A conventional laser beam expander consisting of confocal parabolic mirrors is quite sensitive to misalignment of its components. Beam steering by pivoting the small mirror about its "neutral point" gives a rather limited steering range. The ideal beam expander would be insensitive to misalignment of its components and would also provide substantial amounts of beam steering. At the same time it would be simple and have a substantial field view. This paper describes a two-element system with all these properties, and compares its features with a confocal parabola system counterpart.
"Optimized Alignment-Insensitive Two-Element Laser Beam Expander/Steerer", Proc. SPIE 0251, Optical Alignment I, (31 December 1980); doi: 10.1117/12.959444; https://doi.org/10.1117/12.959444