The need for final form metrology under gravity loading (in the use orientation) is often encountered in the manufacture of large optical reference surfaces and artifacts. This is primarily due to the difficulty in predicting the self-weight deflection at the nm level due to the uncertainty associated with representing the actual boundary conditions in a finite element model. The primary metrology technique for the inspection of such artifacts is Fizeau interferometry, where in the part under test (PUT) is compared to an optical reference of known shape (often referred to as the transmission flat or TF). In a typical Fizeau setup, the optical axis is arranged to be horizontal, i.e., the PUT is oriented vertically. However, in many applications, the use configuration of the PUT is horizontal, i.e., perpendicular to the gravity vector and the final form is desired in this orientation. This is achieved by either orienting the entire interferometer vertically or introducing a fold mirror (Figure 1).
展开▼
