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Micro positionning : EUCLID - Delay line for Synthetic Aperture Telescopes

Contact : André Preumont

Motivation

Optical Aperture Synthetic Telescope (OAST) is based on the use of several independent telescopes whose outputs are combined on a single optical sensor. This technology allows to enhance drastically the resolution of the telescope by increasing its optical aperture. On the other hand, it requires very stringent constraints on the Wave Front Error at the sensor, i.e. Optical Path Delay (OPD) and Tilt between the various telescopes. For example, the OPD between two independent telescopes must be lower than a fraction of the light’s wavelength, typically a few nanometers. This is achieved by means of an Optical Delay Line (ODL) which generally consists in displacing very precisely a plane mirror.

ODL concept

The selected concept for positioning a 2kg mirror with a resolution of a few nanometers over a 5mm range is represented on the figure below. It consists of a linear stage magnetically suspended and driven by a magnetic motor (Voice-Coil). The position of the linear stage is measured by means of a laser interferometer (resolution: 0.3nm). The magnetic suspension is made of an off-the-shelf system previously designed by MECOS for rotating applications. This configuration also allows the control of the tilt of the mirror by means of the magnetic suspension.

Control strategy

Because of the frictionless behavior of the payload, the control algorithm is based on a simple PID. It is implemented digitally on a DSP board (dSPACE DS1102). Experimental results show a resolution of 5 nm rms over a 5 mm range. The closed-loop bandwidth is about 100 Hz and is mainly limited by the control-structure interaction with the flexible modes of the payload.

Related contract

1995 : UEO, Euclid Program RTP 9.2, collaboration with ONERA (Fr), Centre Spatial de Liège (B) and Spacebel Instrumentation (B).