2018AJ....156...43H

Radio astronomical observations below 30 MHz are hampered by the refraction and absorption of the ionosphere as well as the radio frequency interference (RFI), and thus, high angular resolution sky intensity map is not yet 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 arrays, 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 that can deal with a noncoplanar 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 can be reconstructed, even though the sky blocking by the Moon is different for each baseline. We also investigate how the map-making is affected by inhomogeneous baseline distributions.