Different Types of Lenses for Telescopes
A telescope is a fairly simple device—one or more lenses positioned within a tube that collects light and bends, redirects and manipulates it to create a reproduced, enhanced version of the image inside the telescope’s eyepiece. A telescope can be a 25-dollar bit of plastic with two bits of glass in the end, up to a 40-foot monstrosity that requires an entire building to house it in. Regardless of size, they all work along similar principles and every single one of them contains at least one lens.
The most common type of telescope, and the one most commonly associated with lenses, are called refracting telescopes. This name also covers most binoculars, camera lenses and even glasses. Refractors work by bending light through a primary convex lens known as the ‘objective’ lens, and forcing the redirected light rays to converge at a space known as the “:focal point.” From here, the light rays are focused – often by a prism or a second lens – into the eyepiece, where they are usually refracted again to form a prefect replica image.
Reflector telescopes work by using the tube to collect light, and allowing a mirror to bounce it up into the eyepiece, where it is again refracted and converged into the image.
Telescope lenses are not all the same, however. They are classified by the curvature of the optical surfaces at the front and the rear of the lens. The most common types of lenses are the simple convex and concave—one with both sides bulging outwards and the other with the two sides curving back in on itself. However, the one most used in optics is the meniscus, or concave-convex lens, in which one side bulges outward and the other follows it, making a shape like a bracket punctuation mark. If you wear glasses, the chances are they will have a meniscus surface.
Some telescope lenses are a combination of two or three telescope lens styles, often arranged one in front of the other and at specific points along the tube. Lenses that feature this arrangement are called a doublet or a triplet. In a doublet, a convex lens is usually placed in front of a concave lens. This allows different wavelengths of light to be focused in to a single point, producing a much clearer image. Normally, these special lenses are found inside telescope eye pieces rather than the tube of the scope itself, and are a common arrangement in reflector telescopes.
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