Mirror-lens objective

In the mirror-lens objectives for high-quality astrophotography, use is frequently made of systems having a correction plate/correction lens which is of approximately the same size as the concave mirror. In order to avoid this large correction plate/correction lens, use is made of a small correction lens near the focal point and having a positive refractive power. As a result of various measures, the Seidel aberrations of aperature abberation, coma, astigmatism and field curvature are eliminated. In particular, the concave mirror and the surface of the correction lens near the focal point are constructed as hyperboloids. The correction lens can be extended to form an achromatic objective as required for the purpose of improving the colour purity of the image.