Method for simultaneously determining coarse and fine petal piston in a segmented imaging assembly

A method for simultaneously determining coarse and fine petal piston in a segmented imaging assembly. The assembly preferably comprises a source of radiation that outputs a radiation beam; and, a segmented imaging system comprising a reference component and a test component, which components can be used to form an imaged radiation beam. The method comprises a first step of intercepting preselected portions of the imaged radiation beam with a novel composite mask. The composite mask comprises apertures which are dedicated to coarse petal alignment, and four phase retardation regions dedicated to fine petal alignment. The first step creates a real time masked image. A step of comparing the real time masked image against a family of off-line images generated for a sequence of known petal position errors, can provide the magnitude of the fine piston, while a step of interrogating an envelope and a fine structure spacing of the real time masked image, can provide the magnitude of the coarse piston.