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CHAPTER 5 - TELESCOPE BASICS
The Schmidt-Cassegrain 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. Other telescopes, 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 secondary mirror. Once light rays enter the optical system, they travel the length of the
optical tube three times.
The optics of the Ultima 2000 has Starbright coatings - an enhanced multi-layer coating on the primary and secondary
mirrors for increased reflectivity and a fully coated corrector for the finest anti-reflection characteristics.
Inside the optical tube a black tube (not illustrated) 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.
Figure 5-1
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.
