For the post-SELENE project, we are proposing a selenodetic mission, e.g. In-situ Lunar Orientation Measurement (ILOM) to study lunar rotational dynamics by direct observations of the lunar physical libration and the free librations from the lunar surface with an accuracy of 1 millisecond of arc (1 mas). One year-long trajectories of the stars provide the information on various components of the physical librations and they can also be used to possibly detect the lunar free librations in order to investigate the lunar mantle and the liquid core if any. The photographic zenith tube (PZT) supposed to be deployed on the Moon for the ILOM project is similar to that used for latitude observations of the Earth except the half mirror above the objective, a CCD with high well capacity, and the heater beneath the mercury pool. Although in principle, a star position on the focal plane does not change even if the telescope inclines, the tilt of the telescope affects the star position due to aberrations of the objective in the actual case. It is well known that somewhat tube tilt has little influence to the star position for the PZT telescope. We confirmed by the ray tracing that the effect of the tilt less than 100 arc seconds does not exceed 1 mas upon the star position. Thermal test of a tube made of CFRP showed that the tube did not incline by more than 100 seconds of arc even in the thermal condition of the lunar surface. We have a prospect to attain an accuracy of positioning of better than 1 mas from simulated experiments in laboratory using a CCD.
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