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Primary Mirror
Reflecting telescopes use large primary mirrors to gather light. The reflecting telescope uses quartz, glass or a glass and ceramic composite as the construction material for the mirror. The mirror is given a thin metal coating which makes its surface highly reflective to any incoming light. The mirror has a concave shape for gathering light and concentrating it into a narrower beam or "cone." The mirror sits in the bottom of the telescope tube, bracketed to a mount which has adjusting screws to focus the beam.
Secondary Mirror
The reflecting telescope has a secondary mirror that sits up at the top of the telescope tube, mounted in a bracket. The secondary mirror has a much smaller diameter, just large enough to collect the focused cone of light from the primary mirror below it. The secondary mirror has a diagonal surface that directs the light beam from the primary mirror (at an angle) to the side of the telescope, where the focuser and eyepiece is located. Although the secondary mirror angles the light because of its position, it has a very flat surface that permits no distortion.
Secondary Mirror Size and Focal Length
The primary mirror has a diameter rated in inches that determines its size. The distance between the primary mirror and the secondary mirror has a rating measured in inches. A telescope can have a primary mirror diameter of 8 inches with a 48-inch focal length -- the telescope would be an 8-inch f-6 reflector. The formula requires that the focal length be divided by the mirror diameter, arriving at the number 6. All reflecting telescopes have this feature, unique to them.
Focuser
Reflecting telescopes have their focusing device mounted at the top of the telescope tube, unlike refractors, which have them at the bottom. The focuser can hold any type eyepiece in its mount. Most reflector telescope focus devices have knobs attached to them that work by rack and pinion gears. When the knobs turn in either direction, the image is focused through the eye piece. The focuser either pulls the eyepiece in or pulls it out.
Finder Scope
The finder scope on a reflecting telescope sits at the top end of the telescope tube near the eyepiece. The finder scope has a small aperture lens for sighting in initial distant objects, rather than using the main eyepiece with full magnification. The finder scope, although it does not have the intense magnification of the main eyepiece, allows for a much wider field of view to spot a distant object. Once the object has been spotted with the finder scope, which is aligned with the primary focus, the viewer switches to the main eyepiece and increases the magnification.
Telescope Mount
Reflecting telescopes typically use an "Altazimuth" mount: An altazimuth mount allows the telescope tube to move in perpendicular planes, one horizontal and the other vertically. A lever adjusts the up or down motion of the mount until the telescope focuses on an object. When the image has been focused and centered, a locking device secures the telescope tube in a stable position. Some reflecting telescopes use a motor drive, or "clock drive," on the mount, which allows the telescope to track an object in the sky. The motor drive adjusts for the rotation of the earth by keeping the axis of the telescope pointed at the image at all times.