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63-437: Dobsonians are intended to be what is commonly called a "light bucket". Operating at low magnification , and therefore the design omits features found in other amateur telescopes such as equatorial tracking. Dobsonians are popular in the amateur telescope making community, where the design was pioneered and continues to evolve. A number of commercial telescope makers also sell telescopes based on this design. The term Dobsonian

126-402: A m s d e n . {\displaystyle M_{\mathrm {A} }={1 \over M}={D_{\mathrm {Objective} } \over {D_{\mathrm {Ramsden} }}}\,.} With any telescope, microscope or lens, a maximum magnification exists beyond which the image looks bigger but shows no more detail. It occurs when the finest detail the instrument can resolve is magnified to match the finest detail

189-449: A Dobsonian's mount is that it resembles a "gun carriage" configuration with a "rocker box" consisting of a horizontal trunnion style altitude axis and a broadly supported azimuth axis, both making use of material such as plastic, Formica, and Teflon to achieve smooth operation. Many derivative mount designs have kept this basic form while heavily modifying the materials and configuration. Many designs have increased portability by shrinking

252-505: A difference only if the angle is larger than a few degrees). Thus, angular magnification is given by: M A = tan ⁡ ε tan ⁡ ε 0 ≈ ε ε 0 {\displaystyle M_{A}={\frac {\tan \varepsilon }{\tan \varepsilon _{0}}}\approx {\frac {\varepsilon }{\varepsilon _{0}}}} where ε 0 {\textstyle \varepsilon _{0}}

315-402: A high numerical aperture and using oil immersion , the best possible resolution is 200 nm corresponding to a magnification of around 1200×. Without oil immersion, the maximum usable magnification is around 800×. For details, see limitations of optical microscopes . Small, cheap telescopes and microscopes are sometimes supplied with the eyepieces that give magnification far higher than

378-487: 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 of times past, which typically utilized massive German equatorial mounts . John Dobson's telescopes combined several innovations to meet these criteria, including: The design of Dobsonian telescopes has evolved over

441-616: A large surface contact area between the trunnion and the cylinder. In airframe engineering, these are self-contained concentric bearings that are designed to offer fluid movement in a critical area of the steering. The term is also used to describe the wheel that a rotating cylinder runs on. For example, a lapidary (stone-polishing) cylinder runs on a pair of rollers, similar to trunnions. The sugar industry uses rotating cylinders up to 22 feet (7 m) in diameter, 131 ft (40 m) long, and weighing around 1,000 tons . These rotate at around 30 revolutions per hour. They are supported on

504-416: A large van for transport. Designers started coming up with disassembleable or collapsible variants that could be brought to the site with a small SUV , hatchback , or even a sedan . This innovation allowed the amateur astronomy community access to even larger apertures. Many designs have combined the advantages of a light truss tube and a collapsible design. Collapsible "truss tube" Dobsonians appeared in

567-655: A negative magnification implies an inverted image. The image magnification along the optical axis direction M L {\displaystyle M_{L}} , called longitudinal magnification, can also be defined. The Newtonian lens equation is stated as f 2 = x 0 x i {\displaystyle f^{2}=x_{0}x_{i}} , where x 0 = d 0 − f {\textstyle x_{0}=d_{0}-f} and x i = d i − f {\textstyle x_{i}=d_{i}-f} as on-axis distances of an object and

630-551: A psychological effect on the battlefield; owning these giant mortars did not guarantee any army a victory. The French saw the limitations of these massive weapons and focused their efforts on improving their smaller and lighter guns, which used smaller, more manageable projectiles combined with larger amounts of gunpowder. Equipping them with trunnions was key for two reasons. First, teams of horses could now move these cannons fast enough to keep up with their armies and no longer had to stop and dismount them from their carriages to achieve

693-430: A screen, size means a linear dimension (measured, for example, in millimeters or inches ). For optical instruments with an eyepiece , the linear dimension of the image seen in the eyepiece ( virtual image at infinite distance) cannot be given, thus size means the angle subtended by the object at the focal point ( angular size ). Strictly speaking, one should take the tangent of that angle (in practice, this makes

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756-445: A shaft (the trunnion) is inserted into (and turns inside) a full or partial cylinder. In a cannon , the trunnions are two projections cast just forward of the center of mass of the cannon and fixed to a two-wheeled movable gun carriage . As they allowed the muzzle to be raised and lowered easily, the integral casting of trunnions is seen by military historians as one of the most important advances in early field artillery . With

819-822: A team of men or horses could pull them. Due to its capabilities, the French- and Burgundy-designed siege gun, equipped with its trunnions, required little significant modification from around 1465 to the 1840s. King Charles VIII and the French army used this new gun in the 1494 invasion of Italy . Although deemed masters of war and artillery at that time, Italians had not anticipated the innovations in French siege weaponry. Prior to this, field artillery guns were huge, large-caliber bombards: superguns that, along with enormous stones or other projectiles, were dragged from destination to destination. These behemoths could only be used effectively in sieges, and more often than not provided just

882-538: A trunnion which extends into the mass of the dam or lock. The Tainter gate is used in water control dams and locks worldwide. The Upper Mississippi River basin alone has 321 Tainter gates, and the Columbia River basin has 195. In mechanical engineering, it is one part of a rotating joint where a shaft (the trunnion) is inserted into (and turns inside) a full or partial cylinder. Often used in opposing pairs, this joint allows tight tolerances and strength from

945-415: A very large, inexpensive, easy to use, portable telescope, one that could bring deep-sky astronomy to the masses. Dobson's design allows a builder with minimal skills to make a very large telescope out of common items. Dobson optimized the design for observation of faint objects such as star clusters , nebulae , and galaxies (what amateur astronomers call deep sky object s). These dim objects require

1008-416: Is a cylindrical protrusion used as a mounting or pivoting point. First associated with cannons, they are an important military development. Alternatively, a trunnion is a shaft that positions and supports a tilting plate. This is a misnomer, as in reality it is a cradle for the true trunnion. In mechanical engineering (see the trunnion bearing section below), it is one part of a rotating joint where

1071-404: Is always negative, means that, the object and the image move toward the same direction along the optical axis. The longitudinal magnification varies much faster than the transverse magnification, so the 3-dimensional image is distorted. The image recorded by a photographic film or image sensor is always a real image and is usually inverted. When measuring the height of an inverted image using

1134-522: Is currently used for a range of large-aperture Newtonian reflectors that use some of the basic Dobsonian design characteristics, regardless of the materials from which they are constructed. It is hard to classify the Dobsonian telescope as a single invention. In the field of amateur telescope making most, if not all, of its design features had been used before. John Dobson, credited as having invented this design in 1965 pointed out that Dobson identified

1197-690: Is positive and the image is upright. With d i {\textstyle d_{\mathrm {i} }} being the distance from the lens to the image, h i {\textstyle h_{\mathrm {i} }} the height of the image and h o {\textstyle h_{\mathrm {o} }} the height of the object, the magnification can also be written as: M = − d i d o = h i h o {\displaystyle M=-{d_{\mathrm {i} } \over d_{\mathrm {o} }}={h_{\mathrm {i} } \over h_{\mathrm {o} }}} Note again that

1260-405: Is the focal length of the lens in centimeters. The constant 25 cm is an estimate of the "near point" distance of the eye—the closest distance at which the healthy naked eye can focus. In this case the angular magnification is independent from the distance kept between the eye and the magnifying glass. If instead the lens is held very close to the eye and the object is placed closer to

1323-425: Is the focal length of the objective lens in a refractor or of the primary mirror in a reflector , and f e {\textstyle f_{\mathrm {e} }} is the focal length of the eyepiece . Measuring the actual angular magnification of a telescope is difficult, but it is possible to use the reciprocal relationship between the linear magnification and the angular magnification, since

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1386-514: Is the focal length , d o {\textstyle d_{\mathrm {o} }} is the distance from the lens to the object, and x 0 = d 0 − f {\textstyle x_{0}=d_{0}-f} as the distance of the object with respect to the front focal point. A sign convention is used such that d 0 {\textstyle d_{0}} and d i {\displaystyle d_{i}} (the image distance from

1449-605: Is the angle subtended by the object at the front focal point of the objective and ε {\textstyle \varepsilon } is the angle subtended by the image at the rear focal point of the eyepiece. For example, the mean angular size of the Moon 's disk as viewed from Earth's surface is about 0.52°. Thus, through binoculars with 10× magnification, the Moon appears to subtend an angle of about 5.2°. By convention, for magnifying glasses and optical microscopes , where

1512-414: Is the magnification of the objective and M e {\textstyle M_{\mathrm {e} }} the magnification of the eyepiece. The magnification of the objective depends on its focal length f o {\textstyle f_{\mathrm {o} }} and on the distance d {\textstyle d} between objective back focal plane and the focal plane of

1575-401: Is usable. The maximum relative to the minimum magnification of an optical system is known as zoom ratio . Magnification figures on pictures displayed in print or online can be misleading. Editors of journals and magazines routinely resize images to fit the page, making any magnification number provided in the figure legend incorrect. Images displayed on a computer screen change size based on

1638-635: The Central Bureau for Astronomical Telegrams ). Magnification Magnification is the process of enlarging the apparent size , not physical size, of something. This enlargement is quantified by a size ratio called optical magnification . When this number is less than one, it refers to a reduction in size, sometimes called de-magnification . Typically, magnification is related to scaling up visuals or images to be able to see more detail, increasing resolution , using microscope , printing techniques, or digital processing . In all cases,

1701-470: The cartesian sign convention (where the x-axis is the optical axis) the value for h i will be negative, and as a result M will also be negative. However, the traditional sign convention used in photography is " real is positive, virtual is negative". Therefore, in photography: Object height and distance are always real and positive. When the focal length is positive the image's height, distance and magnification are real and positive. Only if

1764-509: The eyepiece (called the tube length): M o = d f o {\displaystyle M_{\mathrm {o} }={d \over f_{\mathrm {o} }}} The magnification of the eyepiece depends upon its focal length f e {\textstyle f_{\mathrm {e} }} and is calculated by the same equation as that of a magnifying glass (above). Note that both astronomical telescopes as well as simple microscopes produce an inverted image, thus

1827-435: The 1970s, today 16 inch systems are common, and huge 32 inch systems not all that rare. In combination with other improvements in observing equipment, such as narrow-pass optical filters , improved eyepieces , and digital visible and infrared photography, the large apertures of the Dobsonian have dramatically increased the number of objects observed as well as the amount of detail in each object observed. Whereas

1890-596: The 1990s, manufactured Dobsonians using the truss tube design have become increasingly popular. The first commercial truss Dobsonian was released into the market by Obsession Telescopes in 1989. Later American manufacturers included StarStructure, Webster Telescopes, AstroSystems, Teeter's Telescopes, Hubble Optics, Waite Research, and New Moon Telescopes. These low-volume builders offer premium objective mirrors, high-end materials and custom craftmanship, as well as optional computer controlled GoTo systems. Some also produce "ultra-light" models that offer greater portability. In

1953-477: The 21st century, truss Dobsonian models are also mass-produced by Meade, Orion, Explore Scientific and others. Mostly manufactured in China, they offer good quality and value while being considerably less expensive than the premium scopes described above. In 2017, Sky-Watcher introduced its line of large Stargate models. Solid tube commercial Dobsonians typically have a maximum aperture of 12 inches (305 mm) due to

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2016-413: The advantage of inexpensive large instruments that could be carried to dark sky locations and star parties in the back of a small car and set up in minutes. The result has been a proliferation of larger telescopes which would have been expensive to build or buy, and unwieldy to operate, using "traditional" construction methods. Whereas an 8 inch Newtonian telescope would have been considered large in

2079-542: The advent of these new weapons. Defensive tactics and fortifications had to be altered since these new weapons could be transported so speedily and aimed with much more accuracy at strategic locations. Two significant changes were the additions of a ditch and low, sloping ramparts of packed earth ( glacis ) that would surround the city and absorb the impact of the cannonballs, and the replacement of round watchtowers with angular bastions . These towers would be deemed trace Italienne. Whoever could afford these new weapons had

2142-443: The altazimuth (rocker box) mount down to a small rotating platform. The altitude trunnion style bearing in these designs becomes a large radius roughly equal to or greater than the radius of the objective mirror, attached to or integrated into the tube assembly which lowers the overall profile of the mount. The advantage of this is that it reduces the total telescope weight, and the telescope's balance becomes less sensitive to changes in

2205-514: The amateur astronomer of the 1970s and 1980s typically did not explore much beyond the Messier and brighter NGC objects, thanks in part to Dobsonians, modern amateur astronomers routinely observe dim objects listed in obscure catalogues, such as the IC , Abell , Kohoutek , Minkowski , and others once considered reference works only for professional astronomers. When mounted on an equatorial platform

2268-441: The amateur telescope making community as early as 1982 and allow the optical tube assembly, the largest component, to be broken down. As the name implies, the "tube" of this design is actually composed of an upper cage assembly, which contains the secondary mirror, and focuser, held in place by several rigid poles over a mirror box which contains the objective mirror. The poles are held in place by quick-disconnecting clamps which allow

2331-437: The back focal plane. This is mounted in front of the telescope eyepiece and used to evaluate the diameter of the exit pupil. This will be much smaller than the object glass diameter, which gives the linear magnification (actually a reduction), the angular magnification can be determined from M A = 1 M = D O b j e c t i v e D R

2394-577: The center of mass to allow the barrel to be elevated to any desired angle, without having to dismount it from the carriage upon which it rested. Some guns had a second set of trunnions placed several feet back from the first pair, which could be used to allow for easier transportation. The gun would recoil causing the carriage to move backwards several feet but men or a team of horses could put it back into firing position. It became easier to rapidly transport these large siege guns, maneuver them from transportation mode to firing position, and they could go wherever

2457-429: The characteristic features of the design as lightweight objective mirrors made from porthole glass, and mountings constructed from plywood , Teflon strips and other low-cost materials. Since he built these telescopes as aids in his avocation of instructional sidewalk astronomy , he preferred to call the design a "sidewalk telescope". Dobson combined all these innovations in a design focused towards one goal: building

2520-540: The creation of larger and more powerful siege guns in the early 15th century, a new way of mounting them became necessary. Stouter gun carriages were created with reinforced wheels, axles, and “trails” which extended behind the gun. Guns were now as long as 2.5 metres (8 ft) in length and they were capable of shooting iron projectiles weighing from 10 to 25 kilograms (25 to 50 lb). When discharged, these wrought iron balls were comparable in range and accuracy with stone-firing bombards . Trunnions were mounted near

2583-484: The difficulties using a Dobsonian for short-exposure (≲ 1 hr) astrophotography are obviated. This has opened up the field of high precision asteroid astrometry (and discovery) to the amateur wishing to contribute minor planet positions to the Minor Planet Center . It also makes possible searches for new, faint objects such as novae / supernovae in local galaxies, and comets (for reports to

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2646-452: The diopter of the eye (making it myopic) so that the object can be placed closer to the eye resulting in a larger angular magnification. The angular magnification of a microscope is given by M A = M o × M e {\displaystyle M_{\mathrm {A} }=M_{\mathrm {o} }\times M_{\mathrm {e} }} where M o {\textstyle M_{\mathrm {o} }}

2709-409: The entire telescope to be easily broken down into its smaller components, facilitating their transport by vehicle or other means to an observing site. These truss tube designs are sometimes incorrectly called a Serrurier truss , but since the main truss is not built with an opposing mirror cell truss it only performs one function of that design, i.e. keeping the optics parallel. The main attribute of

2772-423: The equation for the magnification of a telescope or microscope is often given with a minus sign . The angular magnification of an optical telescope is given by M A = f o f e {\displaystyle M_{\mathrm {A} }={f_{\mathrm {o} } \over f_{\mathrm {e} }}} in which f o {\textstyle f_{\mathrm {o} }}

2835-446: The eye can see. Magnification beyond this maximum is sometimes called "empty magnification". For a good quality telescope operating in good atmospheric conditions, the maximum usable magnification is limited by diffraction . In practice it is considered to be 2× the aperture in millimetres or 50× the aperture in inches; so, a 60 mm diameter telescope has a maximum usable magnification of 120×. With an optical microscope having

2898-578: The first companies to offer Dobsonian telescopes commercially was the now defunct company Coulter Optical (now part of Murnaghan Instruments). In the 1980s, they helped popularize the design with "Odyssey" models of various sizes, with tubes made of Sonotube and following Dobson's original concept of simplicity. By the 1990s, Meade Instruments , Orion Telescopes and other manufacturers began to introduce upgraded Dobsonian models. These imported mass-produced scopes included such niceties as metal tubes and more refined hardware, and are still very affordable. Since

2961-752: The focal length is negative, the image's height, distance and magnification are virtual and negative. Therefore, the photographic magnification formulae are traditionally presented as M = d i d o = h i h o = f d o − f = d i − f f {\displaystyle {\begin{aligned}M&={d_{\mathrm {i} } \over d_{\mathrm {o} }}={h_{\mathrm {i} } \over h_{\mathrm {o} }}\\&={f \over d_{\mathrm {o} }-f}={d_{\mathrm {i} }-f \over f}\end{aligned}}} The maximum angular magnification (compared to

3024-653: The image with respect to respective focal points, respectively. M L {\displaystyle M_{L}} is defined as M L = d x i d x 0 , {\displaystyle M_{L}={\frac {dx_{i}}{dx_{0}}},} and by using the Newtonian lens equation, M L = − f 2 x o 2 = − M 2 . {\displaystyle M_{L}=-{\frac {f^{2}}{x_{o}^{2}}}=-M^{2}.} The longitudinal magnification

3087-409: The lens than its focal point so that the observer focuses on the near point, a larger angular magnification can be obtained, approaching M A = 25   c m f + 1 {\displaystyle M_{\mathrm {A} }={25\ \mathrm {cm} \over f}+1} A different interpretation of the working of the latter case is that the magnifying glass changes

3150-400: The lens) are positive for real object and image, respectively, and negative for virtual object and images, respectively. f {\textstyle f} of a converging lens is positive while for a diverging lens it is negative. For real images , M {\textstyle M} is negative and the image is inverted. For virtual images , M {\textstyle M}

3213-428: The linear magnification is constant for all objects. The telescope is focused correctly for viewing objects at the distance for which the angular magnification is to be determined and then the object glass is used as an object the image of which is known as the exit pupil . The diameter of this may be measured using an instrument known as a Ramsden dynameter which consists of a Ramsden eyepiece with micrometer hairs in

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3276-523: The magnification of the image does not change the perspective of the image. Some optical instruments provide visual aid by magnifying small or distant subjects. Optical magnification is the ratio between the apparent size of an object (or its size in an image) and its true size, and thus it is a dimensionless number . Optical magnification is sometimes referred to as "power" (for example "10× power"), although this can lead to confusion with optical power . For real images , such as images projected on

3339-509: The naked eye) of a magnifying glass depends on how the glass and the object are held, relative to the eye. If the lens is held at a distance from the object such that its front focal point is on the object being viewed, the relaxed eye (focused to infinity) can view the image with angular magnification M A = 25   c m f {\displaystyle M_{\mathrm {A} }={25\ \mathrm {cm} \over f}} Here, f {\textstyle f}

3402-505: The nobility began to pay their taxes and more closely follow their ruler’s mandates. With siege guns mounted on trunnions, stronger and larger states were formed, but because of this, struggles between neighboring governments with consolidated power began to ensue and would continue to plague Europe for the next few centuries. A common floodgate used in dams and canal locks is the Tainter gate . This gate opens and closes by pivoting on

3465-445: The proper range before firing; second, the capability to adjust firing angle without having to lift the entire weight of the gun allowed tactical selection and reselection of targets rather than being deployed solely on the first target chosen. Francesco Guicciardini , an Italian historian and statesman, wrote that the cannons were placed against town walls so quickly, spaced together so closely and shot so rapidly and with such force that

3528-599: The size of the object is a linear dimension and the apparent size is an angle, the magnification is the ratio between the apparent (angular) size as seen in the eyepiece and the angular size of the object when placed at the conventional closest distance of distinct vision: 25 cm from the eye. The linear magnification of a thin lens is M = f f − d o = − f x o {\displaystyle M={f \over f-d_{\mathrm {o} }}=-{\frac {f}{x_{o}}}} where f {\textstyle f}

3591-480: The size of the screen. A scale bar (or micron bar) is a bar of stated length superimposed on a picture. When the picture is resized the bar will be resized in proportion. If a picture has a scale bar, the actual magnification can easily be calculated. Where the scale (magnification) of an image is important or relevant, including a scale bar is preferable to stating magnification. Trunnion A trunnion (from Old French trognon  'trunk')

3654-407: The size of the tube. Truss Dobsonians of 12 to 18 inches (305 to 457 mm) are the most popular sizes, as they offer substantial aperture yet can still be easily set up by one person. Several manufacturers offer models of 24 inch (610 mm) aperture and greater. Truss Dobsonians are the largest telescopes commercially available today. A massive 36 inch (914 mm) aperture Hybrid model from New Moon Telescopes

3717-497: The tactical advantage over their neighbors and smaller sovereignties, which could not incorporate them into their army. Smaller states, such as the principalities of Italy, began to conglomerate. Preexisting stronger entities, such as France or the Habsburg emperors, were able to expand their territories and maintain a tighter control over the land they already occupied. With the potential threat of their land and castles being seized,

3780-427: The time for a significant amount of damage to be inflicted went from a matter of days (as with bombards) to a matter of hours. For the first time in history, as seen in the 1512 battle of Ravenna and the 1515 Battle of Marignano , artillery weaponry played a very decisive part in the victory of the invading army over the city under siege. Cities that had proudly withstood sieges for up to seven years fell swiftly with

3843-524: The weight loading of telescope tube from the use of heavier eyepieces or the addition of cameras etc. Since the late 1990s many innovations in mount design and electronics by amateur telescope makers and commercial manufacturers have allowed users to overcome some of the limitations of the Dobsonian style altazimuth mount. The original intent of the Dobsonian design was to provide an affordable, simple, and rugged large-aperture instrument at low cost. These same attributes facilitate their mass production. One of

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3906-614: The years (see § Derivative designs ), but most commercial or amateur-built "Dobsonian" telescopes follow many or most of the design concepts and features listed above. The Dobsonian design has the following characteristics: From its inception, telescope makers have been modifying the Dobsonian design to fit their needs. The original design fit the needs and available supplies of one person—John Dobson. Other people devised variants that fit their own needs, abilities, and access to parts. This has led to significant diversity in "Dobsonian" design. "Classic" design tube assemblies would require

3969-483: Was displayed at the 2018 Northeast Astronomy Forum. In 2019, a huge 50 inch (1270 mm) aperture folded Newtonian from Canadian based Optiques Fullum was installed in New Jersey. The Dobsonian design is considered revolutionary due to the sheer size of telescopes it made available to amateur astronomers. The inherent simplicity and large aperture of the design began to attract interest through the 1970s since it offered

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