TEST OF A SLOW OFF-AXIS PARABOLA AT ITS CENTER OF CURVATURE

We describe the interferometric testing of a slow (f/16 at the center of curvature) off-axis parabola, intended for use in an x-ray spectrom eter, that uses a spherical wave front matched to the mean radius of t he asphere. We find the figure error in the off-axis mirror by removin g the theoretical difference between the off-axis segment and the sphe rical reference from the measured wave-front error. This center of cur vature test is easy to perform because the spherical reference wave fr ont has no axis and thus alignment is trivial. We confirm that the tes t results are the same as the double-pass null test for a parabola tha t uses a plane autocollimating mirror. We also determine that the off- axis section apparently warped as the result of being cut from the sym metric parent part.