A paper describes the laser truss sensor (LTS) for detecting piston motion between two adjacent telescope segment edges. LTS is formed by two point-topoint laser metrology gauges in a crossed geometry.
A high-resolution (<30 nm) LTS can be implemented with existing laser metrology gauges. The distance change between the reference plane and the target plane is measured as a function of the phase change between the reference and target beams. To ease the bandwidth requirements for phase detection electronics (or phase meter), homodyne or heterodyne detection techniques have been used.
The phase of the target beam also changes with the refractive index of air, which changes with the air pressure, temperature, and humidity. This error can be minimized by enclosing the metrology beams in baffles. For longer-term (weeks) tracking at the micron level accuracy, the same gauge can be operated in the absolute metrology mode with an accuracy of microns; to implement absolute metrology, two laser frequencies will be used on the same gauge. Absolute metrology using heterodyne laser gauges is a demonstrated technology. Complexity of laser source fiber distribution can be optimized using the range-gated metrology (RGM) approach.
This work was done by Duncan T. Liu, Oliver P. Lay, Alireza Azizi, Hernan Erlig, Leonard I. Dorsky, Cheryl G. Asbury, and Feng Zhao of Caltech for NASA’s Jet Propulsion Laboratory. NPO-47753
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Laser Truss Sensor for Segmented Telescope Phasing
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Overview
The document discusses the development of a Laser Truss Sensor (LTS) designed for phasing segmented telescopes, particularly for the Giant Magellan Telescope (GMT). Segmented telescopes require precise control of the positions of their mirror segments to counteract disturbances from gravity, temperature changes, and wind forces. Traditional methods, such as capacitor-based edge sensors, are inadequate for large inter-segment gaps, prompting the need for innovative solutions.
The LTS employs laser metrology to measure the relative positions of mirror segments with high precision. It aims to achieve an accuracy of 30 nanometers for segment positioning, with real-time measurements at approximately 200 Hz. The system utilizes an off-axis guide star for accurate phase measurements between segments, while the edge sensors provide continuous monitoring to ensure stability over time.
One of the key advantages of the LTS is its insensitivity to common thermal changes and in-plane motion of the mirrors, which are significant challenges in maintaining alignment. The document outlines how the LTS can filter out thermal errors due to its operational design, which allows for effective control over a one-minute duration. This capability is crucial for minimizing measurement errors caused by slow-varying thermal distortions.
The document also highlights the use of existing laser metrology gauges developed at NASA's Jet Propulsion Laboratory (JPL), which have demonstrated high resolution and accuracy. The gauges can operate in both relative and absolute metrology modes, allowing for long-term tracking of mirror positions with micron-level accuracy. The implementation of homodyne or heterodyne detection techniques further enhances the system's performance by easing bandwidth requirements for phase detection.
In summary, the Laser Truss Sensor represents a significant advancement in the field of segmented telescope technology, addressing the limitations of previous methods and providing a robust solution for maintaining the alignment of large telescope segments. This innovation is essential for the successful operation of the GMT and similar astronomical instruments, ultimately contributing to the advancement of astronomical research and exploration.
