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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.
Figure 3-1
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The optics of the NexStar have Starbright XLT coatings - enhanced multi-layer coatings 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 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.
Actual image orientation as seen
with the unaided eye
Inverted image, as viewed with the
eyepiece directly in telescope
Reversed from left to right, as
viewed with a Star Diagonal
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The image orientation changes depending on how the eyepiece is inserted into the telescope. When using the star diagonal, the image is
right-side-up, but reversed from left-to-right (i.e., reverted). If inserting the eyepiece directly into the visual back (i.e., without the star
diagonal), the image is upside-down and reversed from left-to-right (i.e., inverted). This is normal for the Schmidt-Cassegrain design.
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