APOLLO: Multiplexed Lunar Laser Ranging

The Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) is a nextgeneration lunar laser ranging (LLR) campaign aimed at order-of-magnitude improvements in tests of gravitational physics via millimeter range precision. We will employ the 3.5 m telescope at the Apache Point Observatory (APO), located in southern New Mexico at an altitude of 2800 m. As a result of the large aperture size and excellent seeing conditions at the site, APOLLO expects to detect 2–10 lunar return photons per pulse. Relative background immunity permits operation in daylight and at full moon, resulting in better sampling of the lunar orbit. We will use avalanche photodiode (APD) arrays as the detector for APOLLO, allowing multiple photons within a single return pulse to be individually time-tagged with high precision. Immediate advantages are shot-by-shot range profiles, as well as guaranteed calibration for each shot. In conjunction with a high time-precision start photodiode, one quickly builds an accurate representation of the convolved laser-detector-electronics temporal response. We describe here the hierarchical, multiplexed APOLLO timing system and its implementation using commercial electronics and a programmable logic controller. We also discuss the various calibration procedures geared toward millimeter precision. There is no practical limit to the number of channels employed in our timing scheme, allowing an upgrade path to larger APD array formats (10×10 or larger) in the future.