This paper presents a recent progress in designing a nanometer-accurate inductive-type of edge sensor. A new
technology of Direct Deposit on Glass (DDG) has been developed. The DDG technology also has high rate of
reproducibility and is suitable for the mass production and still be a very cost-effective solution. The sensor designs are
fully optimized using mechanical and EM modeling for specific metrology needs. We discuss briefly on various
electronic architectures and their impacts on the maintainability of the system. We conclude the paper by showing the
experimental results.
The largest telescopes (ELT) involve a highly segmented primary mirror. The monitoring of the mirror shape use
thousands of sensors, located on segment edges, which measure the relative piston, tip and tilt of all segments.
Today, telescopes with segmented primary mirrors use
capacitance-based edge sensors. Although this technology offers
excellent metrological performances, its practical use is limited by the intrinsic sensitivity to humidity, dust and
condensation, whose effect exceeds the requirements for future ELTs.
Being specialized in both capacitive and inductive metrology, Fogale nanotech has developed a novel concept of edge
sensors using the inductive technology, which does not suffer from humidity, condensation, and dust effects. Cost
effective sensor with specific layout, associated electronics, demonstrated metrological performance (sub-nanometer
resolution and nanometer stability) that outperforms the capacitive concept.
KEYWORDS: Sensors, Mirrors, Sensing systems, Humidity, Temperature metrology, Control systems, Simulation of CCA and DLA aggregates, Actuators, Environmental sensing, Large telescopes
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.
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