An imaging algorithm for a lunar orbit interferometer array

TL;DR

This paper introduces a novel imaging algorithm for a lunar orbit interferometer array that overcomes unique challenges like wide field of view, mirror symmetry, and lunar blocking, enabling high-resolution sky maps in radio frequencies below 30 MHz.

Contribution

The paper presents a new imaging algorithm tailored for lunar orbit interferometer arrays, addressing issues of wide sky coverage, mirror symmetry, and lunar blocking effects.

Findings

The algorithm can produce accurate sky maps despite lunar blocking.

Mirror symmetry is broken by the three-dimensional baseline distribution.

Good quality maps are reconstructed even with inhomogeneous baseline distributions.

Abstract

Radio astronomical observation below 30 MHz is hampered by the refraction and absorption of the ionosphere, and the radio frequency interference (RFI), so far high angular resolution sky intensity map is not available. An interferometer array on lunar orbit provides a perfect observatory in this frequency band: it is out of the ionosphere and the Moon helps to block the RFIs from the Earth. The satellites can make observations on the far side of the Moon and then send back the data on the near side part of the orbit. However, for such array the traditional imaging algorithm is not applicable: the field of view is very wide (almost whole sky), and for baselines distributed on a plane, there is a mirror symmetry between the two sides of the plane. A further complication is that for each baseline, the Moon blocks part of the sky, but as the satellites orbit the Moon, both the direction of the baseline and the blocked sky change, so even imaging algorithms which can deal with non-coplanar baseline may not work in this case. Here we present an imaging algorithm based on solving the linear mapping equations relating the sky intensity to the visibilities. We show that the mirror symmetry can be broken by the three dimensional baseline distribution generated naturally by the precession of the orbital plane of the satellites. The algorithm is applicable and good maps could be reconstructed, even though for each baseline the sky blocking by the Moon is different. We also investigate how the map-making is affected by inhomogeneous baseline distributions.