Comparison of relay zoom and changing transmission spheres to change the lateral magnification of a Fizeau interferometer

Abstract

To obtain higher spatial resolution interferometric measurements, users of optical shop interferometers generally want to obtain the highest possible number of pixels in the field of view. When the optical surface being tested does not fill the interferometer’s field of view, zoom optics in the viewing system can provide a convenient means to fill the detector. Some users employ zoom to measure subapertures of a larger optical surface to observe mid-spatial frequency (MSF) features that may not be seen in a full aperture test. While the zoom obviously enables the detector to be filled, its capability to increase the MSF measurement performance of the instrument is more difficult to assess. To investigate how zoom affects the MSF measurement capability, we measured a spherical surface with significant MSF content over a range of lateral magnifications. Two methods were used to obtain equivalent lateral magnifications: zoom and changing the transmission sphere. Differences in the relative MSF content were observed between the two methods. For further comparison, the same surface was measured with the same transmission spheres on a different interferometer with a fixed magnification coherent viewing system. We report on differences observed in measured MSF content between the two interferometers.