Generally, in order to gain high accuracy in aspheric testing, a piece of high-quality CGH (computer generated hologram) is inserted behind transmission sphere to generate specified wave-front to match aspheric part. According to the difference in function, the CGH is divided into 2 parts: the center region, called as testing hologram, is used to generate specified aspheric wave-front; the outer ring, called as alignment hologram, is used to align the location of CGH behind transmission sphere. Although alignment hologram is used, there is still some adjustment error from both CGH and aspheric part, such as tilt, eccentricity and defocus. Here we will stimulate the effect of these error sources on the accuracy that is rms after the piston, tilt and power are removed, when testing a specified aspheric part. It is easy to conclude that the total measurement error is about 2 nm and the defocus of CGH contributes most.
A convenient method to study the influence of error sources in Fizeau is to build a ray-tracing model to simulate the error sources. In this paper an interferometer model is presented; an extension program is called to simulate the interference; and a preliminary research of several error sources is conducted. These examples demonstrate error analysis based on interferometer models is feasible and provide some guidance for optimizing our interferometer design.