Celestron NexStar 90SLT Computerized Telescope NexStar SLT Series MAKs Manual - Page 25
Focusing, Image Orientation
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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 Maksutov-Cassegrain optical system 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 MaksutovCassegrain system consists of a corrector plate, a spherical primary mirror, and a secondary mirror spot. Once light rays enter the optical system, they travel the length of the optical tube three times. Focusing Once you have found an object in the telescope, turn the focusing knob until the image is sharp. To focus on an object that is nearer than your current target, turn the focusing knob toward the eyepiece (i.e., so that the focusing tube moves away from the front of the telescope). For more distant objects, turn the focusing knob in the opposite direction. To achieve a truly sharp focus, never look through glass windows or across objects that produce heat waves, such as asphalt parking lots. For astronomical viewing, out of focus star images are very diffuse, making them difficult to see. If you turn the focus knob too quickly, you can go right through focus without seeing the image. To avoid this problem, your first astronomical target should be a bright object (like the Moon or a planet) so that the image is visible even when out of focus. Image Orientation The image orientation of any telescope changes depending on how the eyepiece is inserted into the telescope. When observing through the using the diagonal, the image will be right side up, but reversed from left to right. When observing straight through, with the eyepiece inserted directly into the telescope, the image will be inverted. Reversed from left to right, as viewed with a Star Diagonal Inverted image, as viewed with the eyepiece directly in telescope 25