Use of Mangin and aspheric mirrors to increase the FOV in Schmidt-Cassegrain Telescopes

摘要

The Schmidt-Cassegrain configuration has advantages from the point of view of the packaging constraint but doesn'tprovide enough optical quality through the full field of view when a larger F-number (3.6) and a FOV of 1° are necessaryto reach the minimum illumination threshold in the sensor. Moreover, to improve the global performance the telescope'swindow must be spherical instead of flat. All these factors produce a poor image optical quality that must be increased. We had overcome those problems introducing two changes in the traditional Schimdt-Cassegrain configuration. First, wehad changed the spherical primary mirror to a Mangin mirror. This introduces a second surface and an extra thicknessthat can be used to optimize the system without adding new elements. Secondly, as the Mangin mirror is the entrancepupil of the system with a 200 mm diameter, the use of aspherical surfaces on it is too expensive. Instead we haveaspherized the telescope's secondary mirror to obtain the required image quality. This aspheric coefficient of the secondary mirror, introduced in an element with a diameter not larger than 50 mm,replaces the third order coefficient of the second surface of the telescope window.