![]() ![]() Others use flourite doublets or small corrector lenses near the focuser. To eliminate false colour, some apos use triplet lenses with elements of Super ED glass. In f/10 to f/15 focal ratios, chromatic aberration is negligible. Uses a doublet lens with elements made of crown and flint glass. In general, a central obstruction of 20 percent of lower by diameter produces a negligible effect. To make numbers even smaller, some companies state obstruction as a percentage of area (12 percent in this example). ![]() An 8-inch scope with a 2.75-inch-diameter secondary mirror has a central obstruction of 34 percent. As such, central obstruction should be stated as a percentage of the diameter of the aperture. This effect is proportional to the diameter of the secondary mirror. The noticeable effect is the smearing of image contrast caused by the added diffraction of light from the obstruction. While the secondary mirror in a reflector blocks some light, the loss is not significant. Under good conditions, tests have proved that date difference is noticeable, but the performance edge over 1/4 wave optics comes at a high cost. Premium telescopes can do better, with wavefront errors of 1/6 to 1/8 wave. Contrary to some ad claims, diffraction-limited does not mean the optics cannot be improved upon. Anything worse, and planets will look soft, if not blurry. This is equivalent to stating that the optics provide a final error at the eyepiece of only one-quarter of a wavelength of light (the wavefront error), meeting the so-called Rayleigh criterion, a minimum standard for amateur telescopes. Diffraction-LimitedĪ promise of diffraction-limited optics means aberrations in the optics are small enough that image quality is affected primarily by the wave nature of light and not by errors in the optics. But when used visually, image brightness depends solely upon the aperture. For photography, faster f/4 to f/6 systems yield shorter exposure times (therefore, these are known as fast focal ratios). For example, a 100mm telescope with a focal length of 800mm has a focal ratio of f/8. The focal ratio is the focal length divided by the aperture. With Maksutov- and Schmidt-Cassegrains, the optical path is folded back on itself, making the tube shorter than the focal length. The length of the light path from the main mirror or lens to the focal point (the location of the eyepiece) is the focal length. When manufacturers list a resolving power, they are merely stating the Dawes limit for the aperture of telescope, not a measure performance value for that specific model. This is the empirical rule devised by William Dawes in the 19th century. The resolving power of a telescope can be estimated with a simple formula: Resolving power (in arc seconds) = 4.56 divided aperture of telescope (inches) or 116 divided aperture of telescope (mm). In theory, an 8-inch telescope can resolve twice as much detail as can a 4-inch instrument. An 8-inch telescope has four times the surface area, and therefore light-gathering power, of a 4-inch, making its images four times brighter. The larger the lens or mirror, the more light it collects, providing brighter and sharper images. A 4-inch instrument has a main lens or mirror 4 inches in diameter. Claims that such a telescope can magnify 400x are misleading, intended solely to lure the unsuspecting buyer. For example, the maximum usable power for a 60mm telescope is only 120x. The general magnification limit for a telescope is 50x the aperture in inches or 2x the aperture in millimetres. Note: Magnification is one trait that can be ignored. The following terms represent the most important optical specifications of any telescope: ![]()
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