A Cassegrain telescope uses two mirrors as shown in above figure. Such a telescope is built with the mirrors 20 mm apart. If the radius of curvature of the large mirror is
File:Cassegrain-Telescope.jpg - Wikimedia Commons
Miroir sphérique, télescope de Cassegrain concours Mines 05
In a reflecting telescope (Cassegrain), is there a difference between using a concave or convex secondary mirror? - Astronomy Stack Exchange