Under the Defense Advanced Research Projects Agency (DARPA) Zenith program, a novel concept has been developed for a self-assembling ferrofluidic ionic liquid mirror (FILM) telescope utilizing a Halbach array of permanent neodymium magnets. The primary mirror will be constructed from two immiscible liquids containing reflective and magnetic nanoparticles (NPs), which will spontaneously phase separate. To maximize reflectivity, minimize wavefront error (WFE), and anchor the reflective layer, the volume of the upper liquid has been minimized. The system is scalable and self-healing and can be deployed without applied acceleration or rotation. The Halbach array overcomes the force of gravity for a ground-based liquid mirror, providing a Kelvin body force potential parallel to the surface of the array. The liquids are held in place and shaped within the mirror by use of the magnetic array, hydrophilic materials, and the high surface tension and high viscosity of the liquid. By tuning the position of the magnet assembly and application of components that tune the effective magnetic field, the liquid surface is forced to adopt the desired optical shape and allows tilting off-axis and slewing with acceptable imaging quality WFE levels.
We report here on the progress of this work in multiple areas including modeling and simulation of the magnetic fluid system optimized for a 0.5 m diameter demonstration mirror and the supporting development of laboratory 0.25 m × 0.25 m flat prototypes of the fluid and magnetic systems. Analytical and finite element models of the ferrofluid and magnetic array have been developed and these results have informed a PDR-level design for a notional build and demonstration of a 0.5 m diameter F/2 spherical mirror with overall root mean squared (RMS) WFE of λ/6 at λ= 550 nm at Zenith which can be slewed to off-zenith pointing angles of up to 10°.
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.
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