Arc Focus Reference Alignment Patterns (AF)
|Name & IDs||Symbol||Typical Target Used||DOF and Sensitivity||Description|
|Arc Focus (AF-H, AF-V, AF-R, AF-T)||Reflective Tooling Ball||Coarse (visual) to fine interferometric alignment of dX, dY, dZ, and clocking||Projects a focused arc onto a curved reflective surface. Provides alignment in a single degree of freedom (along the line).
AF-H, AF-V, AF-R, and AF-T specify Horizontal, Vertical, Radial, or Tangential
Arc focus reference patterns create a line that comes to focus on the surface of a (typically) curved reflector, like a tooling ball. Rays that focus to a horizontal line along the surface of the tooling ball create a vertical focus at the center of curvature of the tooling ball, and vice versa.
The arc focus pattern functions similarly to the Line Focus (LF) alignment pattern. In this case, the pattern is sensitive to motion along the direction of the line on the surface of the tooling ball, and is insensitive to motion across the line.
AOM adds a few fringes of tilt to this null state to aid in visual alignment of fringes; instead of obtaining a null interferogram, you instead make the “V” fringes parallel. The V fringes also help indicate the direction of misalignment.
A video illustrates dY alignment: Horizontal tilt fringes are reduced until only vertical tilt fringes remain. The objective is to make the remaining vertical tilt fringes parallel.