Adjustable height glass spacers for bonding and aligning X-ray mirror stacks

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.