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The Dobsonian telescope is a design that has become popular amongst Amateur Astronomers because it results in an extremely simple and rugged Large-aperture instrument at low cost. The term "Dobsonian" or "Dob" refers to any Telescope with an Alt-azimuth mount and a Newtonian Telescope tube assembly that feature several innovations made popular by John Dobson . The telescope is a favorite among Amateur Telescope Makers who pioneered many of its original features and has been made increasingly popular by commercial telescope makers. ORIGIN AND DESIGN 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 have been used before. 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." It appears that John Dobson simply combined all these innovations in a design that is focused towards one goal: building a very large, inexpensive, easy to use telescope for the sole purpose of visual observing of astronomical objects. The original “Dobsonian” design built Dobsonians at Stellafane ]] The “classic”, “hard tube” or first generation Dobsonian is the type that was first popularized by John Dobson and the San Francisco Sidewalk Astronomers in the late 1960’s as part of their mission to bring astronomy to the masses. The basic idea driving the original design is to make large Aperture telescopes affordable, easy to make, and portable. It is a combined concept that allows the builder with minimal skill to make an extremely large telescope out of common items found in any hardware store or scrap yard. The design is optimized for Deep Sky observing, i.e. visually observing Star Cluster s, Nebula e, and Galaxies that require a large Objective mirror with a lot of "light gathering" capability. Since deep sky observing requires the observer to travel out to dark locations away from city lights the design had to be more compact, portable, and rugged than the standard large Newtonian telescope. The typical built along John Dobson's original design include:
The features of the Dobsonian's mount are:
The use of Teflon on all bearing surfaces allows for smooth action with just enough "grab" so the telescope does not drift when stationary. The design of Dobsonians has evolved over the years (see Derivative Designs ) but most commercial and Amateur -built "Dobsonian" telescopes incorporate many or most of the features listed above. APPLICATIONS Advantages
Limitations
DERIVATIVE DESIGNS From its inception telescope makers have been modifying the Dobsonian design to fit their needs. The original design fitted one person (John Dobson's) needs and available supplies. Other people started coming up with variants that fitted their own needs/know-how and locally or commercial available parts. This has led to significant trends in “Dobsonian” design. Collapsible tube assemblies “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 SUV , Hatchback , or even a Sedan . This innovation allowed the amateur astronomy community access to even larger apertures. The truss tube Many designs have combined the advantages of a light truss tube and a collapsible design. Collapsible "truss tube" designs allows 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 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 called a '' Serrurier Truss '', although 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. Modifications to the altazimuth mount (rocker box) The Dobsonian use of the altazimuth mount has erroneously led some to call any altazimuth mount a “Dobsonian mount”. The main attribute of a Dobsonian’s mount is that originally resembled a “gun carriage” configuration. Many derivative mount designs have kept this basic form while heavily modifying the materials and configuration. Compact “rocker box” mounts Many designs have increased portability by shrinking the altazimuth (rocker box) mount down to a small rotating platform. The altitude 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. This lowers the overall profile and better distributes the weight of a heavier telescope. Overcoming the limitations of manual operation A number of efforts have been made both by amateurs and commercial organisations to motorise the basic mount and to add 'goto' capability. However for any sort of astrophotography an equatorial mount is needed. Placing the basic Dobson mount onto a 'Poncet Platform' is one way to achieve this at low cost. COMMERCIAL ADAPTATIONS The intent of the Dobsonian design when it was first originated in the 1960s was to provide affordable extremely simple and rugged large-aperture instrument at low cost. These same attributes facilitate their mass production. One of the first companies to offer the Dobsonian commercially was the now defunct company Coulter Optical . They helped popularized the design with models in the 8 inch through 17.5 inch size range in the 1980s and 1990s that were built using Dobson's original concept. Meade and other manufacturers began to introduce their variation on this design by the late 1990s. Although manufactured scopes include such niceties as metal tubes and more refined hardware, these telescopes are still extremely affordable. In late 2006, 6 inch models cost around $300 US and 10 to 12 inch (250–300 mm) scopes can still be had for under $1,000 US. Since the late 1990s, Dobsonians using the truss tube design became increasingly popular with manufacturers. In 1989, the first commercial truss tube Dobsonian was released by Obsession Telescope into the market. Manufacturers of these truss tube Dobsonians have differentiated themselves by going in the opposite direction from John Dobson's original intent by catering toward the upper end of the market and more experienced observers. These telescopes came with full thickness mirrors and use higher end materials such as Baltic birch or similar plywood and cast aluminum fittings. These designs also employed the “rocker box” mount listed above. As of the early 2000s these truss tube Dobsonians are the largest commercially available telescopes, with three manufacturers offering models in the 24 inch (600 mm) to 30 inch (750 mm) range (hard tube and other reflector designs typically stop at 18 inch (450 mm) due to this size of the tube). As these scopes require teams of several people to operate and require the observer to perch on top of a ladder, the markets for these scopes is limited. 15 to 18 inch (375 mm to 450 mm) scopes appear to be the most popular size, as they maximize the amount of aperture yet can still be easily be set up by one person. In late 2006 prices can range from $2000 US for a 12 in instrument to $3500 US plus for 18 in or larger instruments. Popular manufactures include Starmaster, Obsession, Starsplitter, Discovery and others. THE DOBSONIAN'S EFFECT ON AMATEUR ASTRONOMY Although initially shunned by the amateur astronomy community, 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 could be carried to dark sky locations and even Star Parties in the back of a small car and set up in moments. 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 30 years ago, today 16-inch systems are common, and huge 32-inch systems not all that rare. In combination with other improvements such as narrow-pass filters and improved eyepieces, the large apertures of the Dobsonian have dramatically increased the depth of which an amateur astronomer can penetrate into the universe. Whereas the amateur astronomer of the 1970s and 1980s typically did not explore much beyond the Messier and brighter NGC objects, the amateur astronomer of today can routinely observe objects in the IC , Abell , Perek Kohoutek , Minkowski and other obscure catalogues once considered the domain of professional astronomers, thanks in part to Dobsonians. BOOKS ON DOBSONIANS AND OTHER PUBLISHED INFORMATION
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