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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.
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The optics of the CPC 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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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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CPC
The
's focusing mechanism controls the primary mirror which is mounted on a ring that slides back and forth on the primary baffle
tube. The focusing knob, which moves the primary mirror, is on the rear cell of the telescope just below the star diagonal and eyepiece. Turn
the focusing knob until the image is sharp. If the knob will not turn, it has reached the end of its travel on the focusing mechanism. Turn the
knob in the opposite direction until the image is sharp. Once an image is in focus, turn the knob clockwise to focus on a closer object and
counterclockwise for a more distant object. A single turn of the focusing knob moves the primary mirror only slightly. Therefore, it will take
many turns (about 30) to go from close focus (approximately 60 feet) to infinity.
Figure 5-1
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Reversed from left to right, as
viewed with a Star Diagonal
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Figure 5-2
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Inverted image, as viewed with
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