19 July 2010 A novel optical sensor for mirror edge sensing
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The Southern African Large Telescope (SALT) recently (2008) abandoned attempts at using capacitive mirror edge sensors, mainly due to poor performance at a relative humidity above ~60%, a not infrequent occurrence. Different technologies are now being explored for alternative sensors on SALT. In this paper we describe the design and development of a novel prototype optical edge sensor, based on the application of the interferential scanning principle, as used in optical encoders. These prototype sensors were subsequently tested at SAAO and ESO, for potential application on SALT and E-ELT. Environmental tests, conducted in climatic control chambers, looked at temperature and relative humidity sensitivity, long term stability and sensor noise. The temperature sensitivity for height and gap were, respectively, 10nm/°C and 44nm/°C, while for relative humidity they were 4nm/10% and 50nm/10%, respectively. These either met, or were close to, the SALT specification. While there were significant lags in response, this was due to the sensor's relatively large mass (~200 gm per sensor half), which was not optimized. This is likely to improve, should a revised design be developed in future. Impressively the sensor noise was <0.015 nm RMS, over three orders of magnitude better than the specification. Our conclusions are that optical edge sensing is a viable technique for use on segmented mirror telescopes.
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D. A. H. Buckley, D. A. H. Buckley, S. Buous, S. Buous, H. Gajjar, H. Gajjar, J. W. Menzies, J. W. Menzies, F. Schindler, F. Schindler, K. Sändig, K. Sändig, S. Lévêque, S. Lévêque, } "A novel optical sensor for mirror edge sensing", Proc. SPIE 7739, Modern Technologies in Space- and Ground-based Telescopes and Instrumentation, 773912 (19 July 2010); doi: 10.1117/12.858131; https://doi.org/10.1117/12.858131

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