Lunar Exosphere Influence on Lunar-based Near-ultraviolet Astronomical Observations

TL;DR

This study predicts the impact of the lunar exosphere on near-ultraviolet observations from the Lunar-based Ultraviolet Telescope, showing that exosphere emissions can moderately degrade detection capabilities depending on its density.

Contribution

It provides the first quantitative assessment of how lunar exosphere emissions affect UV astronomical observations from a lunar-based telescope.

Findings

Sky brightness due to exosphere is less than 8.7 photons s^{-1} cm^{-2} arcsec^{-2} in 245-340 nm.

Detection of a 13th magnitude source remains feasible with S/N > 8 if OH concentration is below 2×10^8 molecules cm^{-3}.

High exosphere density can significantly impair the telescope's observational performance.

Abstract

The potential effect of the lunar exosphere on the near-ultraviolet sky background emission is predicted for Lunar-based Ultraviolet Telescope (LUT: a funded Chinese scientific payload for the Chang'e-III mission). Using the upper limit on the OH concentration inferred from the recent MIP CHACE results, our calculations show that the sky brightness due to the illuminated exosphere is $<8.7\ \mathrm{photon\ s^{-1}\ cm^{-2}\ arcsec^{-2}}$ within the wavelength range 245-340 nm. By evaluating the signal-to-noise ratios of observations of an AB=13 mag point source at a series of sky background levels, our analysis indicates that the detection performance of LUT can be moderately degraded by the lunar exosphere emission in most cases. An AB=13 mag point source can still be detected by the telescope at a signal-to-noise ratio more than 8 when the OH concentration is less than $2\times10^8\ \mathrm{molecules\ cm^{-3}}$. However, the effect on the performance is considerable when the exosphere is as dense as suggested by CHACE.