Astronomical Telescope
What Are Refracting Telescopes?
<p>Refracting or refractor telescopes are a type of optical telescope. These telescopes use a lens as its objective to form an image. The design was originally utilized in spy glasses as well as astronomical telescopes, but also in long focus camera lenses.</p>
<p>Refractor telescopes do suffer from their share of problems. For instance, they possess a seemingly high degree of residual chromatic and spherical aberration. However, these affect shorter focal lengths to a greater to a greater degree than longer ones.</p>
<p>With very large apertures, a problem of lens sagging can occur due to the result of gravity taking its toll, and deforming the glass. Since lenses are only held in place by their edges, it is the center of large lenses that will begin to sag, therefor distorting the images produced. The largest lens size that a refracting telescope should use, is somewhere around one meter. Anything larger will become impractical.</p>
<p>Another problem caused from glass defects, are small air bubbles that can be trapped in the glass. Plus, glass is opaque to certain wavelengths, with even visible light being dimmed by reflection and absorption as it crosses the air-glass interfaces and passes through the glass itself. Many of these problems can be diminished a great deal or even avoided all together by using a reflecting telescope. These can be made in far larger apertures.</p>
<p>Refracting telescopes all operate under the same principles. By using the combination of an objective lens as well as some type of eyepiece, they succeed in gathering more light than a human eye is capable of collecting on its own. After focusing it, the viewer is presented with a clearer, brighter, magnified virtual image.</p>
<p>Refracting telescopes come in a variety of different configurations to correct for image orientations and different kinds of aberration. Since the images produced are formed by the bending of light, or refraction, these telescopes are known as refracting telescopes or just simply refractors.</p>
<p>A different type of refractor is known as an Apochromatic refractor, and these have objectives built with special, extra-low dispersion materials. What they are designed to do, is bring three wavelengths into focus in the same plane. Usually these will be red, green, and blue. Residual color error (tertiary spectrum) can be up to an order of magnitude less than that of an achromatic lens. They have elements of flourite or extra-low dispersion glass in the objective and they will produce a very clear image with virtually no chromatic aberration.</p>
<p>These telescopes can be found in high-end amateur telescope markets. Apochromatic refractors are sold with objectives that are as high 553 mm in diameter, but the majority of them will be be between 80 and 152 mm.</p>
<p>In 1609, Galileo Galilei invented what is commonly known as the Galilean telescope. It makes use of a convergant objective lens plus a divergant eyepiece lens, and they produce images that are upright. His best telescope was capable of magnifying objects close to thirty times, but because of some flaws in their design, such as the narrow field of view or the shape of the lens, the images appeared blurry and distorted.</p>
<p>Regardless of these design flaws, it was still good enough for Galileo to explore the sky. His telescope was capable of viewing the craters on the Moon, the phases of Venus, and four of the moons of Jupiter, which we now know as the Galilean Moons.</p>
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