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Completed TRL 3 (started at 3, targeting 5)
Observations at high spatial resolution in the X-ray wavelengths represent a critical gap in capability to the heliophysics community. The root cause of this shortcoming is due to the fabrication process of the optics needed to image wavelengths beyond extreme ultraviolet (EUV). X-rays are a particularly important tracer of high-energy phenomena that drive space weather events, yet there are limited resources in this critical regime, which is strongly driven by our lack of capabilities to improve on previous missions. Based on ongoing work at MSFC, we propose to bridge this resolution gap between the EUV and Soft X-Ray (SXR) regimes, and to build upon the plethora of scientific discoveries from the Yohkoh and Hinode observatories. This effort strongly aligns with the Center’s Strategy as it falls within the “Science” Focus Domain and the Agency Mission Pull addressing science exploration missions. The missions benefitting from this technology maturation are critical for fueling space weather research, which ultimately apply to human exploration missions as well.
The MSFC Heliophysics Sounding Rocket Group has a strong heritage of successful research and technology demonstrations spanning instrument and wavelength space. This group has been developing the Marshall Grazing Incidence X-ray Spectrometer (MaGIXS), which combines high-resolution X-ray optics with cutting edge grating technology in order to definitively explore heating of active region loops. Building upon the significant number of lessons learned from the fabrication of the MaGIXS mirrors, we are now preparing to propose the development of the High-Resolution X-ray Imager (Hi-ReX) within the NASA Low Cost Access to Space (LCAS) Sounding Rocket program to address the X-ray spatial resolution gap in solar physics.
In order to successfully compete the Hi-ReX mission concept in the 2019 LCAS selection round (submission July 2019), we need to design the optical system and establish the framework for fabricating the Hi-ReX mirrors. While the MaGIXS mirrors are fabricated via an electroform- nickel-replication (ENR) process under development at MSFC, the Hi-ReX mirrors will be directly polished from a glass (i.e., Zerodur) substrate using the same deterministic polishing methods afforded by the Zeeko 6-axis Intelligent Robotic Polishing (Zeeko IRP) machine. We propose to use the TIP funding opportunity to develop our mirror design within a sounding rocket volume, procure a blank glass substrate with the rough figure machined into the substrate, and to update the metrology methods to provide feedback for deterministic corrective polishing and assessments of the mirror performance. Completion of these tasks (i.e., refining the metrology and glass polishing techniques) will not only assist in the preparatory activities for the Hi-ReX sounding rocket, it will also strengthen Marshall’s growth in optical excellence and have far- reaching applications for the development of the next generation of solar X-ray missions.
To achieve sub-arcsecond spatial resolution (half-power diameter, HPD) of grazing incidence mirrors, the root mean square (RMS) slope error of the surface figure must be ≲0.25 arcseconds. While it has been recently demonstrated that nickel-phosphorus-plated aluminum mandrels can be figured to produce an equivalent of ~2 arcsecond HPD (and perhaps slightly below with careful deterministic polishing), the replication process to create the final mirror, which involves growing a nickel-cobalt shell around the mandrel and then releasing the molded shell from the mandrel, degrades the mirror resolution compared to that of the mandrel. This effect is primarily due to stress imparted within the electroformed material. Regardless of how well a mandrel is polished, the replicated mirror shell will always be worse than the parent mandrel, in terms of performance.
Historically, X-ray tests indicate that the mirror shell resolution is on average two times worse than that of the polished mandrel. Mounting and handling of thin shells imparts extraneous stresses, which further degrades resolution. Therefore, our current methodology and facilities can produce a nickel-replicated optic with approximately 1-4 arcsecond HPD, at best. While there are valuable uses for even this optic and several advantages over glass (e.g. reduced cost and weight, and the ability to nest thin mirrors to build up larger collecting areas), replicated mirrors may be incapable of overcoming these inherent issues, which will ultimately inhibit sub-arcsecond resolution. In comparison to current X-ray observatories, this mature and reliable replication process does not appear to be a favorable candidate to produce X-ray mirrors that will surpass the resolution of Hinode/XRT, which has a resolution of ~2 arcseconds and has been in operation since 2006.
Other methods for fabricating X-ray optics are indeed in use. A team at Goddard Space Flight Center (GSFC) has developed a different replication method that makes use of slumped glass. The process involves polishing a mandrel sector (not a full revolution), then heating the glass to temperature that allows it to form around the mandrel, thus replicating the mandrel’s figure. This process can yield large quantities of thin mirrors, however, they face the same inherent issue with replication - the mirror will always be worse than the mandrel. The Smithsonian Astrophysical Observatory (SAO) is taking this a step further and is developing a system that uses actuators to deform the slumped glass mirrors in an attempt to correct and compensate for figure errors. This method, while novel and cutting edge, is problematic from a systems engineering standpoint, given that our development effort is aimed toward flying a mirror on a sounding rocket experiment. As is the case with nickel-replicated mirrors, these fabrication methods were all developed for the intent of building instruments with large collecting areas by nesting many mirror shells, or stacking many segments. In general, for X-ray astronomy applications, the trade space is heavily weighted toward lightweight mirror assemblies that enable larger collecting areas, and less weighted toward resolving power. In the case for solar physics, we argue that the trade space is more heavily weighted toward resolution over collecting area. This concept is our motivation for direct deterministic polishing of a glass substrate. In the case for high-resolution SXR observations, our goal is to develop a single highly figured mirror, and not one, or many lightweight replicated shells.
For the MaGIXS instrument development effort, we have been optimizing the use of the Zeeko polisher to directly polish the outer surface of cylindrical metallic mandrels down to nearly 3 arcseconds HPD. The final resolution of the three individual flight shells is expected to reach 1-4 arcseconds HPD. To get below an arcsecond, we propose to directly polish the inside surface of a cylindrical glass optic, which alleviates the replication process that degrades the final resolution and minimizes the effects of mounting and handling of the optic. Polishing the glass substrate requires significant adaptation of the Zeeko polishing process: 1) the glass responds differently to polishing tools and slurries and 2) the setup must be adjusted to polish a concave conical surface versus a convex one.
This level of precision not only requires enhancements to the polishing techniques but also improved metrology in order to provide accurate and repeatable measurements of a concave cylindrical surface. Adjustments to the ground support equipment (GSE) will be necessary to kinematically mount and align the mirror to the Zygo interferometer (used for measuring the surface figure). Typically, the measurement accuracy of the Zygo, with only a transmission flat, is ~8 nm +/- 3 nm. Ideally our goal is to improve the accuracy so that we can achieve ~1 nm, with about 10% uncertainty. Therefore, using a computer-generated hologram (CGH), specific to the optical prescription of the mirror, will be necessary. For Zeeko polishing of the MaGIXS spectrometer mirror, a Tech Excellence funded study investigated the advantages of using a CGH in metrology and found that in addition to obtaining true 3D measurements of cylindrical surface, the measurement accuracy improved to ~2.2 nm +/- 0.2 nm, without any characterization, or CGH error correction.
For this effort, we propose to: 1) develop and build GSE for a kinematic interface between mirror substrate and the Zeeko polisher, which must be easily transportable between the polisher and metrology station to reduce risk of damage to the mirror, and 2) complete the optical and opto-mechanical design of the glass substrate such that it meets the effective area requirements of the science objectives, can operate within a sounding rocket skin, and can withstand the flight environment. At closing, the prerequisite facilities needed to begin direct polishing of a grazing incidence glass substrate will be available, and we will have an initial demonstration of capability. Due to the development already underway of the MaGIXS mirrors with the existing facilities available at MSFC, this TRL maturation effort is expected to increase from Level 3 to 4.
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