Optimisation of calibration sources for global 21-cm experiments: the REACH case

TL;DR

This paper presents a novel method for selecting optimal calibration sources in the REACH 21-cm experiment, reducing calibration time and improving calibration accuracy for detecting the Cosmic Dawn signal.

Contribution

It introduces a new approach for choosing calibration sources that optimizes calibration efficiency and accuracy in 21-cm cosmology experiments.

Findings

Optimized calibration sources reduce temperature noise by approximately 15%.

Fewer calibrators can achieve similar or better calibration quality.

Frequency-specific optimization improves calibration precision.

Abstract

The spin-flip 21-cm signal from the Cosmic Dawn and the Epoch of Reionization is an essential probe of the conditions that led to the formation of the first luminous objects in the early Universe. However, its detection remains a major challenge owing to its low strength compared to the bright foregrounds and the requirement of precise calibration of the instrument to prevent systematics that could hinder a detection or lead to false inferences. REACH (Radio Experiment for the Analysis of Cosmic Hydrogen) is a radiometer experiment designed to detect this sky-averaged signal in the frequency range of 50--130~MHz. Using a wide-beam antenna, REACH calibration relies on internal reference sources, covering a broad range of temperatures and reflection coefficients. The choice of type and number of calibrators used significantly influences the quality of the calibration. This work investigates these effects and introduces a novel method for selecting an optimal set of calibration sources. With an optimised set, we aim to reduce calibration time, thereby increasing sky integration time while preserving calibration accuracy. We explore two optimisation strategies: one applied across the full receiver band and another performed on a frequency-by-frequency basis. Finally, we demonstrate that, with a total calibration time comparable to the conventional full-calibrator set, an optimised set with fewer calibrators achieves approximately a $15~\%$ reduction in calibrated temperature noise and improved absolute calibration of the instrument. This has implications for better calibration strategies in similar radiometer experiments.