| Résumé | High precision calibration is essential for the new generation of radio interferometers looking for Baryon Acoustic Oscillation signatures in neutral hydrogen emissions which is a signal buried in foregrounds. Yet, differences in instrument design and non-redundancies in the subsystems consisting such instruments can pose great challenges to proper calibration. For instance, Canadian Hydrogen Intensity Mapping Experiment (CHIME) offers a large instantaneous field of view with fast mapping speed with a sensitivity equivalent to a single dish in terms of total collecting area. However, the calibration of its 1000+ individual receivers poses major obstacle to harness the full capability of the new technology. In future instruments, it is planned to use redundancy at the level required for science experiments (typically below 1%). Although, this idea is against the traditional knowledge of having the radio receivers accurate to 1/20th of the observed wavelength, we plan to meet this goal by solving the redundancy issue in the front-end subsystems using precise metrology and alignment methods. To achieve this goal, we work on a 2-element interferometer called Deep Dish Development Array (D3A), with a targeted precision of 1/1000th of a wavelength at 300 MHz, located at Dominion Radio Astrophysical Observatory, located in Penticton, BC, Canada. The D3A will serve as test bed for dish prototyping, composite dish repeatability, antennae back ends which can be scaled to larger arrays without sacrificing the precision. The two dishes are made out of fiber glass composite and measured in the shop condition and in the field after fabrication. The relative pointing accuracy was obtained as 0.02°. The elevation axes of the dishes were placed in East-West direction within 0.04° of error. The surface RMS error was obtained as 0.389 mm which meets the 1/1000th of observation wavelength. RMS error due to temperature variation and assembly error on the dishes were obtained at 1 mm. This article presents the metrology principles applied to obtain the results and challenges. The tests performed in D3A will be implemented in Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX) and the Canadian Hydrogen Observatory and Radio Transient Detector (CHORD). |
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