A problem arising with refracting telescopes is the frequency dependence of refraction. The amount of refraction at the surface of each lens depends on the wavelength of light. This effect is called chromatic aberration and produces a rainbow of colors around the image. The longer wavelengths (red end of the visible spectrum) bend less than the shorter wavelengths (blue end) as they pass through the lens. By combining several compensating lenses of different optical strengths and materials, chromatic aberration can be reduced.

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The use of molten aluminum droplets is investigated for potential application to precision droplet-based net-form manufacturing (PDM). In the proposed application, final structural components are made from the raw stock in one integrated operation by depositing molten metal droplets, layer after layer, via computer information. This work investigates the feasibility of the proposed technology by investigating the issues associated with generating molten aluminum droplets from capillary stream break-up, and examining the mechanical characteristics of the fabricated aluminum components. New results are presented which illustrate the generation of stable streams of molten aluminum droplets at rates of 24,000 droplets/second for a droplet stream speed of 10.9 m/s, corresponding to throughput rates of 2.3×10−4 kg/s (1.85 lb./hour). The droplets travel 2,500 droplet diameters in an inert environment before impingement with the substrate. Microstructural images are completely devoid of splat boundaries, which have been removed by remelting, and the grain size is approximately uniform throughout the field of view of the image that, in most cases presented, contains easily upwards of 30 splats. Also, it has been found that the presence of aluminum oxide in the melt does not influence the average grain size of the component. An oxide barrier however will encapsulate each grain if the oxides are not removed by filtration in the pre-jetting stage. The presence of aluminum oxide in the melt does not prohibit the removal of the splat boundaries. Mechanical analysis shows that fabrication with molten aluminum droplet deposition results in a 30 percent increase in ultimate tensile strength compared to the raw ingot stock. [S1087-1357(00)02402-3]

The history of refracting telescope goes back to 1608 when German-Dutch eyeglass maker Hans Lippershey unsuccessfully attempted to patent one. He is most often associated with the invention of the telescope. However, the first successful refracting telescope came in 1609 when Italian astronomer, physicist, and engineer Galileo Galilei constructed a version on his own and made remarkable astronomical discoveries. German astronomer and mathematician Johannes Kepler made some modifications to Galileo’s design and contributed immensely to the field of optics.

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Orme , M., and Smith, R. F. (September 1, 1999). "Enhanced Aluminum Properties by Means of Precise Droplet Deposition ." ASME. J. Manuf. Sci. Eng. August 2000; 122(3): 484–493. https://doi.org/10.1115/1.1285914

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They can be heavy, especially if the aperture is large, which requires large and heavy lenses. They can also have a more elongated body due to the long focal length of the objective lens, which can be a challenge for transportation, storage, maintenance, and cleaning. They may be quite expensive, as large high-quality lenses are more costly to produce.

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Many large refracting telescopes have been constructed throughout history. Some notable examples are Lick Observatory, Lowell Observatory, Archenhold Observatory, Royal Greenwich Observatory, Nice Observatory, and Yerkes Observatory, which houses the largest refracting telescope in the world.

There are two main designs of refracting telescope – Galilean Telescope and Keplerian Telescope. The distance between the objective and the eyepiece is the sum of their focal lengths. The magnification of a refracting telescope is equal to the focal length of the objective divided by the focal length of the eyepiece. The brightness of an image depends partially on the amount of light collected by the telescope, which is directly proportional to the area of the objective lens.

Contributed by the Manufacturing Engineering Division for publication in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING. Manuscript received Nov. 1998; revised Sept. 1999. Associate Technical Editor: K. Stelson.

Many scientific telescopes today use lenses to collect more light than the human eye could collect on its own. Their role is to focus the light and make distant objects appear brighter, more transparent, and magnified. A refracting telescope, or a refractor, uses a combination of lenses to produce images of distant objects, e.g., stars and planets that would otherwise not be visible with the human eye. A simple refracting telescope is made up of two lenses, which are called the objective and the eyepiece. The principle of a simple refracting telescope is that parallel rays of light from a distant object fall on the objective lens, which produces an image of the object at its focus. The rays from the object pass through the eyepiece allowing the observer to see the image, sometimes magnified.