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.
At the Southern African Large Telescope (SALT), in collaboration with FOGALE Nanotech, we have been testing the recently-developed new generation inductive edge sensors. The Fogale inductive sensor is one
technology being evaluated as a possible replacement for the now defunct capacitance-based edge sensing system.
We present the results of exhaustive environmental testing of two variants of the inductive sensor. In addition to the environmental testing including RH and temperature cycles, the sensor was tested for sensitivity to dust and metals. We also consider long-term sensor stability, as well as that of the electronics and of the glue used to bond the sensor to its supporting structure. A prototype design for an adjustable mount is presented which will allow for in-plane gap and shear variations present in the primary mirror configuration without adversely disturbing the figure of the individual mirror segments or the measurement accuracy.
The SAMS (Segment Alignment Measurement System) is a
capacitance-based edge sensing solution for the active
alignment of the 10m SALT segmented primary mirror. Commissioning and calibrating the system has been an ongoing
task in an attempt to counteract the unfavourable response of the sensors to high humidity conditions and high dust
levels. Several solutions were implemented and tested including
real-time feedback systems and the application of
In parallel with the continuing efforts to improve the performance of the capacitive sensors, we have also been testing a
prototype inductive sensor developed by Fogale Nanotech that is of a very similar flexible plate construction.
In this paper we present the results obtained and performance gains achieved thus far with the capacitive edge-sensing
system as well as a performance comparison of the Fogale inductive sensor to the capacitive edge sensor.
The 10-m class Southern African Large Telescope (SALT) at Sutherland, South Africa, was inaugurated in November 2005, following completion of all its major sub-systems. It is the largest single optical telescope in the southern hemisphere. The SAMS (Segment Alignment Measurement System) is a unique capacitive edge sensing solution for the active alignment of the SALT primary mirror. Twelve thin film edge sensors are bonded directly onto the edges of each of the 91 segments, with heat-generating control electronics housed remotely in temperature-controlled enclosures. The SAMS is capable of measuring the tip/tilt and piston of each segment, as well as the change in global radius of curvature, a mode normally undetected by such a system. The primary objective was to build a system that offered an excellent cost-to-performance ratio without sacrificing measurement accuracy, a very necessary requirement because of the scale and number of sensors required for large segmented mirrors. This paper describes the results obtained during the commissioning and calibration of the completed system.
There are currently several projects for giant telescopes with segmented mirrors under way. These future telescopes will have their primary mirror made of several thousand segments. The main advantage of segmentation is that it enables the active control of the whole mirror, so as to suppress the deformations of the support structure due to the wind, gravity, thermal inhomogeneities etc. ..., thus getting the best possible stigmatism. However, providing active control of segmented mirrors requires numerous accurate edges sensors. It is acknowledged that capacitance-based technology nowadays offers the best metrological performances-to-cost ratio. As the leader in capacitive technology, FOGALE nanotech offers an original concept which reduces the cost of instrumentation, sensors and electronics, while keeping a very high level of performances with a manufacturing process completely industrialised. We present here the sensors developed for the Segment Alignment Measurement System (SAMS) of the Southern African Large Telescope (SALT). This patented solution represents an important improvement in terms of cost, to market the Position Sensors for Segmented Mirrors of ELTs, whilst maintaining a very high performance level. We present here the concept, the laboratory qualification, and the first trials on the 7 central segments of SALT. The laboratory results are good, and we are now working on the on-site implementation to improve the immunity of the sensors to environment.