The content of OH/H2O molecules in the tenuous exosphere of the Moon is still an open issue at present. We here report an unprecedented upper limit of the content of the OH radicals, which is obtained from the in situ measurements carried out by the Lunar-based Ultraviolet Telescope, a payload of Chinese Chang'e-3 mission. By analyzing the diffuse background in the images taken by the telescope, the column density and surface concentration of the OH radicals are inferred to be <10(11) cm(-2) and < 10(4) cm(-3) (by assuming a hydrostatic equilibrium with a scale height of 100 km), respectively, by assuming that the recorded background is fully contributed by their resonance fluorescence emission. The resulted concentration is lower than the previously reported value by about two orders of magnitude, and is close to the prediction of the sputtering model. In addition, the same measurements and method allow us to derive a surface concentration of < 10(2) cm(-3) for the neutral magnesium, which is lower than the previously reported upper limit by about two orders of magnitude. These results are the best known of the OH (MgI) content in the lunar exosphere to date. (C) 2015 Elsevier Ltd. All rights reserved.

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 photons(-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 x 10(8) molecules cm(-3). However, the effect on the performance is considerable when the exosphere is as dense as suggested by CHACE. (C) 2011 COSPAR. Published by Elsevier Ltd. All rights reserved.