The Visible Tunable Filter (VTF) is a narrowband tunable instrument for imaging spectropolarimetry in the wavelength range between 520 and 870 nm. It is based on large-format Fabry Perots with a free aperture of 250 mm. The instrument will be one of the first-light instruments of the 4 m aperture Daniel K. Inoue Solar Telescope (DKIST) that is currently under construction on Maui (Hawaii). To provide stable and repeatable spectral scanning by tuning the air gap distance of the Etalons, a metrology system with 20 pm resolution and drift stability of better 100 pm per hour is needed. The integration of the metrology system must preserve the tight optical specifications of the Etalon plates. The HEIDENHAIN LIP 382 linear encoder system has a selected linear scale for low noise high signal interpolation. The signal period is 128nm and the interpolated signal from the sensor can be read out at 128 nm/ 14 bit = 7.8125 pm. To qualify the LIP 382 system for the VTF, we investigated the resolution and stability under nominal VTF operation conditions and verified a mounting concept for the sensor heads. We present results that demonstrate that the LIP 382 system fulfills the requirements for the VTF Etalons. We also present a design for the sensor head mounts.
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.