Control of flexure in large astronomical spectrographs

This thesis describes the design, construction and testing of an experimental system for improving the imaging stability on the detectors of the Intermediate-dispersion Spectroscopic and Imaging System (ISIS), a Cassegrain spectrograph at the 4.2 metre William Hershel Telescope. This system, called ISAAC (ISIS Spectrograph Automatic Active Collimator) is based on the new concept of active compensation, where spectrum shifts, due to the spectrograph flexing under the effect of gravity, are compensated by the movement of an active optical element. ISAAC is a fine steering tip-tilt collimator mirror. The thesis provides an extensive introduction on astronomical spectrographs, active optics and actuator systems. The new concept of active compensation of flexure is also described. The problem of spectrograph flexure is analyzed, focusing in particular on the case of ISIS and on how an active compensation system can help to solve it. The development of ISAAC is explained, from the component specification and design, to the construction and laboratory testing. The performance and successful testing of the instrument at the William Herschel Telescope is then described in detail. The implications for the future of ISIS and of new spectrograph designs are then discussed, with particular stress on the new High Resolution Optical Spectrograph (HROS) for the 8-metre Gemini telescopes

The Value of Structural Health Monitoring of a Corroded Reinforced Concrete Beam

With the increasing number of aging structures worldwide, structural health monitoring (SHM) has gained a lot of research interest. Structural health monitoring (SHM) can provide real-time information about a structure’s actual condition, thereby mitigating the risk of failure if the structural condition is worse than presumed, or extending the service life and saving the replacement costs if it has an adequate level of safety. Many SHM techniques have been developed in the past 40 years; however, few of them have been successfully implemented on real structures. The limited practical application of SHM has been attributed to the lack of mature and sophisticated SHM techniques and the lack of economic studies to clearly demonstrate the financial benefits to the structural owners. Christensen et. al described the theoretical principle of a surface strain-based SHM technique for reinforced concrete beams in the book “Monitoring Technologies for Bridge Management” in 2011. The SHM technique is designed to estimate the remaining effective cross-sectional area of the reinforcing bars after corrosion, which can then be used to predict the remaining structural capacity and service life, as well as the degree of certainty associated with these predictions. As part of this research project, laboratory experiments were conducted to evaluate the effectiveness of the surface strain-based SHM technique on nine small-scale reinforced concrete beams. The experimental and data processing procedures were first calibrated to obtain more reliable results. The effectiveness of the proposed SHM technique was then determined and quantified using the errors between the predicted beam capacities using the identified optimal procedures and the actual failure loads. It was found that the proposed technique did not achieve accurate estimates of the remaining cross-sectional area of the reinforcing bars or failure load when applied to the small and slender beams. However, it is believed to have potential to provide better result on large-scale beams. The experimental results were also used to demonstrate the value of SHM systems through reliability and economic analyses. Two monitoring systems with different levels of uncertainty were created. The standard monitoring system was composed of strain measuring equipment only, while the enhanced monitoring system included the strain measuring equipment and a cover meter, used to reduce the uncertainty of the reinforcing bar locations. It was demonstrated that, although the enhanced SHM system was associated with a higher cost, it consistently provided a higher reliability index – leading to an extension of service life – and lower annual worth of life cycle costs (AWLCC) when replacement decisions were based on the respective SHM data