Proper collimation by an objective is obtained as soon as the light leaving the secondary light source can be retroreflected through the pinhole into the source. Retroreflection by an adjustable flat mirror that is slightly off axis lets one observe and judge the size of the return focus on the pinhole stop. The size of the return focus is then minimized by adjusting Z, the distance between the pinhole and the objective. The principle of autocollimation is represented in Fig. 4.1(a). Note that this test cannot be performed using a CCR.
With the introduction of autocollimators, the pinhole stop [label c in
Fig. 4.1(a)] is replaced by a reticle, which is more convenient for visual inspection of the coincidence of this reticle’s image with another reticle. An initial test of any collimating unit includes holding a flat mirror against the objective’s retainer ring in order to obtain a first impression of the angular position of the objective. There is no a priori assurance that the axial surface of the retainer for the collimating objective will be orthogonal to the optical axis of the objective, or that the cylinder axis will be parallel to the optical axis. The objective’s axial Z position is controlled by a fine thread on its retainer. The orientation of the collimation objective with no identical curvatures on its outer surfaces should minimize diffraction angles; a plano-convex lens will have its planar side facing the light source. An achromat will be mounted with the side having larger curvature facing the light source.
For the applications described in this book, an autocollimator like the one shown in Fig. 4.1(b) comes close to the preferred setup. The source unit and the return beam observation unit are assembled separately and ruggedly affixed to the housing, thus allowing some later retrofitting to the returned beam observation unit without affecting the collimation, once the assembly is completed. The retroreflected beam will be viewed through a 50/50 beamsplitter. The latter can be a commercially available thin pellicle. These expensive beamsplitters are sensitive to vibrations and attempts to clean off dust.
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