Reflecting telescope

A reflecting telescope is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century, by Isaac Newton, as an alternative to the refracting telescope which, at that time, was a design that suffered from severe chromatic aberration. Although reflecting telescopes produce other types of optical aberrations, it is a design that allows for large diameter objectives. All of the major telescopes used in astronomy research are reflectors. Reflecting telescopes come in many design variations and may employ extra optical elements to improve image quality or place the image in a mechanically advantageous position. Since reflecting telescopes use mirrors, the design is sometimes referred to as a "catoptric" telescope. From the time of Newton to the 1800s, the mirror itself was made of metal- speculum metal; this type included Newton's first designs and the largest telescopes of the 19th century, the Leviathan of Parsonstown with a 1.8 meter wide metal mirror.

In the 19th century a new method using a block of glass coated with thin layer of silver began to become more popular by the turn of the century. A major turning point in reflecting telescopes was the Paris Observatory 1.2 m of 1878, A. A. Common telescopes which lead to the Crossley and Harvard reflecting telescopes, which helped establish a better reputation for reflecting telescopes as the metal mirror designs were noted for their drawbacks. Chiefly the metal mirrors only reflected about 2/3 of the light and the metal would tarnish. After multiple polishings and tarnishings the mirror could lose its precise figuring needed. Reflecting telescopes became extraordinarily popular for astronomy and many famous telescopes such as the Hubble Space Telescope and popular amateur models use this design. In addition, the reflection telescope principle was applied to other wavelengths of light, for example, X-Ray telescopes use the reflection principle to make image forming optics; the idea that curved mirrors behave like lenses dates back at least to Alhazen's 11th century treatise on optics, works, disseminated in Latin translations in early modern Europe.

Soon after the invention of the refracting telescope, Giovanni Francesco Sagredo, others, spurred on by their knowledge of the principles of curved mirrors, discussed the idea of building a telescope using a mirror as the image forming objective. There were reports that the Bolognese Cesare Caravaggi had constructed one around 1626 and the Italian professor Niccolò Zucchi, in a work, wrote that he had experimented with a concave bronze mirror in 1616, but said it did not produce a satisfactory image; the potential advantages of using parabolic mirrors reduction of spherical aberration with no chromatic aberration, led to many proposed designs for reflecting telescopes. The most notable being James Gregory, who published an innovative design for a ‘reflecting’ telescope in 1663, it would be ten years, before the experimental scientist Robert Hooke was able to build this type of telescope, which became known as the Gregorian telescope. Isaac Newton has been credited with building the first reflecting telescope in 1668.

It used a spherically ground metal primary mirror and a small diagonal mirror in an optical configuration that has come to be known as the Newtonian telescope. Despite the theoretical advantages of the reflector design, the difficulty of construction and the poor performance of the speculum metal mirrors being used at the time meant it took over 100 years for them to become popular. Many of the advances in reflecting telescopes included the perfection of parabolic mirror fabrication in the 18th century, silver coated glass mirrors in the 19th century, long-lasting aluminum coatings in the 20th century, segmented mirrors to allow larger diameters, active optics to compensate for gravitational deformation. A mid-20th century innovation was catadioptric telescopes such as the Schmidt camera, which use both a spherical mirror and a lens as primary optical elements used for wide-field imaging without spherical aberration; the late 20th century has seen the development of adaptive optics and lucky imaging to overcome the problems of seeing, reflecting telescopes are ubiquitous on space telescopes and many types of spacecraft imaging devices.

A curved primary mirror is the reflector telescope's basic optical element that creates an image at the focal plane. The distance from the mirror to the focal plane is called the focal length. Film or a digital sensor may be located here to record the image, or a secondary mirror may be added to modify the optical characteristics and/or redirect the light to film, digital sensors, or an eyepiece for visual observation; the primary mirror in most modern telescopes is composed of a solid glass cylinder whose front surface has been ground to a spherical or parabolic shape. A thin layer of aluminum is vacuum deposited onto the mirror, forming a reflective first surface mirror; some telescopes use primary mirrors. Molten glass is rotated to make its surface paraboloidal, is kept rotating while it cools and solidifies; the resulting mirror shape approximates a desired paraboloid shape that requires minimal grinding and polishing to reach the exact figure needed. Reflecting telescopes, just like any other optical system, do not produce "perfect" images.

The need to image objects at distances up to infinity, view them at different wavelengths of light, along with the requirement to have some way to view the image the primary mirror produces, means there is always some compromise in a reflecting telescope's optical design. Because


Ponoko is an online service for manufacturing. It gained noticeable media attention because of its unique business model, as one of the first manufacturers that uses distributed manufacturing and on-demand manufacturing. Ponoko builds on the success of the information age, applies it to digital fabrication. Ponoko receives customers' digital designs has it printed in 3D or cut at the time of purchase by laser cutters or CNC milling machines or 3D printers; the manufacturers exist in a distributed network, growing around the world, the manufacturer closest to the customer is sourced. Customers can sell their objects either via the Ponoko site, or their own retail outlets. While Ponoko uses desktop manufacturers to produce small-scale products, many believe that such distributed, on-demand manufacturing could create a major paradigm shift in manufacturing; as of 2009, the Ponoko site had 20,000 items available. Ponoko was founded by David ten Have and Derek Elley in 2007 and launched in September the same year at TechCrunch 40.

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Globe of Gottorf

The Globe of Gottorf is a 17th-century large globe of the earth in the Kunstkamera museum in St. Petersburg in Russia, it measures 3.1 meters in diameter. The globe details a map of the earth’s surface on the outside and a map of star constellations with astrological and mythological symbols on the inside. Turned by water power, it demonstrates the “movement” of the heavens to those seated inside in candlelight, it was a predecessor of the modern planetarium. Modern German spells the name of Gottor with an F, not a P The original globe was built between 1654 to 1664 in Gottorf on request of Frederick III, Duke of Holstein-Gottorp. Construction was carried out under the supervision of Adam Olearius and completed by Andreas Bösch, both the residents of the Duke, it was put in a palace garden to amaze visitors. The globe was given to Peter the Great during the Great Northern War and brought to St. Petersburg on March 20, 1717, it was placed in a special pavilion on the Tsaristin meadow. It is known that the Tsar examined the Gottorf globe in the morning, such was the interest he took in it.

In 1717, the globe was moved to the tower of the Kunstkamera building. It was damaged in a fire in 1747, its surface was destroyed. Elizabeth of Russia had the globe rebuilt, but speaking it was no more than a replica since not much was left after the fire, it was painted anew using the original wooden ribs. The Germans seized the globe at the Dutch Admiralty during the Second World War, but it was returned to Russia in 1947; the original globe now resides in a museum in St. Petersburg. German charitable foundations agreed to build a near-replica of the globe in the 1990s, but this time in steel, with electric motors and lights, install it at Gottorf Castle. Official russian website of the Kunstkammer with photos and short description of the original globe German website detailing the history of the original AND replica globes Website of the Gottorf castle with details of the replica globe