Instrument Design and Analysis Techniques for Low-Redshift 21 cm Cosmology and Transient Detection with CHORD and CHIME

The Canadian Hydrogen Observatory and Radio-transient Detector (CHORD) is a compact, innovative “large-$N$ , small-$D$” radio interferometric array set for construction in British Columbianext to its older sibling, the Canadian Hydrogen Intensity Mapping Experiment (CHIME). With its 0.3–1.5 GHz band, sub-30 K system temperature, sub-millimeter precision dish surfaces, nearly 15,000 m$^2$ collecting area, and very broad field of view, it is designed to map the Universe’s matter distribution up to redshifts of $$ = 3.7 and boost fast radio burst (FRB) detection rate. This work emphasizes that CHORD’s array elements be located as close as possible to enhance sensitivity to lower k-modes of the matter power spectrum and minimize transient search cost. It also introduces arguments for using very deep dishes ($f/D = 0.21$) based on coupling and ground illumination considerations. Preliminary calibration results from the Deep Dish Development Array (D3A6) suggest that the signal chain’s system temperature matches specifications at lower frequencies ($\nu$ < 0.8 GHz), while further testing is required for higher frequencies. The CHORD feed, designed for the array, is inexpensive and quick to assemble, while maintaining a 30 K system temperature using ambient-temperature analog receiver hardware across the band. The design’s major novelty is an oversized backshort on an exponentially tapered (“Vivaldi”) design, adaptable to an observatory’s impedance and beam shape requirements via a simple optimization algorithm, particularly suitable for very deep dishes ($f/D ≤ 0.25$). In those dishes, the aperture efficiency remains above 50% over the band. Cosmological measurements are complicated by instrumental coupling and foregrounds. We show that coupling can be estimated from electromagnetic (EM) simulations of the array elements using a combination of their S-parameters, time-domain response, and beam shapes. Lowering the focal ratio of the dishes emerged as a powerful method to reduce coupling power. Foregrounds are typically removed from data using their spectral smoothness via a Fourier decomposition along the frequency axis; this can also be done with a singular value decomposition, and we successfully demonstrate this approach on CHIME data.

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