Medium precision optical testing of a fast concave elliptical mirror by a cylindrical null screen

We extend the principles of the null screen method for testing a fast ellipsoidal concave mirror by designing a null cylindrical screen, located around the near focal point of the ellipsoid (being parallel to the optical axis) and the observing CCD camera is near the far focal point. We present the formulae to design the screen in such a way that the image on the CCD is a perfect square grid; the departures of the surface from a perfect ellipsoidal shape are observed as deformations of the grid in the image. We show qualitative experimental results.