A Dobsonian telescope is an alt-azimuth mounted newtonian telescope design popularized by the amateur astronomer John Dobson starting in the 1960s. 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 intended for visually observing "deep sky" objects, a requirement where the observer needs a large objective diameter (i.e. light-gathering power) combined with portability for travel to non-light polluted locations.Dobsonians are intended to be what is generally 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.
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 had 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". Since he built these telescopes as aids in his avocation of instructional sidewalk astronomy, he prefers to call the design a "sidewalk telescope". 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, portable telescope for the sole purpose of visual observing of astronomical objects as a way to bring astronomy to the masses.
Dobson's 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 deep sky objects such as star clusters, nebulae, and galaxies. 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. John Dobson's telescopes combined several innovations to meet these criteria, including:
* Thin mirrors: Instead of costly Pyrex mirror blanks with the standard 1:6 thickness ratios(1 cm thick for every 6 cm in diameter) so they won't flex and sag out of shape under their own weight, Dobson used mirrors made out of surplus glass ship porthole covers usually with 1:16 thickness ratios. Since the telescope design 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.
* Construction tubes: 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 comparatively large sizes. Dobsonian scopes are intended to be transported out to dark sky locations, and Sonotubes are claimed to be more rugged than aluminum or fiberglass tubes which can dent or shatter from impacts. Sonotubes have the added advantage of being thermally stable and non-conductive which minimize unwanted convection currents in the light path caused by handling of the tube assembly.
* A square "mirrorbox": Dobson used a plywood box for the tube base and mirror housing. This gave a rigid flat surface to attach the "carpet" support for the porthole mirrors he was using, and made it easy to attach the altitude bearings.
* A simple alt-azimuth mount: Dobson opted for a simple to build and use altazimuth mount. He used a “gun carriage type” design consisting of a flat platform ("ground board") on which sits a rotating box with semicircular depressions cut in the top for the altitude bearings ("rocker" or "rocker box"). All parts were made from plywood and other common materials. For the azimuth (side-to-side) motion Dobson used a combination of Teflon blocks turning on a flat Formica covered surface. For the altitude (up-and-down) motion Dobson used a large diameter axle "closet flange" turning on Teflon bearing blocks attached to the altitude cutouts. Dobson's use of Teflon on all bearing surfaces and the large diameter of the bearings created a smooth action with a moderate amount of friction, so that a clamp mechanism is not necessary to prevent unintentional motion of the telescope.
The design of Dobsonian telescopes has evolved over 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.
Advantages
* Compact Size: Basically, a Dobsonian's structure as measured in volume and weight is relatively minimal for any given aperture when compared to other designs. From a cost perspective, a user typically gets more aperture per unit of cost with a Dobsonian.
* Simplicity and Portability: Transporting, setting up, and operating a Dobsonian telescope is simpler than for many other designs. The mount system is compact. Setting up for hard tube dobs simply involves placing the mount on the ground, and setting the tube on top of it. The altazimuth mount is easy to point. The weight of the Dobsonian is distributed over large simple bearing surfaces so the telescope can move smoothly under finger pressure with minimal backlash.
* Good Deep Sky Performance: The Dobsonian design allows the user to have a much larger aperture telescope for the same amount of mass, portability, and money. This combined with a fast focal ratio make these telescopes ideal for deep sky objects.
Limitations
* Non equatorial mount: Because the Dobsonian design is optimized to be a portable, large aperture, inexpensive, deep sky instrument geared towards visual observing, an expensive (and massive) equatorial telescope mount with clock drive was intentionally left out of the design. Instead, with an Altazimuth mount, the user must nudge the scope every few minutes along both axes to compensate for the rotation of the Earth to keep an object in view. The Altazimuth mount also did not allow for the use of conventional setting circles to help in aiming the telescope at the coordinates of known objects.
* Zenithal Hole: Altazimuth mounts are known for being difficult to point at objects near the zenith, mainly because a large movement of the azimuth axis is needed to move the telescope pointing by even a small amount. Improvements in the azimuth bearing material and design can minimize the problem, but not eliminate it. Equatorially-mounted telescopes have a similar issue when observing objects near the celestial poles.
* Balance Issues: Since the telescope tube is usually fixed in relationship to its altitude bearings, the addition or subtraction of equipment such as cameras, finderscopes or even unusually heavy eyepieces can render the telescope mount unbalanced unless a counterweight or similar modification is added. However, most Dobsonian telescopes have enough friction in the bearings to resist a moderate amount of imbalance.
Derivative designs
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.
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" 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 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 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.
Another truss form is the "string truss", made of three loops of inelastic flexible cable and two or three spring loaded struts. These allow compact storage and transport with fast and simple assembly.
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 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.
Overcoming the limitations the altazimuth mount
Since the late 1990s many innovations in mount design and electronics by amateurs telescope makers and commercial manufacturers have allowed users to overcome some of the limitations of the altazimuth mount design.
* Digital setting circles: The invention of microprocessor based digital setting circles has allowed any altazimuth mounted telescope to be fitted or retrofitted with the ability to accurately display what coordinates the telescope is pointed at.
* Poncet Platform: The use of the Poncet Platform (a low profile equatorial mount consisting of a simple polar pivot and flat equatorial plane) that is fitted under the altazimuth mount has given users limited equatorial tracking for visual and astrophotographic work.
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 (now part of Murnaghan Instruments). They helped popularize 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 Instruments 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 inches (250 to 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 Telescopes into the market.[citation needed] 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 inches (610 mm) to 30 inches (760 mm) range (hard tube and other reflector designs typically stop at 18 inches (460 mm) due to the 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. Telescopes with a 15 to 18 inches (380 to 460 mm) aperture 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 early 2009 prices can range from $800 for a 12" instrument to $3500 plus for 18" or larger instruments, depending on the quality and features. Popular manufactures include Starmaster, Obsession, Starsplitter, Discovery, Meade and others.
The Dobsonian's effect on amateur astronomy
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 to 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.
