Surface metrology using one-element interferometry

Surface metrology is the key to make and retain high quality surfaces. There are many coordinate measurement methods and machines to measure large 3D surfaces. Some methods are: triangulation, time-of-flight, structured-lighting, photogrammetry and radio-holography. They differ in ease-of-use, economy, complexity and accuracy. Some, e.g. photogrammetry, measure point-by-point while others, e.g. holography, measure a point-cloud at one go. Some are spot-measurements while others are areal averages. Both photogrammetry and radio-holography are used to measure the surfaces of astronomical telescopes. Both phase-referenced and phase-recovered holographic techniques are used. In this paper we present yet another method to measure surfaces of large astronomical telescopes.