Align an Off-Axis Parabolic Mirror Using Its Optical Axis and Focus

Mounting and Aligning an OAP Mirror

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Figure 2: When using an OAP mirror to collimate a point source, a shear plate interferometer placed in the output beam can facilitate the alignment process.

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Figure 1: The shape of the OAP mirror's reflective profile matches a section of the parent parabola that is not centered on the focal point. Due to this, the OAP's reflective surface is not rotationally symmetric. When mounting the mirror, care should be taken to ensure the mirror does not rotate around its optical axis.

OAP mirrors are not rotationally symmetric. This is a consequence of their reflective surfaces being taken from sections of the parent parabola curve located away from the focal point (Figure 1). Due the asymmetry of the reflector, when an OAP mirror rotates, the position of its focal point also rotates. Since this could negatively impact the performance of an optical system, the mirror should be fixed so that the reflective surface cannot rotate around its optical axis.

The optical performance of the mirror is also sensitive to alignment drift with respect to the other five degrees of freedom. One way to protect against alignment drift is to use a fixed, rather than a kinematic, mount.

Using a shear plate interferometer can be helpful when aligning an OAP mirror to an input point source. The shear plate interferometer should intercept the output beam (Figure 2), to assess its collimation quality. Alignment is optimized when the quality of the collimated beam is optimized.