Method for determining large petal piston in a segmented imaging assembly

A method suitable for determining large petal piston in a segmented imaging assembly. The assembly preferably comprises a polychromatic source of radiation that can output a polychromatic radiation beam; and, a segmented imaging system including petals comprising a reference component and a test component, which components can be used to form an imaged radiation beam. The method steps comprise intercepting preselected portions of the imaged radiation beam with a mask comprising two apertures, so that one aperture is dedicated to radiation imaged by the reference component, and the second aperture is dedicated to radiation imaged by the test component, thereby creating a real time masked image; and, interrogating an envelope and a fine structure spacing of the real time masked image, as a measure of the large petal piston of the segmented imaging system.