As seen in Sec. 2.4 wavefront sensors measure any one of three parameters: W, dW, and T. Sensors which measure W determine the wavefront directly, while those which measure dW and T do so indirectly. This chapter discusses the former while the next two chapters address the latter. To obtain the optical path difference W(x,y) a reference wavefront needs to be generated somehow directly from the aberrated wavefront. There are basically two ways of doing this: radial shear interferometry and point diffraction interferometry.
4.2 Radial Shear Interferometry
The radial shear interferometer (RSI) is a modified Mach-Zehnder interferometer with afocal telescopes in each arm. One telescope expands the beam while the other compresses the beam as shown in Fig. 4.1. (Obviously both telescopes must themselves be of high quality in order for this method to work properly.)