Edgewise connectivity: an approach to improving segmented primary mirror performance

Abstract

Abstract. As future astrophysics missions require space telescopes with greater sensitivity and angular resolution, the corresponding increase in the primary mirror diameter presents numerous challenges. Since fairing restrictions limit the maximum diameter of monolithic and deployable segmented mirrors that can be launched, there is a need for on-orbit assembly methods that decouple the mirror diameter from the choice of launch vehicle. In addition, larger mirrors are more susceptible to vibrations and are typically so lightly damped that vibrations could persist for some time if uncontrolled. To address these challenges, we present a segmented mirror architecture in which the segments are connected edgewise by mechanisms analogous to damped springs. These mechanisms can be damped springs, flux-pinning mechanisms, virtual mechanisms, or any other device with the same basic behavior. Using a parametric finite-element model, we show that for low to intermediate stiffnesses, the stiffness and damping contributions from the mechanisms improve both the natural frequency and disturbance response of the segmented mirror. At higher stiffnesses, the mechanisms structurally connect the segments, leading to a segmented mirror that performs comparably to a monolith-or better, depending on the mechanism damping-with the modular design enabling on-orbit assembly and scalability.

Flux-pinning mechanisms for improving cryogenic segmented mirror performance

“…For the example 15-m edgewise-connected mirror, the mechanism stiffness must be on the order of 10 7 N∕m in order for the mirror to behave similarly to the monolith. 13 Since this stiffness is approximately four orders of magnitude larger than the stiffnesses measured in Sec. 3, the mechanisms would need to be stiffened substantially, which may be achievable by using smaller cooling separations or stronger magnets.…”

“…The performance of an edgewise-connected mirror depends on the mechanism stiffness and damping. 13 For a flux-pinning mechanism, these properties are affected by design choices including the cooling separation, the magnet strength, and the presence of conductive materials. To investigate the achievable stiffness and damping for mechanisms with cooling separations on the order of millimeters, rather than centimeters, we conducted a series of static and dynamic measurements for various mechanisms of the type shown in Fig.…”

“…Although investigating the relationship between mechanism properties, such as the cooling separation and choice of magnet, and the resulting stiffness and damping values provides insight into how to design mechanisms with the desired values, an important result of these measurements is determining the order of magnitude for the stiffness and damping values that can be achieved with inexpensive magnets and cooling separations on the order of 1 mm. Previous modeling work 13 has shown that the mechanism stiffness determines whether the segments of an edgewise-connected mirror act as individual rigid bodies or as a cohesive unit. Changing the stiffness by orders of magnitude (by reducing the cooling separation from 5 cm to 1 mm, for example) can therefore alter the fundamental mirror behavior.…”

“…Though the improvements are modest, more substantial gains are possible with higher damping values. 13 These values may be attainable by placing nonmagnetic conductive materials near the mechanisms. Approaches include placing bulk material adjacent to the mechanisms and fabricating the mirror segments from a nonmagnetic conductive material.…”

“…The stiffness and damping contributions from the mechanisms reduce the requirements for the supporting structure, and if the mechanisms are sufficiently stiff, the segmented mirror performs comparably to a monolith even if the mechanisms are the only structural connections to the segments. 13 While the mechanisms can be a collection of damped springs or any other device with similar behavior, flux-pinning mechanisms are uniquely suited for cryogenic mirrors.…”