MRI: Development of A Real Time Antenna Metrology System for the Large Millimeter Telescope

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Observations of the Universe at millimeter wavelengths address a broad range of scientific questions. The 50m-diameter Large Millimeter Telescope Alfonso Serrano (LMT) is essential to this work. LMT was built by a partnership between the country of Mexico and the University of Massachusetts Amherst. It is the largest telescope of its type in the world and provides an important research tool for astronomers in the US. Very large antennas like the LMT must be carefully aligned to operate optimally. A major limitation is the result of the internal alignment of the telescope changing as it heats and cools. This project will build a system to measure the shape of the antenna surface and the internal alignment of the mirrors that will allow the LMT to adjust and maintain its alignment. The improvement will be most significant during daylight hours when solar heating of the structure causes the greatest deviations. The project makes extensive use of students and young LMT staff members to train the next generation of scientists and engineers. Research with the LMT connects astronomers in the US and Mexico encouraging scientific collaboration between the two countries.The ultimate performance of a large single dish radio telescope like the LMT is limited by its response to thermal gradients in the antenna structure that lead to deformations of the primary reflector surface, causing a loss of antenna gain. These deformations also change the relative position of the secondary and primary mirrors, leading to antenna focus and pointing errors. The real time metrology system to be developed by this project will directly measure internal alignments in the antenna and allow the LMT’s control systems to maintain a high level of performance even in the presence of transient deformations. Sixteen absolute distance measurements between points on the primary surface will be used to determine the coefficients of 14 low spatial order Zernike polynomials to characterize surface deformations. An additional eight absolute distance measurements between the surface and the secondary mirror are used to monitor the position and orientation of the mirror. Simulations show that transient primary surface mirror errors can be reduced to a level of ~20 microns rms, which removes thermal deformations as an important error source in the LMT’s overall surface accuracy. The secondary mirror measurement reduces pointing errors due to misalignment to the level of 0.2 arcsec rms.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.