Large, convex surfaces, such as secondary mirrors, have presented challenging metrology problems for many years. Over the years, new metrology approaches have been developed to keep pace with the ever changing definition of "large". The latest class of large secondary mirrors requires a new approach that is practical, scalable and can produce low uncertainty measurements. This paper presents a new configuration that uses a computer generated hologram based Fizeau interferometer to make sub-aperture measurements on large secondary mirrors. One of the key features of this system is that all of the surfaces used in the interferometer are spherical. Another key element is the ability to perform simultaneous phase shift interferometry which reduces sensitivity to vibration. An example system that is capable of measuring the Large Synoptic Survey Telescope secondary mirror is presented along with a sensitivity analysis.