect efficiently off smooth pure nickel surfaces.
- 42,43 Currently, MSFC is funded by NIST to develop grazing-incidence neutron optics for a multi-step demonstration: (1) High-resolution imaging; (2) 1X-magnification imaging; and (3) large-magnification imaging.
- RESEARCH TOWARD HIGH-RESOLUTION X-RAY OPTICS MSFC’s development and production of ENR grazing-incidence mirror systems successfully support numerous space and laboratory applications (§3) requiring good (HEW = 10–30 arcsec) angular resolution.
- Achieving sub-arcsecond imaging—as needed for the Xray Surveyor—will likely require a substantially modified or a totally different approach.
- Here the authors briefly describe three such research topics.
4.1. Differential deposition
- Instead of removing material to correct the surface figure of a mirror, differential deposition 44,45,46 adds material—effectively filling in valleys rather than abrading the hills .
- MSFC is developing this process 47,48 to correct the surface figure of light-weight grazing-incidence mirrors.
- Based upon metrology during a sequence of differential-deposition runs, Figure 12 shows that the process can indeed improve the surface figure of a grazing-incidence mirror.
- Note that the half-power diameter (HPD = HEW, Right panel) is a tworeflection equivalent calculated from the axial-profile metrology (Left panel) without and with filtering on spatial frequency.
- X-ray tests are planned for later this year.
4.2. Control of coating stress
- Coating stress deforms a thin mirror due to a film-on-substrate bimorph-like effect, as calculated using the Stoney equation.
- Hence, it is important to control and to minimize coating stress and to distinguish intrinsic coating stress from thermally induced stresses due to film and substrate having different coefficients of thermal expansion (CTE).
- Accordingly, MSFC is engaged in a detailed research program to measure and to control coating stress, while depositing high-quality films—low surface roughness and good adhesion.
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Citations
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Cites background from "X-ray optics at NASA Marshall Space..."
...Replicated full-shell optics are made by an entirely different process and have a different set of benefits and challenges.(6,41,42) The best performance to date is individual replicated mirror shells that have around 8 arc sec HPD and larger (>1 m) diameter replicated optics have yet to be proven....
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Cites background from "X-ray optics at NASA Marshall Space..."
...MMA – MSFC provides grazing-incidence MMAs [14-21] to focus X-ray photons onto the polarization-sensitive detectors....
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...MSFC provides the grazing incidence, X-ray mirror module assemblies (MMA) [14-21] and the Science Operations Center (SOC) along with mission management and systems engineering....
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References
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