Locating the Optical and Focal Axes of an OAP

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Figure 2: The orientation of the optical axis can be found by noting it is perpendicular to the base of the mirror's substrate. The location of the focal point can be estimated by considering collimated light rays that are directed parallel to the optical axis. These rays reflect symmetrically around the local surface normals and pass through the mirror's focal point.

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Figure 1: OAP mirrors have a flat, round base and a side that varies in height around the circumference. The planar base is normal to the mirror's optical axis. Shown above is the MPD2151-P01.

When working with off-axis parabolic (OAP) mirrors, it can be challenging to identify the optical and focal axes. This is particularly true when the parabolic curvature of the surface is hard to see (Figure 1).

The physical characteristics and dimensions of the mirror's substrate can provide a useful guide when positioning and aligning the mirror.

The mirror's substrate has a flat, round base. The optical axis is oriented normal to this planar base. Therefore, collimated light should be directed normal to the surface of the base.

The substrate has a tall side and a short side, and the reflective surface is sloped between them. The surface normal at different points across the reflector can be roughly estimated by visually examining the surface (Figure 2).

The location of the focal point can be estimated by considering a ray of collimated light, parallel to the optical axis, that reflects from the surface of the mirror. The incident ray reflects symmetrically about the surface normal. The reflected ray will pass through the focal point. By mentally tracing two rays from positions close to the tall and short sides of the mirror, respectively, it should be possible to estimate the location of the focal point.

Mounting and Alignment Features on Thorlabs' OAP Mirrors
Thorlabs' OAP mirrors have an alignment hole and three tapped mounting holes machined into the bottom surface of their bases. The pattern of tapped holes matches the vertices of an equilateral triangle, and the position of the smooth-bore alignment hole indicates the short side of the OAP mirror. The tapped holes are designed to secure the mirror to mounting adapters or mounting platforms.