Benefit of an Off-Axis Parabolic Mirror

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Figure 2: An off-axis parabolic mirror can be thought of as a section of the larger parabolic shape. Both have the same focal point, but it is more accessible in the case of an OAP mirror.

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Figure 1: The focal point of an on-axis parabolic mirror is close to the reflective surface, and typically surrounded by the reflective surface, which makes the focal point difficult to access.

Both symmetric parabolic and off-axis parabolic (OAP) mirrors have a single focal point. The benefit of an OAP mirror is that its focal point is accessible, unlike that of a symmetric parabolic mirror.

The single focal point is a primary benefit parabolic mirrors and is useful for a range of purposes, including imaging and manufacturing applications that require focusing laser light to a diffraction limited spot. 

There are a few negatives associated using with using conventional parabolic mirrors, which are symmetric around the focal point (Figure 1). One is that the sides of the mirror generally obstruct access to the focus. Another is that when the mirror is used to collimate a divergent light source, the housing of the light source blocks a portion of the collimated beam. In particular, light emitted at small angles with respect to the optical axis of the mirror is typically obstructed.

An off-axis parabolic (OAP) mirror (Figure 2) is one solution to this problem. The reflective surface of this mirror is parabolic in shape, but it is not symmetric around the focal point. The reflective surface of the OAP corresponds to a section of the parent parabola that is shifted away from the focal point. The section chosen depends on the desired angle and / or distance between the focal point and the center of the mirror.