A high-efficiency lidar receiver architecture that emphasizes boosting the receiver collection efficiency of resonance-fluorescence and Doppler lidars has opened up new avenues of study for the mesosphere and lower thermosphere-extended (MLT-X) at sites in Boulder, Colorado, USA, and Cerro Pachón, Chile. Described in this work are in-depth considerations in the design, construction, and alignment of Na Doppler lidar receivers that have yielded signal levels typically 5-10 times higher per

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... aperture product than any demonstrated in the literature, to these authors' knowledge, making studies of finescale MLT turbulence and tenuous thermospheric layers in Na possible with temperature and vertical wind capability for the first time. A lowering of the detection threshold by higher receiver collection efficiency at Cerro Pachón has enabled this Na Doppler lidar to extend its measurement range far higher into the thermosphere, to regions with Na density less than 3 cm −3 . With renewed interest in the MLT-X region prompted by recent lidar discoveries of Fe in the thermosphere reaching 170 km at McMurdo, Antarctica, the receiver optimizations we have made now enable addressing an important need in the community. In addition, the higher spatial and temporal resolutions afforded by high signal-to-noise ratio, down to resolutions of ∼20 s and ∼20 m, promise to make the first direct measurements of eddy flux in the mesopause region possible. Results from deployment of optimized receivers at the Table Mountain Lidar Observatory in Boulder, the Andes Lidar Observatory at Cerro Pachón, and the Arecibo Observatory in Puerto Rico are presented to demonstrate the power and portability of our methods that are readily applicable to other lidar varieties, including, but not limited to, the newly developed Fe Doppler lidar and recently upgraded K Doppler lidar.