Astrometric Support for the Lunar-based Ultraviolet Telescope

Abstract

The Lunar-based Ultraviolet Telescope (LUT) is an astronomical instrument aboard Chang’e3, the lunar probe of China’s Lunar Exploration Program that successfully landed on the northern part of the Moon’s Mare Imbrium (340.4884E, 44.1214N) in late 2013. LUTis charting an ultraviolet map of the plane of the MilkyWay and is also providing long-term light variability monitoring for a sample of RR Lyrae stars. However, the principal goal of the computer-controlled landing of the probe was a safe descent to a stable resting-place, and therefore, the precise orientation of LUT was never a priority. For this reason, at least theoretically, touch-down could have oc- curred anywhere and, for LUT, at any attitude, which would make the pointing and tracking of the wanted celestial objects practically impossible. Moreover, to reduce the data transmission load, the whole frame of every exposure could not be downloaded: only the image data containing the objects canmake it to the ground; also, in order to save on electricity, the telescope does not usually track objects, which means that targets’ accurate positions and veloci- ties (within the focal plane CCD) are both needed. This paper presents the astrometric solution devised to solve these problems: feasibility is first shown with experiments done from Earth, and then confirmed with actual LUT ob- servations from the Moon’s surface.