Designing and building X-ray telescopes that observe the high-energy universe with large-effective area and sub-arcsecond angular resolution has proven to be a great optomechanical challenge, as thin, tightly nested grazing-incidence mirrors are easily deformed during assembly. We are developing a bonding and alignment process in which we fabricate spacers, bond them between thin mirrors, and re-align these mirrors after assembly, using a Hartmann test for alignment metrology. We implemented a non-contact slumping method to produce smooth Corning Eagle XG conical mirrors. Using an ultrafast laser, we welded fused silica spacers to flat fused silica mirrors to create mirror segment stacks. We performed a parameter study on ultrafast laser welding Eagle XG to fused silica, which we expect to enable building an X-ray mirror segment stack with our slumped Eagle XG mirrors and fused silica spacers.
Future X-ray telescopes will require measuring thousands of grazing-incidence mirrors with nanometer accuracy and high throughput. Fizeau interferometry will require accurately separating the reference surface from the surface under test, a challenging proposition for optical surfaces that are highly off-axis conics. We present a lateral shifting interferometry approach called Axial Shift Mapping (ASM), in which we measure the surface under test at multiple positions and numerically separate the reference and test surfaces. In this paper, we estimate the measurement accuracy that would be required for an exemplary 0.5 arcsecond half-power diameter angular resolution telescope. We mathematically describe the procedure for using ASM, plus a lateral shift, to extract the most important quantities for X-ray telescope mirrors: axial profile (including the quadratic component) and cone angle variation. We also conclude that the radius and cone angle of a mirror are unknowable with ASM alone.
High-resolution and large-effective-area x-ray telescopes are critical tools for observing the high-energy universe, but the existing quasi-kinematic and over-constrained mounts for aligning mirrors trade off accuracy and strength. We propose a method to fabricate spacers and bond them between thin mirrors, then adjust mirror alignment by focusing ultrafast laser pulses into these spacers. We built a simple mirror stack assembly composed of two flat mirrors bonded to two spacers and wrote laser-induced modifications into these spacers. We interferometrically measured the air gap between the two mirrors after each set of laser-induced modifications. The resulting deformation in the spacers and measured tilt between flat mirrors shows that realignment can be controlled using the ultrafast laser, however there is uncertainty introduced by imperfect mirror stack assemblies. Future work will aim to achieve predictable laser-induced strain. The presented assembly procedure and laser-strain generation process serves as a potential solution to the optomechanically-limited design of current-generation x-ray telescopes.
Concentrated solar power (CSP) plants need to monitor the surface slope error of thousands of heliostats with sub-milliradian accuracy. We present grating embedded mirrors (GEMs) and the accompanying Diffractive AutoStigmatic Hartmann Camera (DASHCam) metrology system for measuring heliostat surface slopes. GEMs have multiple phase diffraction gratings written within a glass mirror substrate using an ultrafast laser. The gratings direct a small fraction of incident light to non-specular directions, which the DASHCam senses from a virtual center of curvature to measure the facet slopes at each grating location. Our results show, in a laboratory environment, 24 μrad RMS measurement repeatability and 47 µrad RMS accuracy, with single-shot image capture. GEMs and DASHCam are a compact, accurate, and high-speed heliostat slope error metrology system that is robust to the harsh environmental conditions at CSP plants.
Future sub-arcsecond resolution, large-effective area x-ray telescopes that use segmented grazing-incidence mirrors will require a bonding and alignment method that simultaneously: 1) achieves the accuracy needed for scientific observations, 2) possesses the strength needed to survive the ride to space, and 3) has high throughput to bond and align tens of thousands of components in a couple years. Current bonding and alignment processes are either over-constrained to achieve high strength with modest accuracy, or quasi-kinematic to achieve high accuracy but with lower strength. We propose an over-constrained mounting approach where spacers separating mirrors have set-and-forget adjustable height. This approach may provide the advantages of high strength and high accuracy simultaneously, while loosening initial assembly tolerances to improve process throughput. In our proposed process, glass spacers are fabricated with μm-accuracy using ultrashort pulsed laser-assisted chemical etching and Bessel beam optics. Their length is adjusted after assembly and bonding, using the same laser and optics. We show examples of our fabricated spacers assembled into stacks of mirrors and bonded using epoxy with spacer beads. In a separate experiment, we show that the length of spacers can be quickly and stably adjusted with μm-range and with nm-resolution as required for aligning x-ray mirrors. This bonding and alignment process may help solve a longstanding and critical challenge for future sub-arcsecond resolution large-effective area x-ray telescopes.
The next generation of high-resolution x-ray telescopes will require mirror segments characterized to 5 nm uncertainty or better. This is difficult to achieve due to the mirror segment’s off-axis hyperbolic and parabolic shape and the challenge of manufacturing and testing a cylindrical null lens. In a typical Fizeau interferometer setup, errors in the assumed perfect null lens will be coupled into the final surface figure, increasing uncertainty. To combat the higher uncertainty of the cylindrical null corrector, we have been developing lateral shift mapping, an absolute metrology technique using a Fizeau interferometer. In this technique, the surface under test is laterally shifted between measurements while the reference surface does not move. Contributions to the interferogram due to the surface under test will move, while contributions due to the reference will stay static. Using this information, we can extract the true surface under test with low uncertainty. There is a quadratic ambiguity that arises due to the extraction method being akin to an integration. We have shown in the past our ability to utilize lateral shift mapping to extract flat surfaces to sub-nanometer uncertainties by comparing our results to a three-flat test. We also demonstrated that we can eliminate the quadratic ambiguity in flats using an external measurement with an autocollimator. We are expanding this method from optical flats to cylindrical surfaces, creating axial shift mapping. We will report on progress toward sub-nanometer measurements of cylindrical mirrors using axial shift mapping.
Optical systems such as X-ray telescopes or micro-optical systems can require alignment of optical components with nanometer-level tolerances, and often with stringent volume and mass requirements. We propose fabricating, bonding, and subsequently adjusting length of glass spacers using ultrafast lasers. Ultrafast laser processing has been industrialized over the last decade for micron-accuracy glass cutting with complex shapes, and for glass-to-glass and glass-to-metal welding. In this paper, we will show experimental results demonstrating the ability to generate stable strain in Corning® Eagle XG® glass samples, which causes permanent nanometer-scale length changes. We demonstrate a total strain of ~10-3, or microns of displacement per millimeter length of laser-modified glass. We also measure stability in laser-modified samples and find that the length changes are nanometer-stable. We also show how this process may be applied for alignment of X-ray mirrors by combining industrialized ultrafast laser processes for glass cutting and glass-to-glass welding with strain generation and control. This powerful and flexible process may enable compact, lightweight set-and-forget alignment of optical systems with nanometer tolerances.
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