Maintaining sub-pixel alignment for a single-camera two-bucket shearing speckle interferometer

In a phase stepped speckle interferometer, phase steps can be realized temporally or spatially. The spatial approach has the advantage of simultaneous acquisition of all phase stepped interferograms which eliminates phase errors due to phase changes that can occur between phase steps when phase is stepped in time. For spatially phase stepped systems based on multiple optical channels, it is important that interferograms are well aligned to the others. Alignment can be achieved and maintained by a system that does not rely strongly on high mechanical stability but is based on measuring the alignment errors and correcting them. The alignment system presented in this work uses speckle correlation to quantify alignment errors with sub-pixel accuracy, and comprises a piezo-electric controlled mirror to achieve precise alignment. The alignment method has been implemented for a two-channel shearing speckle interferometer. It allows for precise initial alignment both for translation and rotation of the two phase stepped speckle patterns on the CCD. Translation adjustments to maintain sub-pixel alignment during operation can be realized very quickly, preceding a test session. The importance of adequate alignment, and the effects of insufficient alignment, are illustrated by experimental results.