Liquid mirrors are an established technology; they have proven to be a viable low cost alternative to conventional glass optics in several applications. We are working to expand the capabilities of liquid optics by developing a material that can be deformed rapidly in a precise manner as is the case for conventional solid optics while maintaining the low cost and relative ease of construction of current liquid mirror devices. In this paper we discuss the application of a new ferrofluid based technology to the problem of deformable mirrors in adaptive optics.
The trend towards ever larger telescopes and more advanced adaptive optics systems such as multi-conjugate adaptive optics is driving the need for deformable mirrors with a large number of low cost actuators. Other applications require strokes larger than those readily available from conventional mirrors. Magnetically deformable liquid mirrors are a potential solution to both these problems. Depositing a thin silver colloid known as a metal liquid-like film (MELLF) on the ferrofluid surface solves the problem of low reflectivity of pure ferrofluids. This combination provides a liquid optical surface that can be precisely shaped in a magnetic field. We have demonstrated a reflective coating that is stable for more than 30 days with a reflectivity of 50% in the near infrared. Additional experiments indicate that MELLF coatings can provide near infrared reflectivity values in excess of 80%. We also report on recent response time measurements of liquid deformable mirrors. We have demonstrated liquid mirror actuators with slew rates of 800 μm/s, corresponding to an actuator bandwidth of approximately 40 Hz and 80 Hz for strokes of 10 μm and 5 μm respectively.
In this paper we present preliminary results on a new type of optical material. By combining a thin reflective colloidal film with a superparamagnetic liquid known as a ferrofluid, it is possible to produce an optical quality surface that can be shaped by the application of a magnetic field. Ferrofluids are colloidal suspensions of nanometer-sized magnetic particles and are considered a well established, low-risk technology. We have demonstrated deformations of several microns at frequencies exceeding 100 Hz, making the material useful as a deformable mirror for adaptive optics and also of potential interest in numerous other optical devices. Liquid optics are relatively inexpensive when compared to conventional glass surfaces of similar quality and are free of mechanical constraints such as resonance and limits on the displacement of adjacent actuators. We present results to date and discuss some of the potential applications of liquid optics as well as the challenges remaining in realising practical devices based on this technology.
The trend towards ever larger telescopes and more advanced adaptive optics systems is driving the need for deformable mirrors with a large number of low cost actuators. Liquid mirrors have long been recognized a potential low cost alternative to conventional solid mirrors. By using a water or oil based ferrofluid we are able to benefit from a stronger magnetic response than is found in magnetic liquid metal amalgams and avoid the difficulty of passing a uniform current through a liquid. Depositing a thin silver colloid known as a metal liquid like film (MELLF) on the ferrofluid surface solves the problem of low reflectivity of pure ferrofluids. This combination provides a liquid optical surface that can be precisely shaped in a magnetic field. We present experimental results obtained with a prototype deformable liquid mirror based on this combination.