This paper discusses a method of adjusting a two-mirror collimator whose output mirror is a concave off-axis paraboloidal mirror, while its input mirror is either a flat mirror or a convex off-axis aspheric mirror. Two laser beams, a system of stops, and also two beamsplitters and a flat mirror are initially used during the adjustment to specify the direction of the axes of the aspheric base surfaces of the off-axis mirrors and the direction of the axis of the pencil of rays formed by the collimator, along with the position of the focal plane of the collimator. The optical system of a Twyman-Green interferometer is then formed whose signal branch includes the mirror elements of the adjustable collimator, as well as a flat mirror for implementing an autocollimation ray path in it. The adjustment is done with enhanced sensitivity, accuracy, and reliability because of the double pass of the signal beam through the adjustable collimator and by monitoring the state of the adjustment from the interference pattern.
N. P. Larionov, "Adjustment of two-mirror collimators with off-axis aspheric mirrors," J. Opt. Technol. 74, 401-406 (2007)