Research On Application Of Adaptive Optics For Lunar Laser Ranging

Lunar Laser Ranging(LLR) is an challenging experiment which relates to many fields such as optics,mechanism and electronics. The returned photon numbers are few (less than 1 photon) in LLR than in SLR(Satellite Laser Ranging) because of the long distance from earth to moon and other reasons. Adaptive optics is considered to be used in LLR to compensate the effect of the atmosphere and the other reasons such as the tracking error of the telescope. The tip-tilt mirror will be used in LLR to compensate the laser beam "jitter" so that the laser beam will be focused on the reflector, more energy will be received and reflected by the reflector so that the returned photon numbers will be increased.Several questions which relate to the application of the Adaptive Optics technic in LLR were discussed: First, The returned photon numbers were estimated when AO is used in LLR according to the performance of the 61 units adaptive optics system and 1.2m telescope of Yunnan observatory. Second, In order to compensate the laser jitter in time, Lunar surface image was used as the beacon to test the character of the atmosphere and the telescope. Absolute difference algorithm and cross-correlation algorithm were used to compute the relative image offsets from the low contrast extended lunar surface images. In order to test the two algorithms, Data of lunar surface near APOLLO 11 and APOLLO 15 were collected. Relative offsets between the reference image and the live images were computed by matlab software package. Long exposure images with and without removing the relative offsets are compared to test the effect of the two algorithms. Results reveal that the algorithms work well.Third, Field rotation Based on the 1.2m Alt-Az telescope of Yunnan observatory is computed in different conditions. It shows that field rotation shouldn't be ignored in LLR. In order to compensate the effect of the field rotation, The reference image should be changed in a short time or the detector should be rotated in the same velocity and the same direction. Last, the layout of the system according to 1.2m telescope was put forward. This is the primary discussion for adaptive optics application in LLR, there are a lot of work need to be done next and we hope our work could be helpful.