  Grazing incidence interferometry has been applied to rough planar and
cylindrical surfaces. As suitable beam splitters diffractive optical phase
elements are quite common because these allow for the same test sensitivity for
all surface points. But a rotational-symmetric convex aspheric has two
curvatures which reduces the measurable region to a meridian through the vortex
of the aspheric, which is in contrast to cylindrical surfaces having a
one-dimensional curvature which allows the test of the whole surface in gracing
incidence. The meridional limitation for rotational-symmetric aspherics
nevertheless offers the possibility to measure single meridians in a one-step
measurement. An extension to the complete surface can be obtained by rotating
the aspheric around its vortex within the frame of the test interferometer.
