A Cassegrain telescope uses two mirrors as shown in the above figure. Such a telescope is built with the mirrors $20mm$ apart. If the radius of curvature of the large mirror is
File:Cassegrain-Telescope.jpg - Wikipedia
Two-mirror telescopes: Cassegrain, Gregorian and variants
Télescope Cassegrain ASA C 400/3600
Le télescope de Cassegrain
Cassegrain reflecting telescope, 1762-1774 | Science Museum Group Collection