Astronomy and Space Domain Awareness are limited by the size of available telescope optics, the cost for which scales steeply due to the exquisitely ground and polished primary mirrors, typically made of glass or other light-weight substrates. Liquid mirrors (LMs) may break this unfavorable cost scaling. When rotated at a constant angular velocity, it has been shown that fluid surfaces take the form of a paraboloid, which can function as a primary mirror. However, current LMs cannot slew or tilt off-zenith due to gravity, greatly limiting the viewing area in the sky. To overcome these limitations while also enabling low-cost, very-large-aperture telescopes, DARPA launched the Zenith program. Zenith is developing entirely new LM design-for-build approaches that can create large optical surfaces and maintain optical quality during tilt and slew by correcting transient liquid surface aberrations in real time. The development of these new designs is being supported by multi-physics models, materials, surface and field controls, and structures. This paper discusses key and fundamental aspects of four new design and modeling approaches for this new class of LMs. The software and simulation tools developed by the Zenith program to design tiltable and size-scalable liquid mirrors are also available to the astronomical community as an open-source repository.
Enabling tilt and imaging-while-slewing for ground-based liquid mirror telescopes would allow for very large-scale, lowcost mirror solutions. To explore how to accomplish this, our team has taken a systems approach leveraging optical, manufacturing, material, and modeling expertise to address the key technical challenges of creating a stable liquid surface. These technical challenges include maintaining optical quality surface figures while slewing and tilting, creating high optical reflectance concomitantly with controllable liquids, characterizing large-scale deformable optics, and manufacturing and scalability of proposed designs to greater than 50 m configurations. In this paper, we present both a solution framework as well as preliminary modeling results to demonstrate mirror feasibility for a ferrofluid, magnetically-actuated approach.
In parallel with the overall telescope liquid mirror design effort, we discuss nanomaterial synthesis techniques for reflective ferrofluid as well as manufacturing development of a magnetically permeable metallic paraboloid shell with surface wicking structure and electromagnetic control coil arrays. The forces from the coils and capillarity from the wick establish the requisite control and stability to deliver required wavefront performance and maintain fluid stability. We share our initial model and small-scale coupon test results for baseline ferrofluid, wicking structure, and actuation inputs to demonstrate feasibility. We also outline next steps for our optical and ferrofluid modeling and material synthesis for a prototype 50 cm mirror we anticipate building in the near future.
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