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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.
The optics of the Schmidt-Cassegrain telescopes have Starbright 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.
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Image Orientation
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., mirror image). 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.
Actual image orientation as seen
with the unaided eye
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Reversed from left to right, as
viewed with a Star Diagonal
Figure 4-1
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Figure 4-2
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Inverted image, as viewed with
the eyepiece directly in telescope
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