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Laser launch telescopes are optical systems forming a reference beacon in the sky, to be used as a reference for ground telescope adaptive optics. One of the requirement of the system is that the laser beam should have a radius as small as possible at the target distance where the telescope detects the beacon. This is not trivial given the Gaussian nature of laser beams. Traditionally, laser launch telescopes consists of a simple afocal. Its magnification sets the output beam waist to an optimal radius that limits the laser beam divergence. We propose a configuration consisting of a combination of two afocal systems, each with a controlled defocus. Our system provides enough degrees of freedom to displace the waist at any distance in the sky and to achieve a smaller beam radius at the target distance. After presenting the theoretical concept of the combination, this paper describes the practical implementation of the system. We start by describing the design process and the expected performances. Then, the alignment and experimental verification process are emphasized. In particular, the verification of the controlled defocus is challenging as it cannot be done simply by putting a detector at several km from the optical system. Instead, the verification in laboratory is performed by performing interferometric measurement for different defocus and comparing them to theoretical simulations. The laser launch telescope will be used in various meteorological conditions. We describe the impact of thermal variations and how it can be compensated.
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