Zonal Bimorph Deformable Mirror Feasibility Study

Lead Research Organisation: BAE Systems
Department Name: Sensor Technology

Abstract

The atmosphere is in a constant state of flux. Different pockets of air have different temperatures and densities which affect the light passing through it in different ways. This 'turbulence' limits how well you can see distant objects, and effectively puts a limit on the resolution that can be achieved with ground based telescopes. However, at the size of ground-based telescopes has gradually been increasing due to the advent of adaptive optics; this can be used to correct for the blurring caused by turbulence. There are now a number of Very Large Telescopes (VLTs), with 8-10 meter diameters that have been commissioned with adaptive optics. Based on the success of these systems, a much larger facility, the Extremely Large Telescope, (ELT), with a diameter of 30 meters is now being considered by a European Consortium lead by the European Southern Observatory (ESO) The Zonal Bimorph Deformable Mirror (ZBDM) is an important new AO technology that has resulted from five years of investment at BAE SYSTEMS Advanced Technology Centre. ZBDMs exploit the benefits normally associated with bimorph mirrors; simple construction, cost effective manufacture, and inherent ruggedness, but at the same time, they have the potential to be scaled up to large apertures with many thousands of elements. In addition, the low element capacitance (typically two orders of magnitude less than that of an equivalent stacked actuator DM), will make the driver for the mirror significantly easier to implement with much less heat dissipation. The main objective of this proposal is to carry out a risk reduction study on the development of this technology for medium (2-5mm pitch, 200-400mm aperture) sized deformable mirrors for the Astronomical Instrumentation which is to be included in the planned European Extremely Large Telescope facility (e.g. for multi-conjugate adaptive optics and for the planet-finder instrument). ZBDM technology has the flexibility to fulfil a number of AO roles. One particular area is to offer a solution to the limitations in the maximum stroke available with high density (2-5mm) DM's. For example, 2mm pitch mirrors based on stacked actuator technology typically provide a maximum stroke of 2µm. The thin, lightweight ZBDM structure should enable the integration of both 'tweeter' and 'woofer' DMs to realise a dual-stage, high PV stroke 'planet-finder' class device. A single layer, 61 element test mirror fabricated using internal funds has recently demonstrated an inter-actuator stroke of ±1.4µm, coupled with a lowest device resonance of 5 kHz and a PV stroke of ±8µm. Analytical modeling suggests that a dual stage ZBDM with a tweeter pitch of 2mm could deliver a maximum stroke of ±5µm. This proposal covers a two year risk reduction programme that would be exploited through a bid into the FP7 programme to progress the technology towards a full scale prototype. The main objective of this first stage is to undertake a series of risk reduction exercises which will increase the maturity of the technology, and to fabricate a small scale demonstrator which will provide a firm basis for the full scale prototype phase. o Device models will be used to determine the characteristic trade-off between maximum peak-valley (PV) stroke and bandwidth, the range of influence functions that can be achieved, and the inter-dependence of layers for a dual stage mirror. o Assembly and polishing trials will be undertaken in order to determine the best way to reduce / eliminate the risk of print through and maximize mirror flatness. o The current design is based on standard 'soft' PZT, which exhibits hysteresis; a number of strategies for reducing this effect have already been identified. These will be assessed, and the most promising technique will be used in the small scale demonstrator. While ZBDM technology can also be applied to larger mirror types, these applications are being addressed directly by the ELT Large AO Mirror Progr

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