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The separation of cosmological signal from astrophysical foregrounds is afundamental challenge for any effort to probe the evolution of neutral hydrogenduring the Cosmic Dawn and epoch of reionization (EoR) using the 21 cmhyperfine transition. Foreground separation is made possible by their intrinsicspectral smoothness, making them distinguishable from spectrally complexcosmological signal even though they are ~5 orders of magnitude brighter.Precisely calibrated radio interferometers are essential to maintaining thesmoothness and thus separability of the foregrounds. One powerful calibrationstrategy is to use redundant measurements between pairs of antennas with thesame physical separation in order to solve for each antenna's spectral responsewithout reference to a sky model. This strategy is being employed by theHydrogen Epoch of Reionization Array (HERA), a large radio telescope in SouthAfrica that is now observing while being built out to 350 14-m dishes. However,the deviations from perfect redundancy inherent in any real radio telescopecomplicate the calibration problem. Using simulations of HERA, we show howcalibration with antenna-to-antenna variations in dish construction andplacement generally lead to spectral structure in otherwise smooth foregroundsthat significantly reduces the number of cosmological modes available to a 21cm measurement. However, we also show that this effect can be largelyeliminated by a modified redundant-baseline calibration strategy that reliespredominantly on short baselines.

Mitigating the Effects of Antenna-to-Antenna Variation on Redundant-Baseline Calibration for 21 cm Cosmology