The absolute flatness measurements of two aluminum coated mirrors based on the skip flat test

Two methods based on the skip flat test are proposed to understand the absolute flatnesses of two aluminum coated mirrors, one with a circular aperture of 69 mm diameter, and the other with an elliptical aperture whose major and minor axes are 60 mm and 42 mm, respectively. For the mirror with the circular aperture, three measurements including one cavity measurement and two skip flat measurements are conducted. The measuring results are 0.079λ (PV) and 0.019λ (RMS). For the mirror with the elliptical aperture, two cavity measurements and one skip flat test are used. The measured results are 0.059λ (PV) and 0.014λ (RMS). The results agree well with those obtained with the mirrors measured at normal incidence, using high precision transmission flat as the reference. The agreements of the results indicate that the new methods could be used as two reliable ways to understand the absolute flatnesses of the aluminum coated mirrors.