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.