OSIRIS (Optical System for Imaging and low/intermediate-Resolution Integrated Spectroscopy) is an instrument designed to obtain images and low-resolution spectra of astronomical objects in the optical domain (from 365 to 1000 nm). It will be installed on Day One at the Nasmyth focus of the 10 m Spanish GTC Telescope. One of its key elements is two ICOS ET100 tunable filters (see Fig. 1), which will be used for several scientific programs. The tunable filters are basically two partially plated parallel mirrors acting as an Etalon. When a polychromatic light crosses the cavity between the mirrors only specific wavelengths are able to pass through, depending on the gap width. One of the mirrors is movable which permits changing of the cavity width. The mirror is moved by three high voltage piezoelectric actuators, while the gap width and parallelism is measured using several capacitors on the internal side of the mirrors. An electronic module, called the CS100 controller, controls the mirrors' positions. This controller has several specifications that do not meet the OSIRIS project requirements. We have developed an improved plug-in electronic module in order to improve both the CS100 performance and to meet the OSIRIS specifications.
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