1.
Cassegrain reflector
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The Cassegrain reflector is a combination of a primary concave mirror and a secondary convex mirror, often used in optical telescopes and radio antennas. Alternatively, as in radio telescopes, the final focus may be in front of the primary. In an asymmetrical Cassegrain, the mirror may be tilted to avoid obscuration of the primary or to avoid the need for a hole in the primary mirror, the classic Cassegrain configuration uses a parabolic reflector as the primary while the secondary mirror is hyperbolic. Modern variants often have a primary for increased performance, or the primary and/or secondary are spherical or elliptical for ease of manufacturing. The Cassegrain reflector is named after a published reflecting telescope design that appeared in the April 25,1672 Journal des sçavans which has been attributed to Laurent Cassegrain. Similar designs using convex secondaries have been found in the Bonaventura Cavalieris 1632 writings describing burning mirrors, james Gregorys 1662 attempts to create a reflecting telescope included a Cassegrain configuration, judging by a convex secondary mirror found among his experiments. The Cassegrain design is used in catadioptric systems. The Classic Cassegrain has a primary mirror, and a hyperbolic secondary mirror that reflects the light back down through a hole in the primary. Folding the optics makes this a compact design, on smaller telescopes, and camera lenses, the secondary is often mounted on an optically flat, optically clear glass plate that closes the telescope tube. This support eliminates the diffraction effects caused by a straight-vaned support spider. The closed tube stays clean, and the primary is protected and it makes use of the special properties of parabolic and hyperbolic reflectors. A concave parabolic reflector will reflect all incoming light rays parallel to its axis of symmetry to a single point, a convex hyperbolic reflector has two foci and will reflect all light rays directed at one of its two foci towards its other focus. The parabolic mirror reflects parallel light rays entering the telescope to its focus, the hyperbolic mirror then reflects those light rays to its other focus, where the image is observed. The Ritchey-Chrétien is a specialized Cassegrain reflector which has two hyperbolic mirrors and it is free of coma and spherical aberration at a flat focal plane, making it well suited for wide field and photographic observations. It was invented by George Willis Ritchey and Henri Chrétien in the early 1910s and this design is very common in large professional research telescopes, including the Hubble Space Telescope, Keck Telescopes and VLT telescope, it is also found in high-grade amateur telescopes. Ingalls, the astronomy editor at the time. It uses a concave primary mirror and a convex spherical secondary. While this system is easier to polish than a classic Cassegrain or Ritchey-Chretien system, because this is less noticeable at longer focal ratios, Dall-Kirkhams are seldom faster than f/15
2.
Earth's rotation
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Earths rotation is the rotation of the planet Earth around its own axis. The Earth rotates from the west towards east, as viewed from North Star or polestar Polaris, the Earth turns counter-clockwise. The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where the Earths axis of rotation meets its surface and this point is distinct from the Earths North Magnetic Pole. The South Pole is the point where the Earths axis of rotation intersects its surface. The Earth rotates once in about 24 hours with respect to the sun, Earths rotation is slowing slightly with time, thus, a day was shorter in the past. This is due to the effects the Moon has on Earths rotation. Atomic clocks show that a modern-day is longer by about 1.7 milliseconds than a century ago, analysis of historical astronomical records shows a slowing trend of 2.3 milliseconds per century since the 8th century BCE. Among the ancient Greeks, several of the Pythagorean school believed in the rotation of the rather than the apparent diurnal rotation of the heavens. Perhaps the first was Philolaus, though his system was complicated, in the third century BCE, Aristarchus of Samos suggested the suns central place. However, Aristotle in the fourth century criticized the ideas of Philolaus as being based on rather than observation. He established the idea of a sphere of fixed stars that rotated about the earth and this was accepted by most of those who came after, in particular Claudius Ptolemy, who thought the earth would be devastated by gales if it rotated. In the 10th century, some Muslim astronomers accepted that the Earth rotates around its axis, treatises were written to discuss its possibility, either as refutations or expressing doubts about Ptolemys arguments against it. At the Maragha and Samarkand observatories, the Earths rotation was discussed by Tusi and Qushji, in medieval Europe, Thomas Aquinas accepted Aristotles view and so, reluctantly, did John Buridan and Nicole Oresme in the fourteenth century. Not until Nicolaus Copernicus in 1543 adopted a heliocentric world system did the earths rotation begin to be established, Copernicus pointed out that if the movement of the earth is violent, then the movement of the stars must be very much more so. He acknowledged the contribution of the Pythagoreans and pointed to examples of relative motion, for Copernicus this was the first step in establishing the simpler pattern of planets circling a central sun. Tycho Brahe, who produced accurate observations on which Kepler based his laws, in 1600, William Gilbert strongly supported the earths rotation in his treatise on the earths magnetism and thereby influenced many of his contemporaries. Those like Gilbert who did not openly support or reject the motion of the earth about the sun are often called semi-Copernicans, however, the contributions of Kepler, Galileo and Newton gathered support for the theory of the rotation of the Earth. The earths rotation implies that the bulges and the poles are flattened
3.
Telescope mount
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A telescope mount is a mechanical structure which supports a telescope. Telescope mounts are designed to support the mass of the telescope, many sorts of mounts have been developed over the years, with the majority of effort being put into systems that can track the motion of the stars as the Earth rotates. Fixed-altitude mounts usually have the primary optics fixed at an angle while rotating horizontally. They can cover the sky but only observe objects for the short time when that object passes a specific altitude. Transit mounts are single axis mounts fixed in azimuth while rotating in altitude and this allows the telescope to view the whole sky, but only when the Earths rotation allows the objects to cross through that narrow north-south line. This type of mount is used in Transit telescopes, designed for precision astronomical measurement, Transit mounts are also used to save on cost or where the instruments mass makes movement on more than one axis very difficult, such as large radio telescopes. Altazimuth, altitude-azimuth, or alt-az mounts allow telescopes to be moved in altitude, or azimuth and this meant until recently it was normally used with inexpensive commercial and hobby constructions. Since the invention of digital tracking systems, altazimuth mounts have come to be used in all modern large research telescopes. Digital tracking has also made it a popular telescope mount used in amateur astronomy, besides the mechanical inability to easily follow celestial motion the altazimuth mount does have other limitations. The mount also has blind spot or zenith hole, a spot near the zenith where the rate in the azimuth coordinate becomes too high to accurately follow equatorial motion. These mounts also require a third axis to de-rotate the field as the telescope tracks, alt-alt mounts, or altitude-altitude mounts, are designs similar to horizontal equatorial yoke mounts or Cardan suspension gimbals. This mount is an alternative to the mount that has the advantage of not having a blind spot near the zenith. It has the disadvantage of having all the mass, complexity and these mounts may include a third azimuth axis to rotate the entire mount into an orientation that allows smoother tracking. Slewing or mechanically driving the mounts polar axis in a direction to the Earths rotation allows the telescope to accurately follow the motion of the night sky. Tilting the polar axis adds a level of complexity to the mount, mechanical systems have to be engineered to support one or both ends of this axis. Designs such as German equatorial or cross axis mounts also need large counter weights to counterbalance the mass of the telescope, larger domes and other structures are also needed to cover the increased mechanical size and range of movement of equatorial mounts. Because of this, equatorial mounts become less viable in very large telescopes and have been pretty much replaced by altazimuth mounts for those applications, instead of the classical mounting using two axles, the mirror is supported by six extendable struts. This configuration allows moving the telescope in all six degrees of freedom
4.
Camera
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Thumb|A2016 Nikon D810 A camera is an optical instrument for recording or capturing images, which may be stored locally, transmitted to another location, or both. The images may be individual still photographs or sequences of images constituting videos or movies, the camera is a remote sensing device as it senses subjects without physical contact. The word camera comes from camera obscura, which means dark chamber and is the Latin name of the device for projecting an image of external reality onto a flat surface. The modern photographic camera evolved from the camera obscura, the functioning of the camera is very similar to the functioning of the human eye. A camera may work with the light of the spectrum or with other portions of the electromagnetic spectrum. A still camera is a device which creates a single image of an object or scene. All cameras use the basic design, light enters an enclosed box through a converging lens/convex lens. A shutter mechanism controls the length of time that light can enter the camera, a display, often a liquid crystal display, permits the user to view scene to be recorded and settings such as ISO speed, exposure, and shutter speed. A movie camera or a video camera operates similarly to a camera, except it records a series of static images in rapid succession. When the images are combined and displayed in order, the illusion of motion is achieved, the forerunner to the photographic camera was the camera obscura. The oldest known record of this principle is a description by Han Chinese philosopher Mozi, Mozi correctly asserted that the camera obscura image is inverted because light travels in straight lines from its source. In the 11th century Arab physicist Ibn al-Haytham s wrote very influential essays about experiments with light through an opening in a darkened room. The use of a lens in the opening of a wall or closed window shutter of a room to project images used as a drawing aid has been traced back to circa 1550. Since the late 17th century portable camera obscura devices in tents, before the development of the photographic camera, it had been known for hundreds of years that some substances, such as silver salts, darkened when exposed to sunlight. The first person to use this chemistry to create images was Thomas Wedgwood, to create images, Wedgwood placed items, such as leaves and insect wings, on ceramic pots coated with silver nitrate, and exposed the set-up to light. These images werent permanent, however, as Wedgwood didnt employ a fixing mechanism and he ultimately failed at his goal of using the process to create fixed images created by a camera obscura. The first permanent photograph of an image was made in 1826 by Joseph Nicéphore Niépce using a sliding wooden box camera made by Charles. Niépce had been experimenting with ways to fix the images of a camera obscura since 1816, the photograph Niépce succeeded in creating shows the view from his window
5.
Celestial object
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An astronomical object or celestial object is a naturally occurring physical entity, association, or structure that current astronomy has demonstrated to exist in the observable universe. In astronomy, the object and body are often used interchangeably. Examples for astronomical objects include planetary systems, star clusters, nebulae and galaxies, while asteroids, moons, planets, and stars are astronomical bodies. A comet may be identified as both body and object, It is a body when referring to the nucleus of ice and dust. The universe can be viewed as having a hierarchical structure, at the largest scales, the fundamental component of assembly is the galaxy. Galaxies are organized groups and clusters, often within larger superclusters. Disc galaxies encompass lenticular and spiral galaxies with features, such as spiral arms, at the core, most galaxies have a supermassive black hole, which may result in an active galactic nucleus. Galaxies can also have satellites in the form of dwarf galaxies, the constituents of a galaxy are formed out of gaseous matter that assembles through gravitational self-attraction in a hierarchical manner. At this level, the fundamental components are the stars. The great variety of forms are determined almost entirely by the mass, composition. Stars may be found in systems that orbit about each other in a hierarchical organization. A planetary system and various objects such as asteroids, comets and debris. The various distinctive types of stars are shown by the Hertzsprung–Russell diagram —a plot of stellar luminosity versus surface temperature. Each star follows a track across this diagram. If this track takes the star through a region containing a variable type. An example of this is the instability strip, a region of the H-R diagram that includes Delta Scuti, RR Lyrae, the table below lists the general categories of bodies and objects by their location or structure. International Astronomical Naming Commission List of light sources List of Solar System objects Lists of astronomical objects SkyChart, Sky & Telescope Monthly skymaps for every location on Earth
6.
Clock drive
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In astronomy a clock drive is a regulatory mechanism used to move an equatorial mounted telescope along one axis to keep the telescope in exact sync with the apparent motion of the celestial sky. This allows the telescope to stay fixed on a point in the sky without having to be constantly re-aimed due to the Earths rotation. The mechanism itself used to be clockwork but nowadays is usually electrically driven, clock drives can be light and portable for smaller telescopes or can be exceedingly heavy and complex for larger ones such as the 60-inch telescope at the Mount Wilson Observatory. Clock-driven equatorial platforms are used in non-tracking type mounts, such as altazimuth mounts. The original inventor of the drive is unknown, being an idea that was tried in several ways throughout history. In China in 1094, during the Song Dynasty, Su Song built a clock tower that had many features. Early telescope clock drives used a clock work with falling weights and pendulums, over time the falling weights and pendulums were replaced by an electric motor
7.
Right ascension
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Right ascension is the angular distance measured eastward along the celestial equator from the vernal equinox to the hour circle of the point in question. When combined with declination, these astronomical coordinates specify the direction of a point on the sphere in the equatorial coordinate system. Right ascension is the equivalent of terrestrial longitude. Both right ascension and longitude measure an angle from a direction on an equator. Right ascension is measured continuously in a circle from that equinox towards the east. Any units of measure could have been chosen for right ascension, but it is customarily measured in hours, minutes. Astronomers have chosen this unit to measure right ascension because they measure a stars location by timing its passage through the highest point in the sky as the Earth rotates. The highest point in the sky, called meridian, is the projection of a line onto the celestial sphere. A full circle, measured in units, contains 24 × 60 × 60 = 86 400s, or 24 × 60 = 1 440m. Because right ascensions are measured in hours, they can be used to time the positions of objects in the sky. For example, if a star with RA = 01h 30m 00s is on the meridian, sidereal hour angle, used in celestial navigation, is similar to right ascension, but increases westward rather than eastward. Usually measured in degrees, it is the complement of right ascension with respect to 24h and it is important not to confuse sidereal hour angle with the astronomical concept of hour angle, which measures angular distance of an object westward from the local meridian. The Earths axis rotates slowly westward about the poles of the ecliptic and this effect, known as precession, causes the coordinates of stationary celestial objects to change continuously, if rather slowly. Therefore, equatorial coordinates are inherently relative to the year of their observation, coordinates from different epochs must be mathematically rotated to match each other, or to match a standard epoch. The right ascension of Polaris is increasing quickly, the North Ecliptic Pole in Draco and the South Ecliptic Pole in Dorado are always at right ascension 18h and 6h respectively. The currently used standard epoch is J2000.0, which is January 1,2000 at 12,00 TT, the prefix J indicates that it is a Julian epoch. Prior to J2000.0, astronomers used the successive Besselian Epochs B1875.0, B1900.0, the concept of right ascension has been known at least as far back as Hipparchus who measured stars in equatorial coordinates in the 2nd century BC. But Hipparchus and his successors made their star catalogs in ecliptic coordinates, the easiest way to do that is to use an equatorial mount, which allows the telescope to be aligned with one of its two pivots parallel to the Earths axis
8.
Perpendicular
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In elementary geometry, the property of being perpendicular is the relationship between two lines which meet at a right angle. The property extends to other related geometric objects, a line is said to be perpendicular to another line if the two lines intersect at a right angle. Perpendicularity can be shown to be symmetric, meaning if a first line is perpendicular to a second line, for this reason, we may speak of two lines as being perpendicular without specifying an order. Perpendicularity easily extends to segments and rays, in symbols, A B ¯ ⊥ C D ¯ means line segment AB is perpendicular to line segment CD. A line is said to be perpendicular to an if it is perpendicular to every line in the plane that it intersects. This definition depends on the definition of perpendicularity between lines, two planes in space are said to be perpendicular if the dihedral angle at which they meet is a right angle. Perpendicularity is one instance of the more general mathematical concept of orthogonality, perpendicularity is the orthogonality of classical geometric objects. Thus, in advanced mathematics, the perpendicular is sometimes used to describe much more complicated geometric orthogonality conditions. The word foot is used in connection with perpendiculars. This usage is exemplified in the top diagram, above, the diagram can be in any orientation. The foot is not necessarily at the bottom, step 2, construct circles centered at A and B having equal radius. Let Q and R be the points of intersection of two circles. Step 3, connect Q and R to construct the desired perpendicular PQ, to prove that the PQ is perpendicular to AB, use the SSS congruence theorem for and QPB to conclude that angles OPA and OPB are equal. Then use the SAS congruence theorem for triangles OPA and OPB to conclude that angles POA, to make the perpendicular to the line g at or through the point P using Thales theorem, see the animation at right. The Pythagorean Theorem can be used as the basis of methods of constructing right angles, for example, by counting links, three pieces of chain can be made with lengths in the ratio 3,4,5. These can be out to form a triangle, which will have a right angle opposite its longest side. This method is useful for laying out gardens and fields, where the dimensions are large, the chains can be used repeatedly whenever required. If two lines are perpendicular to a third line, all of the angles formed along the third line are right angles
9.
Coordinate axis
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The order of the coordinates is significant, and they are sometimes identified by their position in an ordered tuple and sometimes by a letter, as in the x-coordinate. The coordinates are taken to be real numbers in elementary mathematics, the use of a coordinate system allows problems in geometry to be translated into problems about numbers and vice versa, this is the basis of analytic geometry. The simplest example of a system is the identification of points on a line with real numbers using the number line. In this system, an arbitrary point O is chosen on a given line. The coordinate of a point P is defined as the distance from O to P. Each point is given a unique coordinate and each number is the coordinate of a unique point. The prototypical example of a system is the Cartesian coordinate system. In the plane, two lines are chosen and the coordinates of a point are taken to be the signed distances to the lines. In three dimensions, three perpendicular planes are chosen and the three coordinates of a point are the distances to each of the planes. This can be generalized to create n coordinates for any point in n-dimensional Euclidean space, depending on the direction and order of the coordinate axis the system may be a right-hand or a left-hand system. This is one of many coordinate systems, another common coordinate system for the plane is the polar coordinate system. A point is chosen as the pole and a ray from this point is taken as the polar axis, for a given angle θ, there is a single line through the pole whose angle with the polar axis is θ. Then there is a point on this line whose signed distance from the origin is r for given number r. For a given pair of coordinates there is a single point, for example, and are all polar coordinates for the same point. The pole is represented by for any value of θ, there are two common methods for extending the polar coordinate system to three dimensions. In the cylindrical coordinate system, a z-coordinate with the meaning as in Cartesian coordinates is added to the r and θ polar coordinates giving a triple. Spherical coordinates take this a further by converting the pair of cylindrical coordinates to polar coordinates giving a triple. A point in the plane may be represented in coordinates by a triple where x/z and y/z are the Cartesian coordinates of the point
10.
Declination
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In astronomy, declination is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declinations angle is measured north or south of the celestial equator, the root of the word declination means a bending away or a bending down. It comes from the root as the words incline and recline. Declination in astronomy is comparable to geographic latitude, projected onto the celestial sphere, points north of the celestial equator have positive declinations, while those south have negative declinations. Any units of measure can be used for declination, but it is customarily measured in the degrees, minutes. Declinations with magnitudes greater than 90° do not occur, because the poles are the northernmost and southernmost points of the celestial sphere, the Earths axis rotates slowly westward about the poles of the ecliptic, completing one circuit in about 26,000 years. This effect, known as precession, causes the coordinates of stationary celestial objects to change continuously, therefore, equatorial coordinates are inherently relative to the year of their observation, and astronomers specify them with reference to a particular year, known as an epoch. Coordinates from different epochs must be rotated to match each other. The currently used standard epoch is J2000.0, which is January 1,2000 at 12,00 TT, the prefix J indicates that it is a Julian epoch. Prior to J2000.0, astronomers used the successive Besselian Epochs B1875.0, B1900.0, the declinations of Solar System objects change very rapidly compared to those of stars, due to orbital motion and close proximity. This similarly occurs in the Southern Hemisphere for objects with less than −90° − φ. An extreme example is the star which has a declination near to +90°. Circumpolar stars never dip below the horizon, conversely, there are other stars that never rise above the horizon, as seen from any given point on the Earths surface. Generally, if a star whose declination is δ is circumpolar for some observer, then a star whose declination is −δ never rises above the horizon, as seen by the same observer. Likewise, if a star is circumpolar for an observer at latitude φ, neglecting atmospheric refraction, declination is always 0° at east and west points of the horizon. At the north point, it is 90° − |φ|, and at the south point, from the poles, declination is uniform around the entire horizon, approximately 0°. Non-circumpolar stars are visible only during certain days or seasons of the year, the Suns declination varies with the seasons. As seen from arctic or antarctic latitudes, the Sun is circumpolar near the summer solstice, leading to the phenomenon of it being above the horizon at midnight
11.
Setting circles
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Setting circles are used on telescopes equipped with an equatorial mount to find astronomical objects in the sky by their equatorial coordinates often used in star charts or ephemerides. Setting circles consist of two graduated disks attached to the ascension and declination axis of an equatorial mount. The right ascension disk is graduated into hours, minutes, the declination disk is graduated into degrees, minutes, and seconds. Since right ascension coordinates are fixed to the sphere the RA disk is usually driven by a clock mechanism in sync with sidereal time. Locating an object on the sphere with setting circles is similar to finding a location on a terrestrial map using latitude and longitude. Sometimes the right ascension setting circle has two scales on it, one for the Northern and one for the Southern hemisphere, historically setting circles have rivaled the telescopes optics as far as difficulty in construction. Making a set of setting circles required a lot of precision crafting on a dividing engine, setting circles usually had a large diameter and when combined with a vernier scale could point a telescope to nearly an arc minute of accuracy. In the 20th century setting circles were replaced with electronic encoders on most research telescopes, Polaris is roughly at the north pole, while Sigma Octantis is roughly at the south pole. Setting Right Ascension - After polar alignment, the uses a calculator or a known star to synchronize the right ascension circle with Sidereal Time. Accuracy of pointing the telescope can be hard to achieve and these sources of error add up and cause the telescope to point far from the desired object. They are also hard to control, for example, Polaris is often used as the north pole for alignment purposes. Also, even the finest graduations on setting circles are more than a degree apart. Nothing can be if the optical tube is not perpendicular to the declination axis or if the R. A. and Dec axes are not perpendicular. In the southern hemisphere the Right Ascension scale operates in reverse from in the Northern Hemisphere, the term Right Ascension took its name from early northern hemisphere observers for whom ascending stars were on the east or right hand side. In the southern hemisphere the east is on the left when a mount is aligned on the south pole. Many Right Ascension setting circles therefore carry two sets of numbers, one showing the value if the telescope is aligned in the northern hemisphere, the other for the southern. Alternatively, it is possible to point to a star very close to the object, rotate the circles to match the stars coordinates. Digital setting circles consist of two rotary encoders on both axis of the mount and a digital readout
12.
Celestial coordinates
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In astronomy, a celestial coordinate system is a system for specifying positions of celestial objects, satellites, planets, stars, galaxies, and so on. Coordinate systems can specify a position in 3-dimensional space, or merely the direction of the object on the celestial sphere, the coordinate systems are implemented in either spherical coordinates or rectangular coordinates. Spherical coordinates, projected on the sphere, are analogous to the geographic coordinate system used on the surface of the Earth. These differ in their choice of fundamental plane, which divides the sphere into two equal hemispheres along a great circle. Rectangular coordinates, in units, are simply the cartesian equivalent of the spherical coordinates, with the same fundamental plane. Each coordinate system is named after its choice of fundamental plane, the following table lists the common coordinate systems in use by the astronomical community. The fundamental plane divides the sphere into two equal hemispheres and defines the baseline for the latitudinal coordinates, similar to the equator in the geographic coordinate system. The poles are located at ±90° from the fundamental plane, the primary direction is the starting point of the longitudinal coordinates. The origin is the distance point, the center of the celestial sphere. The horizontal, or altitude-azimuth, system is based on the position of the observer on Earth, the positioning of a celestial object by the horizontal system varies with time, but is a useful coordinate system for locating and tracking objects for observers on Earth. It is based on the position of relative to an observers ideal horizon. The equatorial coordinate system is centered at Earths center, but fixed relative to the celestial poles, the coordinates are based on the location of stars relative to Earths equator if it were projected out to an infinite distance. The equatorial describes the sky as seen from the solar system, the equatorial system is the normal coordinate system for most professional and many amateur astronomers having an equatorial mount that follows the movement of the sky during the night. Celestial objects are found by adjusting the telescopes or other instruments scales so that they match the equatorial coordinates of the object to observe. There are also subdivisions into mean of date coordinates, which average out or ignore nutation, and true of date, the fundamental plane is the plane of the Earths orbit, called the ecliptic plane. The geocentric ecliptic system was the coordinate system for ancient astronomy and is still useful for computing the apparent motions of the Sun, Moon. The heliocentric ecliptic system describes the orbital movement around the Sun. The system is used for computing the positions of planets and other solar system bodies
13.
Altazimuth mount
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An altazimuth or alt-azimuth mount is a simple two-axis mount for supporting and rotating an instrument about two perpendicular axes – one vertical and the other horizontal. Rotation about the vertical axis varies the azimuth of the direction of the instrument. Rotation about the horizontal axis varies the altitude of the pointing direction and these mounts are used, for example, with telescopes, cameras, radio antennas, heliostat mirrors, solar panels, and guns and similar weapons. Several names are given to this kind of mount, including altitude-azimuth, azimuth-elevation, a gun turret is essentially an alt-azimuth mount for a gun, and a standard camera tripod is an alt-azimuth mount as well. When used as a telescope mount, the biggest advantage of an alt-azimuth mount is the simplicity of its mechanical design. The primary disadvantage is its inability to follow objects in the night sky as the Earth spins on its axis. On the other hand, an equatorial mount only needs to be rotated about an axis, at a constant rate. Altazimuth mounts need to be rotated about both axes at variable rates, achieved via microprocessor based two-axis drive systems, to track equatorial motion and this imparts an uneven rotation to the field of view that also has to be corrected via a microprocessor based counter rotation system. The design also does not allow for the use of setting circles to locate astronomical objects although modern digital setting circles have removed this shortcoming. Another limitation is the problem of gimbal lock at zenith pointing, when tracking at elevations close to 90°, the azimuth axis must rotate very quickly, if the altitude is exactly 90°, the speed is infinite. Thus, altazimuth telescopes, although they can point in any direction, cannot track smoothly within a zenith blind spot, typical current applications of altazimuth mounts include, Research telescopes Mounts for very large telescopes. In the largest telescopes the mass and cost of a mount is prohibitive. The simple structure of an alt-azimuth mount allows significant cost reductions, an altazimuth mount also reduces the cost in the dome structure covering the telescope since the simplified motion of the telescope means the structure can be more compact. Altazimuth mounts are cheap and simple to use
14.
Astrophotography
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Astrophotography is a specialized type of photography for recording photos of astronomical objects and large areas of the night sky. The first photograph of an object was taken in 1840. This is done by long time exposure since both film and digital cameras can accumulate and sum light photons over long periods of time. Astrophotography is a subdiscipline in amateur astronomy usually seeking aesthetically pleasing images rather than scientific data. Amateurs use a range of special equipment and techniques. With a few exceptions, astronomical photography employs long exposures since both film and digital imaging devices can accumulate and light photons over long periods of time, the amount of light hitting the film or detector is also increased by increasing the diameter of the primary optics being used. Since the Earth is constantly rotating, telescopes and equipment are rotated in the direction to follow the apparent motion of the stars overhead. This is accomplished by using either equatorial or computer-controlled altazimuth telescope mounts to keep celestial objects centered while the earth rotates, all telescope mount systems suffer from induced tracking error due to imperfect motor drives and mechanical sag of the telescope. Tracking errors are corrected by keeping a selected aiming point, usually a bright guide star, sometimes the object to be imaged is moving, so the telescope has to be kept constantly centered on that object. Guiding was formerly done manually throughout the exposure with an observer standing at the telescope making corrections to keep a cross hair on the guide star, since the advent of computer-controlled systems this is accomplished by an automated systems in professional and even amateur equipment. These all require specialized equipment such as telescopes designed for imaging, for wide field of view. Astronomical CCD cameras may use cryogenic cooling to reduce thermal noise, specialized filters are also used to record images in specific wavelengths. The development of astrophotography as a tool was pioneered in the mid-19th century for the most part by experimenters and amateur astronomers. Because of the long exposures needed to capture relatively faint astronomical objects. Early photographic processes also had limitations, the daguerreotype process was far too slow to record anything but the brightest objects, and the wet plate collodion process limited exposures to the time the plate could stay wet. The first-known attempt at astronomical photography was by Louis Jacques Mandé Daguerre, inventor of the process which bears his name. Tracking errors in guiding the telescope during the long exposure meant the photograph came out as an indistinct fuzzy spot, the Sun may have been first photographed in an 1845 daguerreotype by the French physicists Léon Foucault and Hippolyte Fizeau. No photographic alteration was caused by the light of the corona condensed by a lens for two minutes, during totality, on a sheet of paper prepared with bromide of silver, the Suns solar corona was first successfully imaged during the Solar eclipse of July 28,1851
15.
Cardinal point (optics)
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In Gaussian optics, the cardinal points consist of three pairs of points located on the optical axis of a rotationally symmetric, focal, optical system. These are the points, the principal points, and the nodal points. The only ideal system that has achieved in practice is the plane mirror. Cardinal points provide a way to simplify a system with many components. The cardinal points lie on the axis of the optical system. Each point is defined by the effect the optical system has on rays that pass through that point, the paraxial approximation assumes that rays travel at shallow angles with respect to the optical axis, so that sin θ ≈ θ and cos θ ≈1. Aperture effects are ignored, rays that do not pass through the stop of the system are not considered in the discussion below. The front focal point of a system, by definition, has the property that any ray that passes through it will emerge from the system parallel to the optical axis. The rear focal point of the system has the reverse property, the front and rear focal planes are defined as the planes, perpendicular to the optic axis, which pass through the front and rear focal points. An object infinitely far from the optical system forms an image at the focal plane. For objects a finite distance away, the image is formed at a different location, but rays that leave the object parallel to one another cross at the rear focal plane. A diaphragm or stop at the focal plane can be used to filter rays by angle, since. No matter where on the object the ray comes from, the ray will pass through the aperture as long as the angle at which it is emitted from the object is small enough. Note that the aperture must be centered on the axis for this to work as indicated. Using a sufficiently small aperture in the plane will make the lens telecentric. Similarly, the range of angles on the output side of the lens can be filtered by putting an aperture at the front focal plane of the lens. This is important for DSLR cameras having CCD sensors, the pixels in these sensors are more sensitive to rays that hit them straight on than to those that strike at an angle. A lens that does not control the angle of incidence at the detector will produce pixel vignetting in the images and this means that the lens can be treated as if all of the refraction happened at the principal planes
16.
Catadioptric system
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A catadioptric optical system is one where refraction and reflection are combined in an optical system, usually via lenses and curved mirrors. Catadioptric combinations are used in focusing systems such as lights, headlamps, early lighthouse focusing systems, optical telescopes, microscopes. Other optical systems use lenses and mirrors are also referred to as catadioptric such as surveillance catadioptric sensors. Catadioptric combinations have been used in many optical systems. In the 1820s, Augustin-Jean Fresnel developed several catadioptric lighthouse reflectors, léon Foucault developed a catadioptric microscope in 1859 to counteract aberrations of using a lens to image objects at high power. In 1876 a French engineer, A. Mangin, invented what has come to be called the Mangin mirror, the two surfaces of the reflector have different radii to correct the aberration of the spherical mirror. Light passes through the glass twice, making the system act like a triplet lens. Mangin mirrors were used in searchlights, where they produced a true parallel beam. Catadioptric telescopes are optical telescopes that combine specifically shaped mirrors and lenses to form an image and their designs can have simple all-spherical surfaces and can take advantage of a folded optical path that reduces the mass of the telescope, making them easier to manufacture. Catadioptric dialytes are the earliest type of catadioptric telescope and they consist of a single-element refractor objective combined with a silver-backed negative lens. The first of these was the Hamiltonian telescope patented by W. F. Hamilton in 1814, there are several telescope designs that take advantage of placing one or more full-diameter lenses in front of a spherical primary mirror. Some of these designs have been adapted to create compact, long-focal-length catadioptric cassegrains, the Schmidt corrector, the first full-diameter corrector plate, was used in Bernhard Schmidts 1931 Schmidt camera. The relatively thin and lightweight corrector allows Schmidt cameras to be constructed in diameters up to 1.3 m, the design has lent itself to many Schmidt variants. Popular sub-types Schmidt–Cassegrain telescopes are one of the most popular commercial designs on the amateur astronomical market, the design replaces the Schmidt Camera film holder with a Cassegrain secondary mirror, making a folded optical path with a long focal length and a narrow field of view. Wartime secrecy kept these inventors from knowing about each others designs, albert Bouwers built a prototype meniscus telescope in August 1940 and patented it in February 1941. It used a spherically concentric meniscus and was suitable as a monochromatic astronomical camera. In a later design he added a cemented doublet to correct chromatic aberration, Dmitri Maksutov built a prototype for a similar type of meniscus telescope, the Maksutov telescope, in October 1941 and patented it in November of that same year. His design corrected spherical and chromatic aberrations by placing a weak negative-shaped meniscus corrector closer to the primary mirror, popular sub-types Maksutov–Cassegrain telescopes are the most commonly seen design that uses a meniscus corrector, a variant of the Maksutov telescope
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Mount Wilson Observatory
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The Mount Wilson Observatory is an astronomical observatory in Los Angeles County, California, United States. The MWO is located on Mount Wilson, a 1, 740-metre peak in the San Gabriel Mountains near Pasadena, the observatory was conceived and founded by George Ellery Hale, who had previously built the 1 meter telescope at the Yerkes Observatory, then the worlds largest telescope. The Mount Wilson Solar Observatory was first funded by the Carnegie Institution of Washington in 1904, among the conditions of the lease was that it allow public access. There are three solar telescopes at Mount Wilson Observatory, today, just one of these telescopes, the 60 foot Solar Tower, is still used for solar research. The Snow Solar Telescope was the first telescope installed at the fledgling Mount Wilson Solar Observatory and it was the worlds first permanently mounted solar telescope. Solar telescopes had previously been portable so they could be taken to solar eclipses around the world, the telescope was donated to Yerkes Observatory by Helen Snow of Chicago. George Ellery Hale, then director of Yerkes, had the telescope brought to Mount Wilson to put it into service as a scientific instrument. Stellar research soon followed as the brightest stars could have their spectra recorded with very long exposures on glass plates, today the Snow solar telescope is mostly used by undergraduate students who get hands on training in solar physics and spectroscopy. It was also used publicly for the May 9,2016 transit of Mercury across the face of the sun, the 60-foot Solar Tower soon built on the work started at the Snow telescope. At its completion in 1908, the tower design of the 60 foot focal length solar telescope allowed much higher resolution of the solar image. The higher resolution came from situating the optics higher above the ground, on June 25,1908, Hale would record Zeeman splitting in the spectrum of a sunspot, showing for the first time that magnetic fields existed somewhere besides the earth. A later discovery was of the polarity in sunspots of the new solar cycle of 1912. The success of the 60 foot Tower prompted Hale to pursue yet another, taller tower telescope, in the 1960s, Robert Leighton discovered the sun had a 5-minute oscillation and the field of heliosiesmology was born. The 60 foot Tower is operated by the Department of Physics, the 150-foot focal length solar tower expanded on the solar tower design with its tower-in-a-tower design. An inner tower supports the optics above, while an outer tower and this design allowed complete isolation of the optics from the effect of wind swaying the tower. Two mirrors feed sunlight to a 12-inch lens which focuses light down at the ground floor and it was first completed in 1910, but unsatisfactory optics caused a two-year delay before a suitable doublet lens was installed. Research included solar rotation, sunspot polarities, daily sunspot drawings, the solar telescope would be the worlds largest for 50 years until the McMath-Pierce Solar telescope was completed at Kitt Peak in Arizona in 1962. In 1985, UCLA took over operation of the tower from the Carnegie Observatories after it was decided to stop funding the observatory
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Hale Telescope
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The Hale Telescope is a 200-inch, f/3.3 reflecting telescope at the Palomar Observatory in California, US, named after astronomer George Ellery Hale. With funding from the Rockefeller Foundation, he orchestrated the planning, design, and construction of the observatory and it is still in active use. It was the largest optical telescope in the world from its completion in 1948 until the BTA-6 was built in 1976, the chief optical designer for Hales previous 100-inch telescope was George Willis Ritchey, who intended the new telescope to be of Ritchey–Chrétien design. Compared to the parabolic primary, this design would have provided sharper images over a larger usable field of view. However, Ritchey and Hale had a falling out, with the project already late and over budget, Hale refused to adopt the new design, with its complex curvatures, and Ritchey left the project. The Mount Palomar Hale telescope turned out to be the last world-leading telescope to have a primary mirror. The Corning Glass Works was assigned the task of making a 200-inch primary mirror, construction of the observatory facilities and dome started in 1936, but because of interruptions caused by World War II, the telescope was not completed until 1948 when it was dedicated. Due to slight distortions of images, corrections were made to the telescope throughout 1949 and it became available for research in 1950. The 200-inch Hale saw first light on January 26,1949 under the direction of American astronomer Edwin Powell Hubble, targeting NGC2261, the photographs made then were published in the astronomical literature and in the May 7,1949 issue of Colliers Magazine. It is equipped with optical and infrared array imagers, spectrographs. It has also used lucky cam imaging, which in combination with adaptive optics pushed the mirror close to its resolution for certain types of viewing. One of the Corning Labs glass test blanks for the Hale was used for the C. Donald Shane telescopes 120-inch primary mirror, the collecting area of the Hale is about 31,000 square inches. The optical tube assembly uses a Serrurier truss, then newly invented by Mark U, Serrurier of Caltech in Pasadena in 1935, designed to flex in such a way as to keep all of the optics in alignment. Theodore von Karman designed the system to avoid potential issues with turbulence during tracking. Pyrex was chosen for its low expansion qualities so the mirror would not distort the images produced when it changed shape due to temperature variations. The mirror was cast in a mold with 36 raised mold blocks, the shape of a central hole was also part of the mold so light could pass through the finished mirror when it was used in a Cassegrain configuration. The defective mirror was used to test the annealing process, after the mold was re-engineered, a second mirror was successfully cast. After cooling several months, the mirror blank was transported by rail to Pasadena
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Equatorial platform
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They are used to give equatorial tracking to any device sitting on them, from small cameras up to entire observatory buildings. They are often used with altazimuth mounted telescopes, such as the common Dobsonian telescope type, many types of equatorial platform have been used over the years. The mid-1960s saw the introduction of the Russian AFU-75 satellite-tracking camera which consisted of a 3 axis altitude-altitude-azimuth mount mounted on a three-point equatorial platform, two of the points were aligned on a polar axis while the third was a jackscrew actuator to drive the platform. This gave the mount a few minutes of equatorial tracking to allow stars in the field of view to be imaged as points for accurate measurement. One was Alan Gees design that uses a bearing surface. Poncet based platforms are designed to track for 1 hour since longer tracking exceeds their range of motion. At the end of their drive limit the mount has to be pivoted back to the east to reset the drive mechanism. Amateur telescope making - Stephen F. Tonkin -1999, Chapter 9 Equatorial Platforms List of telescope parts and construction List of telescope types equatorial plattform
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Amateur astronomy
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Amateur astronomy is a hobby whose participants enjoy watching the sky, and the abundance of objects found in it with the unaided eye, binoculars, or telescopes. Many amateurs are beginners or hobbyists, while others have a degree of experience in astronomy and often assist. Amateur astronomers often look at the sky using nothing more than their eyes, people have studied the sky throughout history in an amateur framework, without any formal method of funding. It is only within about the past century, however, that amateur astronomy has become an activity clearly distinguished from professional astronomy, collectively, amateur astronomers observe a variety of celestial objects and phenomena. Common targets of amateur astronomers include the Moon, planets, stars, comets, meteor showers, and a variety of deep sky objects such as clusters, galaxies. Many amateurs like to specialise in observing particular objects, types of objects, one branch of amateur astronomy, amateur astrophotography, involves the taking of photos of the night sky. Most amateur astronomers work at visible wavelengths, but a small minority experiment with wavelengths outside the visible spectrum, non-visual amateur astronomy includes the use of infrared filters on conventional telescopes, and also the use of radio telescopes. Some amateur astronomers use home-made radio telescopes, while others use radio telescopes that were built for astronomy research but have since been made available for use by amateurs. The One-Mile Telescope is one such example, Amateur astronomers use a range of instruments to study the sky, depending on a combination of their interests and resources. Commercial telescopes are available, new and used, but it is common for amateur astronomers to build their own custom telescopes. Some people even focus on amateur telescope making as their primary interest within the hobby of amateur astronomy, although specialized and experienced amateur astronomers tend to acquire more specialized and more powerful equipment over time, relatively simple equipment is often preferred for certain tasks. Amateur astronomers also use star charts that, depending on experience and intentions, Amateur astronomers often like to keep records of their observations, which usually takes the form of an observing log. Observing logs typically record details about which objects were observed and when, sketching is sometimes used within logs, and photographic records of observations have also been used in recent times. The information gathered is used to help studies and interactions between amateur astronomers in yearly gatherings, although not professional information or credible, it is a way for the hobby lovers to share their new sightings and experiences. The Internet is a tool of amateur astronomers. The popularity of CCD imaging among amateurs has led to large numbers of web sites being written by individuals about their images, much of the social interaction of amateur astronomy occurs on mailing lists or discussion groups. Discussion group servers host numerous astronomy lists, a great deal of the commerce of amateur astronomy, the buying and selling of equipment, occurs online. Many amateurs use online tools to plan their observing sessions using tools such as the Clear Sky Chart
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Dobsonian telescope
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Dobsons telescopes featured a simplified mechanical design that was easy to manufacture from readily available components to create a large, portable, low-cost telescope. The design is optimized for visually observing faint, deep-sky objects such as nebulae and this type of observation requires a large objective diameter of relatively short focal length and portability for travel to relatively less light-polluted locations. Dobsonians are popular in the telescope making community, where the design was pioneered. A number of commercial telescope makers also sell telescopes based on this design and it is hard to classify the Dobsonian Telescope as a single invention. In the field of amateur telescope making most, if not all, john Dobson, credited as having invented this design in the 1950s, points out that for hundreds of years, wars were fought using cannon on Dobsonian mounts. Since he built these telescopes as aids in his avocation of instructional sidewalk astronomy, Dobsons design allows a builder with minimal skill to make an extremely large telescope out of common items. Dobson optimized the design for visual observation of faint objects such as clusters, nebulae. These dim objects require a large objective mirror able to gather a large amount of light, because deep sky observing often requires travel to dark locations away from city lights, the design benefits from being more compact, portable, and rugged than standard large Newtonian telescopes. Construction tubes, Dobson replaced the traditional aluminum or fiberglass telescope tube with the thick compressed paper tubes used in construction to concrete columns. Sonotubes, the leading brand employed by Dobson, are less expensive than commercially available telescope tubes and are available in large sizes. Dobsonian scopes are intended to be transported out to dark sky locations, Sonotubes have the added advantage of being thermally stable and non-conductive which minimizes unwanted convection currents in the light path caused by handling of the tube assembly. A square mirrorbox, Dobson used a box for the tube base. This gave a rigid surface to attach the carpet support for the porthole mirrors he was using. A simple alt-azimuth mount, Dobson opted for a simple to build and he used a “gun carriage type” design consisting of a flat platform on which sits a rotating box with semicircular depressions cut in the top for the altitude bearings. All parts were made from plywood and other common materials, for the azimuth motion Dobson used a combination of Teflon blocks turning on a flat Formica covered surface. For the altitude motion, Dobson used a large diameter axle closet flange turning on Teflon bearing blocks attached to the altitude cutouts. The design of Dobsonian telescopes has evolved over the years, but most commercial or amateur-built Dobsonian telescopes follow many or most of the design concepts, the Dobsonian design has the following characteristics, Altazimuth mount, An equatorial telescope mount with clock drive was left out of the design. Equatorial mounts tend to be massive, expensive, complicated, and have the characteristics of putting the eyepiece of Newtonian telescopes in very hard to access positions
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Barn door tracker
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A barn door tracker, also known as a Haig or Scotch mount, is a device used to cancel out the diurnal motion of the Earth for the observation or photography of astronomical objects. It is an alternative to attaching a camera to a motorized equatorial mount. The barn door tracker was created by George Haig and his plans were first published in Sky & Telescope magazine in April 1975. Modified versions of the tracker were published in the magazines February 1988, a simple single-arm barn door tracker can be made by attaching two pieces of wood together with a hinge. A camera is mounted on the top board, usually some sort of ball joint to allow the camera to be pointed in any direction. The hinge is aligned with a pole and the boards are then driven apart at a constant rate. This is called a tangent drive and this type of mount is good for approximately 5–10 minutes before tracking errors become evident when using a 50 mm lens. This is due to the tangent error and that length of time can be increased to about 20 minutes when using an isosceles mount. A curved drive bolt in lieu of either a straight tangent or isosceles mount will greatly extend the useful tracking time and these designs were further improved upon by Dave Trott, whose designs were published in the February 1988 issue of Sky & Telescope. By using a second arm to drive the camera platform - thus adding complexity to the fabrication - tracking accuracy was greatly increased, the most accurate of these designs is the Type-4. A modified double arm design minimizes tangent error by raising the point of rotation of the arm on which the camera is mounted and this has the effect of tilting the arc traced by the camera arm backwards causing it to follow a better path. A basic geometrical analysis of the tangent error shows that it can be compensated for by inserting a specially shaped piece between the threaded rod and the top board. Such solution was already known for a time before the original G. Haig publication. The most basic of these designs are manually operated, although some have added electric motors to automate and improve the accuracy of the tracking process
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Equatorial room
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An equatorial room, in astronomical observatories, is the room which contains an equatorial mounted telescope. Equatorial rooms tend to be large rooms to accommodate all the range of motion of a long telescope on an equatorial mount and are usually topped with a dome to keep out the weather. In some cases an observatory would move to a new location, the space would then be converted, for example, into use as a classroom or library. These peculiar rooms can sometimes be found in buildings at old colleges and towns, nature Nov 19,1885, page 57, By University of Wisconsin--Madison
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Hexapod-Telescope
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The Hexapod-Telescope is a telescope located at Cerro Armazones Observatory in northern Chile. The 1.5 metres Ritchey-Chrétien reflecting telescope is notable for the design of telescope mount, instead of the typical mounting where the telescope moves on two rotating axes, the mirror end of the telescope is supported by six extensible struts, an arrangement known as a Stewart platform. This configuration allows the telescope to move in all six degrees of freedom. As a result, the ratio of bearing pressure and its own weight is very high, furthermore, the six-leg structure allows for a very precise positioning and repeatability. The disadvantage of the system is that controlling and aiming a telescope is much more complex than with conventional telescope mounts. The mounting was designed by engineers of the company Vertex, in collaboration with astronomers of the Astronomy Institute of the Ruhr University Bochum in Germany, led by Prof. Rolf Chini, the HPT was thoroughly tested at AIRUB. In 2006, it was moved to its new location on Cerro Armazones Observatory in the Atacama Desert in Chile, AIRUB, in collaboration with the Astronomy Department of the Catholic University of the North, is developing an observatory there. A new control building was next to the HPT on the summit of Cerro Murphy. Together with the Landessternwarte Heidelberg-Königstuhl, AIRUB developed the Bochum Echelle Spectrograph for the Optical for the HPT and it is a copy of the Fiber-fed Extended Range Optical Spectrograph operated by the European Southern Observatory. Array for Microwave Background Anisotropy Telescope mount Theodor Schmidt-Kaler, The Hexapod Telescope,117 AIRUB - Observatorio Cerro Armazones at Ruhr-Universität Bochum Observatorío Cerro Armazones at Universidád Católica del Norte Hexapod for ALMA Radio Telescopes secondary mirror
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Polar alignment
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Polar alignment is the act of aligning the rotational axis of a telescopes equatorial mount or a sundial in parallel with that of the Earth. The method to use depending on whether the alignment is taking place in daylight or in night. Furthermore, the method differs if the alignment is done in the Northern Hemisphere or Southern Hemisphere, the purpose of the alignment also must be considered. For example, the demand for accuracy is more significant in astrophotography than in occasional stargazing. In the Northern hemisphere, sighting Polaris the North Star is the procedure for aligning a telescope mounts polar axis parallel to the Earths axis. Polaris is approximately three quarters of a degree from the North Celestial Pole and is seen by the naked eye. σ Octantis, sometimes known as the South Star, can be sighted in the Southern hemisphere to perform polar alignment, at magnitude +5.6, it is difficult for inexperienced observers to locate in the sky. Its declination of -88° 57′ 23″ places it within 1° 2′37 of the South Celestial Pole, an even closer star BQ Octantis of magnitude +6.9 lies 10 from the South Pole as of 2016. Although not visible to the eye, it is easily visible in most polar scopes. It will be closest to the South Pole in the year 2027, in the Northern hemisphere, rough alignment can be done by visually aligning the axis of the telescope mount with Polaris. This method can sometimes be adequate for general observing through the eyepiece or for very wide angle astro-imaging with a tripod-mounted camera and this method is often used by newcomers to amateur astronomy equipped with an equatorially-mounted telescope. There are ways to improve the accuracy of this method, for example, instead of reading the latitude scale directly, a calibrated precision inclinometer can be used to measure the altitude of the polar axis of the mount. Typically, this provides enough accuracy to allow tracked telephoto images of the sky, for astro-imaging through a lens or telescope of significant magnification, a subsequent drift alignment is necessary to refine the rough alignment. Drift alignment is a method to refine the polar alignment after an alignment is done. The method is based on attempting to track stars in the sky using the clock drive, adjustments are then made to reduce the drift, and the process is repeated until the tracking is satisfactory. For the polar axis altitude adjustment, one can attempt to track a star low in the east or west. For the azimuth adjustment, one attempts to track a star close to the meridian, with declination about 20° from the equator. A crosshair eyepiece is an ordinary ocular with the difference that it has a crosshair for aiming
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Poncet Platform
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A Poncet Platform or Poncet mount is a type of equatorial platform that uses a simple polar pivot and an inclined plane. The motion of the mount allows any device sitting on that platform to track the apparent motion of the stars in the night sky, the Poncet Platform was invented in the 1970s by Adrien Poncet. Poncets original design was a simple type of equatorial platform that uses pivot as one support. It was publicized in the January 1977 issue of Sky & Telescope magazine and it has been used to add equatorial tracking to everything from small cameras to entire observatory buildings. Its simple design and low profile has made it a useful retrofit for altazimuth mounted telescopes such as the popular Dobsonian telescope. Users simply place their telescope on top of it to get the feature of tracking in the direction of right ascension accurate enough for work at higher magnification or astrophotography. Poncet Platforms do have design limitations and they are usually designed to track for 1 hour since longer tracking could cause the instrument on top of it to topple off. After that hour the mount has to be pivoted back to the east to reset the drive mechanism. Since the Poncet platform has no roller bearing surfaces that can be driven, straight line drives such as threaded nut/bolt drives change drive rate when converted to a circular motion. It is also time consuming to reset via spinning the nut back to the starting point, amateurs have tackled this by employing curved bolt designs and even using specially shaped cams to convert the straight line motion to a variable speed motion. High mechanical loadings from heavy telescopes or using them at low geographic latitudes can cause the mount to bind up, List of telescope parts and construction List of telescope types equatorial platform
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Universe Today
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Universe Today is a moderately popular North American based non-commercial space and astronomy news site. By early September 2005, the forum section merged with Bad Astronomy as a site with the BAUT forum. During April 2011, the Association of British Science Writers noted that Universe Today decided not to make preparations for reporting on embargoed stories until they are public knowledge, emily Lakdawalla said that she relies on Universe Today and Bad Astronomy to give. An independent look at big news stories, Universe Today has published two books, which are available both as e-books and on physical media, Plotner, Tammy, Barbour, Jeff. Whats Up 2006,365 Days of Skywatching, whats Up 2007,365 Days of Skywatching. Astronomy Cast Bad Astronomy Space. com Official website
