1. THE OSIRIS TUNABLE FILTERS  OSIRIS uses two 100 mm aperture Fabry-Perot tunable filters. One of them is optimized for short wavelengths, and one for long wavelengths  These tunable filters were manufactured by ICOS (formerly Queensgate, UK). Only ICOS supplies large aperture tunable filters  The mirror movement is performed by three piezoelectric actuators  The gap width and parallelism are measured using several capacitors plated in the internal side of the mirrors  Each tunable filter is controlled by an electronic unit: The CS100 controller

2. THE CS100 CONTROLLER  This electronic unit is offered by ICOS as the standard equipment to control their tunable filters  It is a rather old design (1987). ICOS does not offer any other alternative controller, neither the possibility of developing custom improvements  Its specifications are insufficient for the OSIRIS requirements: The CS100 is only capable to remotely control the gap width a range within 2  m... OSIRIS needs to control the whole filter range (8-10  m) The CS100 allows to remotely control some of the frontal panel switches and buttons... OSIRIS needs to control more things, as the quadrature control and gauges OSIRIS also needs to change the cavity width quickly, and synchronized with an external CCD controller trigger signal. This possibility is not allowed by the CS100 controller

3. THE SOLUTION: AN IMPROVED CS100 CONTROLLER  The CS100 control electronics has been modified, keeping its useful modules : paralellism servocontrol, position sensing and high voltage circuitry  A new control electronics has been developed in order to improve the CS100 performance and suited to the OSIRIS requirements  This new electronic module is connected to the original CS100 taking advantage of some already existing testing connectors. This permits an easy installation, without modification of the internal electronics  The new module has two main elements: The Application Board and the Control Board

4. THE APPLICATION BOARD  The Application Board is the main component of the new design, and has been designed to overcome all the mentioned limitations  It includes several functional modules: 16 bit Position Control module Automatic Quadrature Control module Frontal Panel Control module Gauge Reading module CCD Synchronization module Interface and Communication module  All the functional modules are controlled by the Control Board

5. THE CONTROL BOARD  As Control Board for this prototype, an Avnet FPGA evaluation board has been used. This board is based on a Xilinx Spartan IIE FPGA. It is powerful and flexible  The use of microcontroller embedded technology avoids the purchasing of an “off the shelf” microcontroller development environment, and speeds up the software development  This board carries out the control of the new capabilities of the Application Board, allowing the communication of the improved CS100 with a remote terminal using a complete set of commands

6. THE TEST BENCHES  Several test bench arrangements have been developed in order to test the functionality and validate the specifications of the new CS100 electronics

7. GOOD RESULTS!  It is possible the remote control of all the frontal panel elements, as well as the remote reading of the gauges  It is possible the synchronization of the Tunable Filter with an external trigger signal. The set of gap widths are stored in a configurable table in memory, allowing quick gap changes  It is possible to control the Tunable Filters over their whole range, and with a resolution better than 0.5 nm  The quadrature error is compensated in an automatic way, without the use of the frontal panel quadrature control

8. MAXIMUM RESOLUTION: 0.15nm!  Measurements carried out in two different ways (optical spectrum analyzer and photometry), have shown that the 16-bit resolution of the improved CS100 controller can be resolved. This resolution is equivalent to 0.15 nm of mirror displacement  Based on this prototype, we are currently developing the final version of the “plug-in” improvement module for the CS100 controller, planned to be finished and tested by the end of 2005