The Schmidt-Cassegrain Optical System - Celestron Ultima 11 Instruction Manual

THE SCHMIDT-CASSEGRAIN OPTICAL SYSTEM

A telescope is an instrument that collects and focuses light. The nature of the optical design determines
how the light is focused. Some telescopes, known as refractors, use lenses while others, known as
reflectors, use mirrors. The Schmidt-Cassegrain optical system (or Schmidt-Cass for short) uses a
combination of mirrors and lenses and is referred to as a compound or catadioptric telescope. This unique
design offers large diameter optics while maintaining very short tube lengths, making them extremely
portable. The Schmidt-Cassegrain system consists of a zero power corrector plate, a spherical primary
mirror, and a convex secondary mirror. Once light rays enter the optical system, they travel the length of
the optical tube three times.
Inside the optical tube you will notice a black tube (not illustrated) that extends out from the center hole in
the primary mirror. This is the primary baffle tube and it prevents stray light from passing through to the
eyepiece or camera without striking the primary or secondary mirrors.
Figure 1-1
This cross-sectional diagram shows the light path of the Schmidt-Cassegrain optical system. Note
that the light rays travel the length of the telescope tube three times, making this a compact optical
design. Note that the curve of the corrector plate is greatly exaggerated
6 Telescope Basics