Effects of model incompleteness on the drift-scan calibration of radio telescopes

Abstract

Precision calibration poses challenges to experiments probing the redshifted 21-cm signal of neutral hydrogen from the Cosmic Dawn and Epoch of Reionization (z ∼ 30 − 6). In both interferometric and global signal experiments, systematic calibration is the leading source of error. Though many aspects of calibration have been studied, the overlap between the two types of instruments has received less attention. We investigate the sky based calibration of total power measurements with a HERA dish and an EDGES style antenna to understand the role of auto-correlations in the calibration of an interferometer and the role of sky in calibrating a total power instrument. Using simulations we study various scenarios such as time variable gain, incomplete sky calibration model, and primary beam model. We find that temporal gain drifts, sky model incompleteness, and beam inaccuracies cause biases in the receiver gain amplitude and the receiver temperature estimates. In some cases, these biases mix spectral structure between beam and sky resulting in spectrally variable gain errors. Applying the calibration method to the HERA and EDGES data, we find good agreement with calibration via the more standard methods. Although instrumental gains are consistent with beam and sky errors similar in scale to those simulated, the receiver temperatures show significant deviations from expected values. While we show that it is possible to partially mitigate biases due to model inaccuracies by incorporating a time-dependent gain model in calibration, the resulting errors on calibration products are larger and more correlated. Completely addressing these biases will require more accurate sky and primary beam models.

A Bayesian approach to high fidelity interferometric calibration − II: demonstration with simulated data

“…To address this, a model can be introduced for the temporal evolution of the calibration solutions (e.g. Gehlot et al 2021). The parameters of this model can be constrained by their own priors.…”

The Impact of Beam Variations on Power Spectrum Estimation for 21 cm Cosmology. I. Simulations of Foreground Contamination for HERA

“…Although these gain errors are largely kept to about 1%, when convolved with bright foregrounds this could easily swamp the intrinsic 21 cm signal, if the gain errors have sufficient structure. Gehlot et al 2021). In other words, the difference in the sky observed by unperturbed and perturbed primary beams is an additional potential source of chromatic gain errors (Orosz et al 2019;Barry & Chokshi 2022).…”

“…There are calibration techniques adopted by past and current interferometric experiments, including sky-based calibration (Pearson & Readhead 1984;Rau et al 2009) and redundant baseline calibration (Wieringa 1992;Liu et al 2010;Dillon et al 2020). The former relies on precise prior information about the sky, and an inaccurate or incomplete sky model can cause artificial frequency structure in calibration solutions (Barry et al 2016;Ewall-Wice et al 2017;Mouri Sardarabadi & Koopmans 2018;Gehlot et al 2021). The latter is accompanied by a key assumption of the redundancy in the measurements, and nonredundancy resulting from nonuniform primary beams can be a source of chromatic gain error (Byrne et al 2019;Orosz et al 2019;Choudhuri et al 2021).…”