Method for quantitatively determining petal piston in a segmented imaging assembly

A method suitable for determining 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 the steps of intercepting preselected portions of the imaged radiation beam with a mask, for masking at least one portion of at least one of the reference component and the test component, so that a portion of the imaged radiation beam incurs a predetermined and differential phase retardation, relative to another portion of the beam passing through the mask. These actions create 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 a determination of the magnitude of the real time test component piston.