274:: Dobson replaced the traditional aluminum or fiberglass telescope tube with the thick compressed paper tubes used in construction to pour concrete columns. "Sonotubes", the leading brand employed by Dobson, are less expensive than commercially available telescope tubes and are available in a wide variety of sizes. For protection against moisture, the tubes were usually painted or coated with plastic. Sonotubes are claimed to be more rugged than aluminum or fiberglass tubes which can dent or shatter from impacts during transport. They 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.
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telescope and keep the eyepiece in a relatively easy to access position on the side of the telescope. The altazimuth mount design used in
Dobsonian designs also adds to simplicity and portability; there is no added mass or need to transport counter weights, drive components, or tripods/pedestals. Setting up for hard tube dobs simply involves placing the mount on the ground, and setting the tube on top of it. The weight of the Dobsonian style altazimuth mount is distributed over large simple bearing surfaces so the telescope can move smoothly under finger pressure with minimal backlash.
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center-bolt (Fig.1, dark green) keeps the rocker box centered and allows it to pivot above the ground board. On opposing sides of the rocker box, semicircular depressions are cut out from the top edge of each wall (the rocker box is open on the top and on the back). Each depression has a widely spaced pair of supports installed in the cut (Fig.1, top yellow). The telescope optical tube assembly (OTA, Fig.1, light blue) has two large round
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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 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
210:: Instead of a standard mount using axial bearings, Dobson opted for a very stable design that was simple to build, and which had fewer mechanical limitations when used with large and heavy telescopes. He modified the classical fork mount into a free-standing three-piece construction, which holds the telescope steady on seven discrete support points and allows for easy and safe repositioning of a large and heavy telescope.
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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 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.
355:: The Dobsonian design of maximized objective diameter combined with portability makes the design ideal for observing dim star clusters, nebulae, and galaxies (deep sky objects), an activity that requires large objectives and travel to dark sky locations. Since these objects are relatively large, they are observed at low magnifications that do not require a clock-driven mount.
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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 the 1970s, today 16 inch systems are common,
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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 the first companies to offer Dobsonian telescopes commercially was the now defunct company Coulter Optical
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Many designs have increased portability by shrinking 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
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mirror blanks with the standard 1:6 thickness ratios (1 cm thick for every 6 cm in diameter) so they don't flex and sag out of shape under their own weight, Dobson used mirrors made out of glass from surplus ship porthole covers usually with 1:16 thickness ratios. Since the telescope design
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The use of Teflon over textured material combines with gravity-produced wedging forces to create a unique smooth action, transitioning from rock steady to smooth motion and back. Thus a clamp mechanism is not needed to prevent unintentional motion of the telescope, unlike most other telescope mounts.
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Solid tube commercial
Dobsonians typically have a maximum aperture of 12 inches (305 mm) due to 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
369:. Most Dobsonian telescopes have enough friction in the bearings to resist a moderate amount of imbalance; however, this friction can also make it difficult to position the telescope accurately. To correct such imbalance, counterweights are sometimes hooked or bolted onto the back of the mirror box.
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tube and a collapsible design. Collapsible "truss tube" Dobsonians appeared in 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
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The altazimuth mount does have its own limitations. Un-driven altazimuth mounted telescopes need to be "nudged" every few minutes along both axes to compensate for the rotation of the Earth to keep an object in view (as opposed to one axis for un-driven equatorial mounts), an exercise that becomes
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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 the weight loading of telescope tube from the use of heavier eyepieces or the addition of cameras etc.
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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
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of the telescope OTA. The trunnions (commonly known as altitude bearings) rest atop the aforementioned four supports in the top cutouts of the rocker box. To raise the telescope (altitude), just lift the tube and the trunnions will slide over the four supports. To move the telescope left or right
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The classical
Dobsonian mount (refer to Fig.1) consists of a flat horizontal "ground board" platform (Fig.1, black) on top of which are attached three of the seven supports (Fig.1, bottom yellow). Upon these three supports rests a box construction called a "rocker box" (Fig.1, dark blue). A loose
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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 the advantage of inexpensive large instruments that
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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
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with clock drive was left out of the design. Equatorial mounts tend to be massive (less portable), expensive, complicated, and have the characteristics of putting the eyepiece of
Newtonian telescopes in very hard to access positions. Altazimuth mounts cut the size, weight and cost of the total
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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 the amateur astronomer of the 1970s and 1980s typically did not explore much beyond the Messier and brighter
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Classical Dobsonian mount parts are typically made from plywood and other cheap materials which are glued, screwed, or even nailed together. In contrast to other telescope mount types, no precision-machined mechanical parts are required. For smooth sliding motions, small
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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.
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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
239:(PTFE) blocks are used for the seven supports. Their surface sizes can be precisely calculated for the particular OTA weight. To improve the smoothness and steady position-holding, the bottom of the rocker box is typically covered with micro-textured
476:-based digital setting circles has allowed any altazimuth mounted telescope to be fitted or retrofitted with the ability to accurately display the coordinates of the telescope direction. These systems not only give the user a digital read-out for
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The steadiness of the classical Dobsonian is unparalleled, as the telescope is actually not rotating on two axes as other mounts but instead statically standing on seven solid blocks (until pushed to a new position). Only the
141:, he preferred to call the design a "sidewalk telescope". Dobson combined all these innovations in a design focused towards one goal: building a very large, inexpensive, easy to use, portable telescope, one that could bring
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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.
554:(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,
61:. Dobson's 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 observing faint,
282:: Dobson often used a plywood box for the tube base and mirror housing, into which the Sonotube was inserted. This gave a rigid flat surface to attach the mirror supports, and made it easy to attach the trunnions.
243:. The altitude trunnions often have a large diameter, and can also be covered with textured material. For larger telescopes, semicircular wood pieces or arc-shaped rails can be used instead of round trunnions.
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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.
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for ease of tracking, and with careful polar alignment sub-arc second precision CCD imaging is entirely possible. Roeser Observatory, Luxembourg (MPC observatory code 163) have contributed hundreds of
403:"Classic" design tube assemblies would require a large van for transport. Designers started coming up with disassembleable or collapsible variants that could be brought to the site with a small
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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.
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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
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has an alt-azimuth mount, the mirror only has to be supported in a simple cell with a backing of indoor/outdoor carpet to evenly support the weight of the much thinner mirror.
1225:– The book treats the "why" as importantly as the "how", sharing Dobson's philosophy on the importance of popular access to astronomy for proper appreciation of the universe.
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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
361:: Designs that have the telescope tube fixed in relationship to its altitude bearings can be put out of balance by the addition or subtraction of equipment such as cameras,
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Since the 1990s, manufactured Dobsonians using the truss tube design have become increasingly popular. The first commercial truss Dobsonian was released into the market by
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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,
324:, mainly because a large movement of the azimuth axis is needed to move the telescope pointing by even a small amount. Altazimuth mounts are also not well suited for
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This ratio ensures that per inch of objective diameter, a Dobsonian will weigh less and require less space than other types and is thus the most portable design.
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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
223:(azimuth), push or pull the top rim of the OTA (some have a dedicated handle) so that the pivoting rocker box slides over the ground board's three supports.
434:, 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.
341:: The Dobsonian design's structure as measured in volume and weight is relatively minimal for any given objective diameter when compared to other designs.
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A modified Dobsonian from 1983 that features a collapsible open tube assembly with integrated bearing surface and a very compact "rocker box" mount.
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The main attribute of a Dobsonian's mount is that it resembles a "gun carriage" configuration with a "rocker box" consisting of a horizontal
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objects, thanks in part to Dobsonians, modern amateur astronomers routinely observe dim objects listed in obscure catalogues, such as the
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to help in aiming the telescope at the coordinates of known objects. They are known for being difficult to point at objects near the
290:), but most commercial or amateur-built "Dobsonian" telescopes follow many or most of the design concepts and features listed above.
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218:(or arc-shaped rails for larger telescopes) secured on the left and right sides (Fig.1, red). Their common axis intersects the
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to give a current graphical representation of where the telescope is pointing, allowing the user to quickly find an object.
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Two Dobsonians that combine a truss tube, compact "rocker box", large radius altitude bearings, and a collapsible design.
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347:: From a cost perspective, a user typically gets more objective diameter per unit of cost with the Dobsonian design.
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1189:"Digital Setting Circles Revisited – Lumicon Sky Vector I – JMI NGC-MAX – Celestron Advanced Astro Master"
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s). These dim objects require a large objective mirror able to gather a large amount of light. Because "
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more difficult with higher magnifications. The altazimuth mount does not allow the use of conventional
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Dobson identified the characteristic features of the design as lightweight objective mirrors made from
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415:. This innovation allowed the amateur astronomy community access to even larger apertures.
199:. John Dobson's telescopes combined several innovations to meet these criteria, including:
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Dobsonians are intended to be what is commonly called a "light bucket". Operating at low
1603:
And the Skylark Sings with Me: Adventures in Homeschooling and Community-based Education
1532:
1525:
Classic Telescopes: A guide to collecting, restoring, and using telescopes of yesteryear
1650:
Seeing in the Dark: How Amateur Astronomers Are Discovering the Wonders of the Universe
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473:
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A light bucket is a telescope with a large aperture, used mainly for visual observing.
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In the 21st century, truss Dobsonian models are also mass-produced by Meade, Orion,
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strips and other low-cost materials. Since he built these telescopes as aids in his
92:, and therefore the design omits features found in other amateur telescopes such as
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662:(and discovery) to the amateur wishing to contribute minor planet positions to the
170:
1670:
The Dobsonian Telescope: A practical manual for building large aperture telescopes
647:, and others once considered reference works only for professional astronomers.
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systems. Some also produce "ultra-light" models that offer greater portability.
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in 1965 and credited with vastly increasing the size of telescopes available to
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The Largest Portable Dobsonian Telescope in the World (42 inch - 107 cm)
117:"for hundreds of years, wars were fought using cannon on 'Dobsonian' mounts".
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Visual Astronomy Under Dark Skies: A new approach to observing deep space
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the difficulties using a Dobsonian for short-exposure (≲ 1 hr)
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In combination with other improvements in observing equipment, such as
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are obviated. This has opened up the field of high precision asteroid
81:) of relatively short focal length and portability for travel to less
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1548:
A Buyer's and User's Guide to Astronomical Telescopes and Binoculars
1419:"16-inch f/4.5 collapsible-truss Dobsonian from New Moon Telescopes"
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mount invented later can rival the steady smoothness of a Dobsonian.
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The design of Dobsonian telescopes has evolved over the years (see
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666:. It also makes possible searches for new, faint objects such as
1065:. South East Queensland Astronomical Society Inc. Archived from
1060:"Recommendations for Beginning Amateur Astronomers, page 4"
861:
DiBona, Chris; Stone, Mark; Cooper, Danese (21 October 2005).
225:
1090:
Hamberg, Ivar (Fall 1982). "An Extremely Portable 17⁄
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using a home-built 20" dobsonian on an equatorial platform.
1465:"News & Views from the 2018 Northeast Astronomy Forum"
1174:"Arduino bluetooth digital setting circles for telescopes"
484:(dec.), they also interface with digital devices such as
1451:"Sky-Watcher's Stargate Truss-Tube Dobsonian Telescopes"
298:
The Dobsonian design has the following characteristics:
1527:. Springer Science & Business Media. p. 127.
1495:
Bunch of Amateurs: A search for the American character
1480:"Huge 50" Newtonian Telescope Installed in New Jersey"
1242:
Know Thy Dob: Choosing and Using a Dobsonian Telescope
1211:
How and Why to Make a User-Friendly Sidewalk Telescope
27:
Type of Newtonian telescope popularized by John Dobson
1719:
1584:. Springer Science & Business Media. p. 21.
1512:. Springer Science & Business Media. p. 156.
832:; Aguirre, E. (September 1995). "Walden of the Sky".
423:
Many designs have combined the advantages of a light
1600:
1386:"Review of Hubble Optics UL16 Dobsonian Telescope"
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1027:
740:
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603:in the back of a small car and set up in minutes.
460:Overcoming the limitations of the altazimuth mount
438:Modifications to the altazimuth mount (rocker box)
1705:Complete Plans for Building a Dobsonian Telescope
607:and huge 32 inch systems not all that rare.
1571:A Salesman for the Heavens Wants to Rope You In
1235:
1233:
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969:Newton, Jack; Teece, Philip (19 January 1995).
745:Newton, Jack; Teece, Philip (19 January 1995).
508:: The use of equatorial platforms (such as the
1213:. Everything in the Universe. pp. 61–62.
586:introduced its line of large Stargate models.
334:Large objective diameter compared to mass/cost
549:Commercially custom-built 16" truss Dobsonian
8:
1014:Dobson ... promoting causal visual observing
496:using live ephemeris calculating / charting
73:. This type of observation requires a large
975:. Cambridge University Press. p. 288.
751:. Cambridge University Press. p. 287.
599:could be carried to dark sky locations and
594:The Dobsonian's effect on amateur astronomy
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680:Central Bureau for Astronomical Telegrams
541:Commercially mass-produced 10" Dobsonian
96:tracking. Dobsonians are popular in the
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867:. "O'Reilly Media, Inc.". p. 351.
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516:work. Such platforms can incorporate a
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1668:Kriege, David; Berry, Richard (1997).
1371:"Teeter's Telescopes Custom Dobsonian"
1147:. Cambridge University Press. p.
948:. New Society Publishers. p. 93.
824:
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125:glass, and mountings constructed from
1562:35 Who Made a Difference: John Dobson
802:Star Clusters and how to Observe Them
191:" observing often requires travel to
7:
1698:San Francisco Sidewalk Astronomers'
1626:Woodruff, John (2003). "Dobsonian".
1497:. Crown/Archetype. pp. 231–234.
1139:Newton, Jack; Teece, Philip (1995).
34:A Dobsonian telescope on display at
1467:. Sky & Telescope. 23 Apr 2018.
1453:. Sky & Telescope. 30 Oct 2017.
1373:. Sky & Telescope. 18 Jan 2016.
1715:World's Largest Backyard Telescope
1653:. Simon and Schuster. p. 37.
1607:. New Society Publishers. p.
799:Allison, Mark (21 December 2005).
778:. Simon and Schuster. p. 39.
148:
25:
1694:Sidewalk Astronomers Organization
1172:Andico, Orly (January 11, 2012).
942:Albert, David H. (January 1999).
395:diversity in "Dobsonian" design.
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1478:Parkerson, Stuart (2 Jul 2019).
1437:"Obsession 18-inch UC telescope"
1399:Harrington, Phil (27 Jul 2015).
1384:Parkerson, Stuart (2 Jun 2019).
1209:(1991). Sperling, Norman (ed.).
345:Low cost to objective size ratio
339:Low mass to objective size ratio
229:Fig.1 Dobsonian mount schematics
1340:Bonandrini, Dave (8 Aug 2009).
772:Ferris, Timothy (8 July 2003).
181:(what amateur astronomers call
1560:Moser, Don (November 1, 2005)
1143:The Guide to Amateur Astronomy
972:The Guide to Amateur Astronomy
894:. Inc NetLibrary. p. 17.
748:The Guide to Amateur Astronomy
1:
1632:. Firefly Books. p. 60.
1482:. Astronomy Technology Today.
1435:Shibley, John (19 May 2009).
1424:. Astronomy Technology Today.
1417:Allis, Damian G. (May 2013).
1388:. Astronomy Technology Today.
1187:Ward, Peter (December 1993).
996:Knoll, Peter (30 July 1996).
945:And the Skylark Sings with Me
915:Knoll, Peter (30 July 1996).
1629:Firefly Astronomy Dictionary
1401:"Waite Research Renegade 20"
1355:Shinn, Brett (16 Dec 2008).
1304:Nelson, Anthony (May 2005).
888:Fred Watson (October 2007).
612:narrow-pass optical filters
451:Compact "rocker box" mounts
399:Collapsible tube assemblies
149:Dobson's design innovations
1816:
1581:The New Amateur Astronomer
1578:Mobberley, Martin (2004).
891:Why is Uranus Upside Down?
307:equatorial telescope mount
1599:Albert, David H. (1999).
1569:Moser, Don (April 1989) "
1550:(1 ed.). p. 38.
1034:. Firefly Books. p.
353:Good "Deep Sky" telescope
288:§ Derivative designs
145:astronomy to the masses.
1647:Ferris, Timothy (2003).
1573:", Smithsonian Magazine.
1546:Mullaney, James (2009).
1342:"Webster Telescopes D18"
1002:. Springer. p. 25.
921:. Springer. p. 25.
805:. Springer. p. 74.
365:or even unusually heavy
197:German equatorial mounts
98:amateur telescope making
1700:telescope building page
848:1995S&T....90c..84L
692:List of telescope types
674:in local galaxies, and
470:Digital setting circles
1523:English, Neil (2012).
1508:Cooke, Antony (2006).
1357:"Astrosystems Telekit"
1291:"Obsession Telescopes"
1240:English, Neil (2011).
999:Induced Seismic Events
918:Induced Seismic Events
550:
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533:Commercial adaptations
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383:
230:
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53:design popularized by
39:
1439:. Astronomy Magazine.
1406:. Astronomy Magazine.
1111:"The 17.5" Dobsonian"
1030:Moon Observer's Guide
1026:Grego, Peter (2004).
548:
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79:light-gathering power
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1564:– Smithsonianmag.com
1266:"The Odyssey 8 Site"
678:(for reports to the
571:computer controlled
568:Obsession Telescopes
524:measurements to the
498:planetarium software
280:A square "mirrorbox"
261:: Instead of costly
161:built Dobsonians at
1533:2013ctgc.book.....E
1493:Hitt, Jack (2012).
835:Sky & Telescope
664:Minor Planet Center
652:equatorial platform
650:When mounted on an
526:Minor Planet Center
505:Equatorial platform
472:: The invention of
59:amateur astronomers
51:Newtonian telescope
44:Dobsonian telescope
1313:Astronomy Magazine
775:Seeing in the Dark
580:Explore Scientific
551:
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392:
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374:Derivative designs
272:Construction tubes
231:
167:
165:in the early 1980s
139:sidewalk astronomy
48:altazimuth-mounted
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38:in the early 1980s
1672:. Willmann-Bell.
1660:978-0-684-86580-5
1618:978-0-86571-401-4
1591:978-1-85233-663-9
1251:978-1-4419-8785-3
1158:978-0-521-44492-7
1045:978-1-55297-888-7
556:Meade Instruments
514:astrophotographic
220:center of gravity
137:of instructional
108:Origin and design
16:(Redirected from
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560:Orion Telescopes
490:tablet computers
326:astrophotography
303:Altazimuth mount
63:deep-sky objects
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1193:Sky & Space
1191:. Test Report.
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478:right ascension
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294:Characteristics
204:Nontraditional
184:deep sky object
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1784:Solar System
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1324:. Retrieved
1317:the original
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1274:. Retrieved
1270:the original
1260:
1244:. Springer.
1241:
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1201:
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1134:
1123:. Retrieved
1119:the original
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1074:. Retrieved
1067:the original
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842:(3): 84–87.
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1772:Outer space
1760:Spaceflight
830:Levy, David
614:, improved
584:Sky-Watcher
522:astrometric
518:clock drive
494:smartphones
488:computers,
482:declination
206:alt-azimuth
85:locations.
55:John Dobson
1326:2020-06-22
1276:2020-06-22
1125:2009-02-25
1076:2009-02-25
727:References
672:supernovae
660:astrometry
249:Ball-Scope
163:Stellafane
94:equatorial
36:Stellafane
1736:Astronomy
1115:telia.com
645:Minkowski
616:eyepieces
480:(RA) and
409:hatchback
367:eyepieces
216:trunnions
135:avocation
102:Dobsonian
75:objective
18:Dobsonian
1794:Category
686:See also
641:Kohoutek
624:infrared
444:trunnion
189:deep sky
179:galaxies
143:deep-sky
123:porthole
71:galaxies
65:such as
1722:Portals
1529:Bibcode
844:Bibcode
620:visible
492:, and
241:Formica
175:nebulae
159:Amateur
127:plywood
67:nebulae
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676:comets
486:laptop
322:zenith
237:Teflon
177:, and
131:Teflon
46:is an
1748:Stars
1422:(PDF)
1404:(PDF)
1320:(PDF)
1309:(PDF)
1070:(PDF)
1063:(PDF)
698:Notes
668:novae
637:Abell
425:truss
413:sedan
305:: An
263:Pyrex
208:mount
1674:ISBN
1655:ISBN
1634:ISBN
1613:ISBN
1586:ISBN
1246:ISBN
1215:ISBN
1153:ISBN
1040:ISBN
1004:ISBN
977:ISBN
950:ISBN
923:ISBN
896:ISBN
869:ISBN
807:ISBN
780:ISBN
753:ISBN
622:and
573:GoTo
157:Two
69:and
1149:289
682:).
629:NGC
405:SUV
1796::
1611:.
1609:92
1311:.
1230:^
1151:.
1113:.
1038:.
1036:60
1012:.
840:90
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821:^
735:^
670:/
643:,
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633:IC
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42:A
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20:)
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