We have planned the Lunar observatory project, TSUKUYOMI aiming to meter-wavelength observations on the Moon. One of the scientific objectives is to observe the 21 cm global signal from the Dark Ages using the 1–50 MHz observing frequency range. The receiving system must have a noise temperature sufficiently lower than the foreground noise and also requires the flat bandpass response. To cover the ultra-wide bandwidth, an electrically-short dipole antenna and a preamplifier with high input impedance will be employed. This paper focuses on a feasibility study of the system performance. The environment of and around the observation site, such as the lunar surface dielectric constant and the antenna height from the ground plane, affects the sensitivity because it alters important parameters such as the antenna beam pattern and impedance. The investigation results of relationship between the surrounding environment and the sensitivity will be also reported.
Low-frequency radio observations below 50 MHz on the Moon are not subject to some radio interference, allowing for the study before the formation of the first star, which is impossible from the Earth. Our lunar observatory project, TSUKUYOMI, aims to observe the 21cm global signal from the Dark Ages, requiring wideband observations covering 1-50MHz to spot absorption features of ∼ 40 mK relative to the CMB. Considering the radiation from the Milky Way, which is the main foreground noise source and the reception characteristics of the short dipole antenna, a pre–amplifier with a noise lower than 2nV/√ Hz and an input capacitance of 25pF will result in a system noise well below foreground noise over the entire bandwidth and a roughly flat wideband response. Managing the input/floating capacitance and using a lumped constant circuit is crucial for wideband performance. This paper outlines the wideband system and delves into the system performance requirements.
Aim to Japan's participation in the Artemis program in the 2030s in mind, we pursue the feasibility studies of lunar telescope, including astronomical observations. Focusing on the meter-wavelength observations (observing frequency of lower than 50MHz), which cannot be observed in the harsh environments on the ground from the Earth, including the ionosphere and radio frequency interference, we have reported on conceptual design based on the results of our feasibility studies in Japan. The main scientific objectives we have studied so far are broadly covering the following three areas: astronomy and astrophysics, planetary science, and lunar science. In astrophysics, the observing frequency range of 1- 50MHz gives us an opportunity to observe the 21 cm global signal (spatial average temperature) from the Dark Ages, which is determined purely by cosmology and is not affected by first-generation star formation and cosmic reionization. In astronomy, it provides the images of the Milky Way galaxy at meter wavelengths. In planetary science, it will be possible to study the environments of exoplanets through 1) radio waves from auroras on gas giant exoplanets like Jupiter and 2) stellar radio-wave bursts. In lunar science, it has the potential to observationally study the lunar ionosphere, subsurface structure, and dust environment. At present, we plan the meter-wavelength interferometric array as lunar telescope, including the single-dish observations. In this paper, focused on the scientific requirements from cosmology, we will report the design concepts of Japanese lunar telescope project, including the advanced feasibility studies of antenna, receiver, signal chain and spectrometer that are compared as other studies in US, China and Europe. We named this project TSUKUYOMI.
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