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

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.