A problem can arise when a set of optics must be aligned but intrinsic surface errors
dominate alignment wavefront errors. Aligning diamond-turned optics interferometrically at
visible wavelengths is one such example. Diamond-turned optics can exhibit a high-spatialfrequency
surface ripple from the machining process, which, in many cases, can render an
interferogram unintelligible to the point that even serious alignment errors cannot be detected.
In an alignment demonstration conducted last year this problem was encountered head on
and two techniques were applied to extract meaningful wavefront data. The first relied on
spatial filtering of the return wavefront to smooth out the effects of high-slope surface errors.
This approach showed potential in that it is a simple method that can be easily applied. The
second approach used a new software algorithm, available as part of the Zygo Mark lV phase
measuring interferometer, where regions of fringe discontinuities are discarded and the
resultant piece-wise phase map is reconstructed as a continuous wavefront. Using this latter
approach, we were able to precisely align a three-mirror telescope comprised of diamondturned
mirrors and used in a double-pass configuration. The approaches to be described are
applicable to the alignment of infrared and visible sensors and metrology of single surfaces.