Stemmed Mirrors

Unique Design Minimizes Mounting Stress and Deformation

Mounting flat mirrors by their edges in a kinematic mount imparts stress onto the mirror surface. This results in distortion and reduced quality of the reflected wavefront, which is especially noticeable when using high-quality mirrors. Stemmed mirrors, on the other hand, are mounted from a smaller diameter “stem” protruding from the back of the mirror, resulting in significantly reduced stress on the mirror surface, high stability, and cost reduction and can be used as a replacement for a more expensive and complex kinematic mount and a conventional mirror.

 What’s Wrong with Conventional Mirrors?

The flatness specified for a first surface mirror may not reflect the actual flatness of it in its end application. Positioning the mirror in a mount and applying pressure to secure it distorts the surface, degrading the quality of the reflected wavefront. Such was the case when engineers from Edmund Optics^® mounted a conventional mirror with a flatness of λ/10 and secured it with up to 12 in-oz of torque (“hand-tight”). The flatness degraded to λ/5, changing the specification by a factor of 2.

 The Solution: Stemmed Mirrors

Stemmed mirrors feature monolithic glass substrates shaped into their characteristic geometry. All contact between a stemmed mirror and its mechanical mount is through the mirror’s stem, significantly decreasing the stress imparted on the mirror surface (Figure 1). A simple kinematic mount can secure stemmed mirrors with a high degree of positional stability without noticeably impacting performance. In order to maintain a similar flatness, conventional mirrors would need to be secured using much more complex and expensive kinematic mounts. The total cost of a stemmed mirror and a simple mount is less than that of a conventional mirror and the complex mount that would be required to maintain a similar flatness.

Figure 1: The mirror stem is secured in a kinematic mount, decoupling mounting stability from the flatness of the mirrored surface.

To quantify the level of aberrations introduced to both a λ/10 stemmed mirror and a λ/10 conventional first surface mirror, both were placed in an identical kinematic mount while a controlled amount of torque was applied. As shown in Figure 2, the stemmed mirror maintained its λ/10 flatness specification through up to 12 in-oz of torque (which is considered “hand-tight”) while the conventional mirror went out of specification under the same torque with a final flatness of λ/5. Figure 2 also separates the contributions of different classifications of optical aberrations, revealing that the most detrimental aberration was astigmatism, followed by power error.

Figure 2: Stemmed Mirrors were shown to maintain surface flatness better than conventional mirrors by a factor of 2.