Extensible Cassegrain reflector telescope has concave main mirror with transparent window section at centre

摘要

The extensible Cassegrain telescope has a main concave mirror which is pierced at its centre to provide a transparent window which allows the passage of a cone of light returned by a convex secondary mirror via an eyepiece disposed along the cone axis. The main mirror can be a glass disc mounted on a thick base and with a vacuum plenum between them.