Testing a fast off-axis parabolic mirror by using tilted null screens

We propose the design of tilted null screens for testing off-axis segments of conic surfaces. The tilt allows us to control the size of the screen and the sensitivity of the test. For positive tilt angles the sensitivity is increased while the size of the screen is reduced in the sagittal caustic region and vice versa in the tangential caustic region. Further analysis and preliminary experimental results are presented for a fast off-axis concave parabolic mirror with an elliptical aperture. An offset distance of XC = 25.4 mm yields radius of curvature at the vertex R = 20.4 mm; major axis of the mirror DM = 49.4 mm; and minor axis Dm = 29.5 mm.