Several proposed future X-ray missions will require thin (≤0.5 mm thick) mirrors with precise surface figures to maintain high angular resolution (≤0.5 arcsec). To study methods of meeting these requirements, adjustable X-ray optics have been fabricated with thin-film piezoelectric actuators to perform figure correction. The fabrication and actuator performance for an adjustable X-ray mirror that forms a conical approximation to a Wolter-I telescope are reported. The individual responses of actuator cells were measured and shown to induce a figure change of 870 nm peak-to-valley on average. These measured responses were compared with predicted responses generated using a finite-element analysis algorithm. On average, the measured and predicted cell responses agreed to within 60 nm root mean square. A set of representative mirror distortions and the measured cell responses were used to simulate figure corrections and calculate the half-power diameter (HPD, single reflection at 1 keV) achieved. These simulations showed an improvement in 4.5 to 9 arcsec mirrors to 0.5 to 1.5 arcsec HPD. The disagreements between the predicted and measured cells’ performance in actuation and figure correction were attributed to a high spatial frequency metrology error and differences in mirror bonding considerations between the finite-element analysis model and the as-built mirror mount.
Many current outstanding questions in x-ray astronomy were addressed by Lynx, an observatory concept that concluded its study phase in 2019. High-effective area, high angular resolution x-ray missions like Lynx require thin (≤ 0.5 mm thick) mirrors with precise surface figures to maintain high angular resolution (≤ 0.5 arcsec). To study methods of meeting these requirements, adjustable x-ray optics have been fabricated with thin-film piezoelectric actuators to perform figure correction. These adjustable x-ray optics serve to correct low spatial frequency figure errors (⪆ 0.1 mm−1 ). The fabrication and actuator performance for an adjustable x-ray mirror that forms a conical approximation to a Wolter-I telescope are reported. This mirror has a BCB insulating layer with a top level of Ti traces to address its 288 actuator cells. The individual responses of cells are measured and on average they induce a figure change of 0.87 μm Peak-to-Valley (PV) with an associated Root Mean Square (RMS) of 0.10 μm. These measured cell responses are compared to predicted responses generated using a Finite-Element (FEA) analysis algorithm. On average the measured and predicted cell responses agree to within 0.06 μm RMS. The disagreement between predicted and measured cell responses is posited as being due to differences in radial constraints points between the FEA model and the as-built mirror mount.
Next-generation x-ray observatories require lightweight, high throughput optics that maintain a <0.5 arcsec resolution to probe the physics of black holes and gain understanding of the early universe. One potential type of x-ray mirror consists of a 400-μm thick curved Corning EAGLE XG® glass substrate with a Cr/Ir x-ray mirror coating deposited on the front (concave) side and an array of radio frequency sputtered Pb0.995 ( Zr0.52Ti0.48)0.99Nb0.01O3 piezoelectric thin film actuators on the back (convex) side to enable correction of figure errors. A stress-balancing process was developed to correct the figure distortion arising from thin film stresses in the actuator layers. Compressively stressed SiO2 films were deposited on the convex side of the mirror to balance the tensile integrated stress of the actuator array while also matching the film thickness distribution. Finite-element methods were used to assess the impact of film thickness distributions on the convex and concave substrate surfaces. The resulting models show peak-to-valley figure errors of 105 nm, well within the 1-μm peak-to-valley dynamic range of the piezoelectric adjusters. In contrast, when stress compensation was done with an iridium mirror film deposited on the front side, the mismatched thickness distribution results in peak-to-valley figure errors over 3 μm.
Thin adjustable X-ray mirrors can correct deformations generated from fabrication, gravity release, mounting stresses, drifting stresses in the reflecting layer(s) and thermal variations while maintaining high angular resolution (< 0.5 arcsecond) and large effective area (< 2 m2) required for future X-ray missions. This work presents fabrication process developments for adjustable mirror segments with actuators for the Lynx X-ray observatory mission concept. Piezoelectric actuator arrays were fabricated on the convex side of precision slumped glass or curved silicon mirror segments using a 1.5 μm thick lead zirconate titanate (PZT) film. A two-layer metal routing scheme with a polymeric insulator was used to independently address 288 actuators on the mirror. The two-layer metal allows narrow kerfs between actuators and increased actuator density. Anisotropic conductive film was used to bond thin flexible copper cables to flat edges of the mirror to interface with external control electronics. This prototype mirror has eight cables with a total of 290 connections to access the array. To reduce the cabling complexity for future mirrors, thin film transistors have been fabricated on the curved mirror to function as access switches. To facilitate this, a mask aligner that allows precision alignment on curved mirror segments was developed and arrays of thin film transistors (TFT) on curved substrates have been tested. TFT and actuator integration on future mirrors will reduce external connections to just two cables with a total of 30 connections. Keywords: Lynx, adjustable optics, X-ray optics, thin film piezoelectric, curved substrate aligner
Exquisite angular resolution (< 0.5 arcsec) and high effective area (≥ 2 m^2 @ 1 keV) are requirements for a next-generation X-ray observatory capable of tackling outstanding problems in high energy astrophysics, including understanding how black holes grow over cosmic time and how hot baryonic material is distributed on the largest scales. However, realizing a telescope with this performance is challenging, as the thin optics required are susceptible to fabrication errors, thin film stress, and mounting deformations. One potential method of addressing these errors to fabricate adjustable X-ray optics – mirrors with actuators capable of correcting the optic’s figure following mounting. In this work, we present interferometric measurements of an adjustable X-ray optics prototype with lead titanate zirconate (PZT) actuators. We detail the realized actuator performance and correctability of the mirror prototype, and discuss the implications for the next-generation of adjustable mirrors.
Adjustable X-ray optics represent a potential mirror technology for the NASA Lynx X-ray observatory mission concept. Adjustable optics employ an integrated micron-thick piezoelectric film deposited on the convex side of silicon Wolter-type mirror segments. Discrete, independently addressable electrodes on the convex surface form individual actuators; the applied voltages are used to correct the shape of the mirror segments for figure errors resulting from a change in thermal environment, epoxy creep, or failure of an epoxy bond.
On-orbit correction requires a metrology system to provide real-time feedback of mirror figure. We are examining the use of deposited semiconductor strain gauges to monitor mirror mechanical strains and surface temperatures. To establish requirements for monitoring we modeled a variety of thermal and mechanical disturbances to a mirror segment such as might occur on-orbit or from launch. Models are described and resulting requirements and performance discussed.
Piezoelectric adjustable x-ray optics use magnetron sputtered thin film coatings on both sides of a thin curved glass substrate. To produce an optic suitable for a mission requiring high-angular resolution like “Lynx,” the integrated stresses (stress×thickness) of films on both sides of the optic must be approximately equal. Thus, understanding how sputtered film thickness distributions change for convex and concave curved substrates is necessary. To address this, thickness distributions of piezoelectric Pb0.995(Zr0.52Ti0.48)0.99Nb0.01O3 films are studied on flat, convex, and concave cylindrical substrates with a 220-mm radius of curvature. A mathematical model of the film thickness distribution is derived based on the geometric properties of the sputter tool and the substrate, and film thicknesses deposited with a commercially available sputtering tool are measured with spectroscopic ellipsometry. Experiment and modeled results for flat and convex curved substrates demonstrate good agreement, with average relative thickness distribution difference of 0.19% and −0.10% respectively, and a higher average difference of 1.4% for concave substrates. The calculated relative thickness distributions are applied to the convex and concave sides of a finite-element analysis (FEA) model of an adjustable x-ray optic prototype. The FEA model shows that, left uncorrected, the relative film thickness variation will yield an optic with an optical performance of 2.6 arc sec half power diameter (HPD) at 1 keV. However, the mirror figure can be corrected to diffraction-limited performance (0.3 arc sec HPD) using the piezoelectric adjusters, suggesting that the tolerances for applying a balanced integrated stress on both sides of a mirror are alleviated for adjustable x-ray optics as compared to traditional static x-ray mirrors. Furthermore, the piezoelectric adjusters will also allow changes in mirror figure over the telescope lifetime due to drift in the stress states of the x-ray surfaces to be corrected on orbit.
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