Teak

Teak does not grow in the rainforest and its consumption encompasses a different set of environmental concerns, such as the disappearance of rare old-growth teak. This limit depends on the wavelength of the studied light (so that the limit for red light comes much earlier than the limit for blue light) and on the diameter of the telescope mirror. Its popularity has led to growth in sustainable production throughout the seasonally dry tropics in forestry plantations. This absolute limit is called the diffraction limit (or sometimes the Rayleigh criterion, Dawes limit or Sparrow's resolution limit). Teak is grown extensively by forest departments of different states in forest areas. The phenomenon of optical diffraction sets a limit to the resolution and image quality that a telescope can achieve, which is the effective area of the Airy disc, which limits how close two such discs can be placed. The age of the tree can be assessed from the annular rings formed every year inside the trunk. See adaptive optics, speckle imaging and optical interferometry.

The mature teak fetches a very good price. In recent years, some technologies to overcome the distortions caused by atmosphere on ground-based telescopes were developed, with good results. It is very resistant to the attack of termites. Current research telescopes have several instruments to choose from such as:. Teak is used extensively in India to make doors and window panes, furniture and columns and beams in old type houses. After the photographic plate, successive generations of electronic detectors, such as the charge-coupled device (CCDs), have been perfected, each with more sensitivity and resolution, and often with a wider wavelength coverage. Teak cut from old trees grown slowly in natural forests is more durable and harder; teak from young trees grown in plantations is more prone to splitting and water damage. Later, the sensitized photographic plate took its place, and the spectrograph was introduced, allowing the gathering of spectral information.

Teak is easily worked and has natural oils that make it suitable for use in exposed locations, where it is durable even when not treated with oil or varnish. Initially the detector used in telescopes was the human eye. It is also used for indoor flooring and as a veneer for indoor furnishings. the Liverpool Telescope and the Faulkes Telescope North and South), allowing automated follow-up of astronomical events. The timber is used in the manufacture of outdoor furniture, boat decks, and other articles where weather resistance is desired. Many are robotic telescopes, computer controlled over the internet (see e.g. There are three species:. These allow many astronomical targets to be monitored continuously, and for large areas of sky to be surveyed.

The word teak originated from the word 'tekku' in Malayalam, the language of Kerala state in South India. Relatively cheap, mass-produced ~2 meter telescopes have recently been developed and have made a significant impact on astronomy research. They are large trees, growing to 30-40 m tall, deciduous in the dry season. This technology has driven new designs for future telescopes with diameters of 30, 50 and even 100 meters. Teak (Tectona), also called jati is a genus of tropical hardwood trees in the family Verbenaceae, native to the south and southeast of Asia, and is commonly found as a component of monsoon forest vegetation. In this generation of telescopes, the mirror is usually very thin, and is kept in an optimal shape by an array of actuators (see active optics). Tectona philippinensis (Philippine Teak) is endemic to the Philippines, and is also endangered. The largest current ground-based telescopes have primary mirrors of between 6 and 11 meters in diameter.

Tectona hamiltoniana (Dahat Teak) is a local endemic species confined to Myanmar, where it is endangered. Its example was followed by the Keck telescopes with 10 m segmented mirrors. Tectona grandis (Common Teak) is by far the most important, with a wide distribution in India and Indo-China. This has now been replaced by a single 6.5m mirror. A new era of telescope making was inaugurated by the Multiple Mirror Telescope (MMT), with a mirror composed of six segments synthesizing a mirror of 4.5 meters diameter. They have a pierced primary mirror, a Newtonian focus, and a spider to mount a variety of replaceable secondary mirrors.

Most large research telescopes can operate as either a Cassegrain telescope (longer focal length, and a narrower field with higher magnification) or a Newtonian telescope (brighter field). For example:. There are mountings even simpler than altazimuth, typically used for specialized instruments. Modern large telescopes use computer-controlled altazimuth mounts, and for long exposures they rotate the instruments or have variable-rate image rotators in an image of the telescope pupil.

This is known as an equatorial mount. The preferred solution for small astronomical telescopes is to tip the altazimuth mount so that the azimuth axis is parallel with the axis of the Earth's rotation. The last effect makes an altazimuth mount especially impractical for long-exposure photography with small telescopes. Even if this is done by computer control, the image rotates at a rate that varies depending on the angle of the target from the celestial pole.

When using an altazimuth for astronomy, both axes must be continuously adjusted to compensate for the Earth's rotation. A Dobsonian mount is a type of altazimuth mount which has proven to be very popular as it is simple and inexpensive. A fork rotates in azimuth (in the horizontal plane), and bearings on the tips of the fork allow the telescope to vary in altitude (in a vertical plane). It is similar to that of a surveying transit.

A simple telescope mount is an altitude-azimuth or altazimuth mount. These are more useful for astronomical viewing. Newtonian or reflecting telescopes employ the reflective properties of light, and use mirrors and lenses. These can be used for both terrestrial and astronomical viewing.

Galilean or refracting telescopes employ the refractive properties of light, and are constructed of lenses. Optical telescopes are also divided into two types. Telescopes are broadly classified into two main types. Optical interferometer arrays and arrays of radio telescopes were developed much more recently.

Later, Johannes Kepler described the optics of lenses (see his books Astronomiae Pars Optica and Dioptrice), including a new kind of astronomical telescope with two convex lenses (a principle often called Kepler telescope). Galileo's telescope consisted of a convex object lens and a concave eye lens, which is universally called a Galilean Telescope (used as a viewfinder in many simple cameras). Galileo is generally credited with being the first to use a telescope for astronomical purposes. Galileo Galilei made his own telescope in 1609, calling it at first a perspicillum, and then using the terms telescopium in Latin and telescopio in Italian (from which the English word derives).

Even if Lippershey did not make the first one, he publicized it. Some name that person as Hans Lippershey (c1570-c1619), but Jacob Metius and Zacharias Jansen also claimed to have invented a telescope during the same time period. Leonard Digges is sometimes credited with the invention in England in the 1570s, but usually credit for assembling the first telescope is usually given to an unknown Dutch spectacle maker in about 1608. The Visby lenses tentatively suggest that the technology was known to the Arabs and Persians then to the Vikings in the 10th century.

Article. The first telescopes may have been Assyrian crystal lenses. . The mirrors are usually a section of a rotated parabola.

They use ring-shaped "glancing" mirrors, made of heavy metals, that reflect the rays just a few degrees. X-ray and gamma-ray telescopes have a problem because these rays go through most metals and glasses. Aperture synthesis is now also being applied to optical telescopes using optical interferometers (arrays of optical telescopes) and Aperture Masking Interferometry at single telescopes. As of 2005, the current record is many times the width of the Earth, utilizing space-based Very Long Baseline Interferometry (VLBI) telescopes such as the Japanese HALCA (Highly Advanced Laboratory for Communications and Astronomy) [VSOP (VLBI Space Observatory Program) satellite].

Radio telescopes are often operated in pairs, or larger groups to synthesize large "virtual" apertures that are similar in size to the separation between the telescopes: see aperture synthesis. The dish is sometimes constructed of a conductive wire mesh whose openings are smaller than a wavelength. Radio telescopes are focused radio antennas, usually shaped like large dishes. The word "telescope" usually refers to optical telescopes, but there are telescopes for most of the spectrum of electromagnetic radiation.

Telescopes are used for astronomy and in many non-astronomical instruments including theodolites, transits, spotting scopes, monoculars, binoculars, camera lenses and spyglasses. Telescopes work by employing one or more curved optical elements - lenses or mirrors - to gather light or other electromagnetic radiation and bring that light or radiation to a focus, where the image can be observed, photographed or studied. Telescopes increase the apparent angular size of distant objects, as well as their apparent brightness. A telescope (from the Greek tele = 'far' and skopein = 'to look or see'; teleskopos = 'far-seeing') is an optical tool that gathers and focuses electromagnetic radiation.

The 1-meter refracting Swedish Solar Telescope (SST) on La Palma, is currently the highest-resolution solar telescope in the world. It was a failure. The horizontal tube was 60 m long and the objective had 1.25 m in diameter. The telescope was aimed by the aid of a Foucault sidérostat, which is a movable plane mirror with a 2 m diameter, mounted in a large cast-iron frame.

Its lens was stationary, prefigured so as to sag into the correct shape. It was on display at the 1900 Paris Exposition. The largest refractor ever constructed was French. It was exceeded in size one year later by the 0.91 m refractor at the Lick Observatory.

This was the last time the most powerful operational telescope in the world was located in Europe. The 0.76 m Nice refractor (in France) that became operational in 1888 was at that time the world's largest telescope. The 1.02 m Yerkes Telescope (in Wisconsin) is the largest aimable refracting telescope in use. The telescope now has an adaptive optics system, and is still useful for advanced research.

In 1919, the telescope was used for the first stellar diameter measurements using interferometry. The mirror was made of green glass by Saint-Gobain. The 100 inch (2.54 m) Hooker Telescope at the Mount Wilson Observatory was used by Edwin Hubble to discover galaxies, and the redshift. The mounting is a special design of equatorial mount called a yoke mount, which permits the telescope to be pointed at and near the north celestial pole.

It has a single borosilicate (Pyrex™) mirror that was famously difficult to construct. The 200 inch (5.08 m) Hale telescope on Palomar Mountain was the largest conventional research telescope for many years. One of them is the Overwhelmingly Large Telescope (OWL), which is intended to have a single aperture of 100 meters in diameter. There are many plans for even larger telescopes.

The CHARA (Center for High Angular Resolution Astronomy) array is the telescope array that can currently (2005) produce the highest resolution images at near-infrared wavelengths. The Navy Prototype Optical Interferometer is the optical telescope (array) that can currently (2005) produce the highest resolution images at visible wavelengths. The four telescopes, belonging to the European Southern Observatory (ESO) and located in the Atacama desert in Chile, are usually operated independently for faint astronomical observations, but up to three telescopes can be operated together for aperture synthesis observations of bright objects. The Very Large Telescope array (VLT) is currently (2002) the record holder for total collecting area in an array of telescopes, with four telescopes each 8 meters in diameter.

The Keck telescopes are currently (2005) the largest, but will soon be superseded by the Gran Telescopio Canarias and Southern African Large Telescope. In this way the images can be diffraction limited, and used for coverage in the ultraviolet (UV) and infrared. The Hubble Space Telescope is in orbit beyond Earth's atmosphere to allow for observations not distorted by astronomical seeing. polarimeters, that detect light polarization.

spectrographs, useful in different regions of the spectrum. imagers, of different spectral responses. ball-and-socket (ancient and useless for astronomy). fixed with movable plane mirror for solar observing.

meridian transit (altitude only). Newtonian reflecting telescopes. Galilean refracting telescopes. Radio telescopes.

Optical telescopes.