Dobsonian telescope

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 [5] points out that "for hundreds of years, wars were fought using cannon on "'Dobsonian' mounts". Dobson himself identified the characteristic features of the design as lightweight objective mirrors made of porthole glass, and mountings constructed from plywood, Teflon strips and other low-cost materials.[4] Since he built these telescopes as aids in his avocation of instructional sidewalk astronomy, he prefers to call the design a "sidewalk telescope".[6] 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[7] telescope for the sole purpose of observing astronomical objects as a way to bring astronomy to the masses.[8]

Dobson's design innovations

Two more Amateur built Dobsonians at Stellafane

Dobson's design allows a builder with minimal skill to make an extremely 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 objects). 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] (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.[9] 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 minimizes 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.

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 (less portable), expensive, complicated, and have the characteristics of putting the eyepiece of Newtonian telescopes in very hard to access positions.[10] Altazimuth mounts cut the size, weight and cost of the total 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.

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 more difficult with higher magnifications.[11] The altazimuth mount does not allow the use of conventional setting circles to help in aiming the telescope at the coordinates of known objects. They 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. Altazimuth mounts are also not well suited for astrophotography.

• Large objective diameter compared to mass/cost
  • Low mass to objective size ratio: The Dobsonian design's structure as measured in volume and weight is relatively minimal for any given objective diameter when compared to other designs.[lower-alpha 1]
  • Low cost to objective size ratio: From a cost perspective, a user typically gets more objective diameter per unit of cost with the Dobsonian design.[12]
• Good "Deep Sky" telescope: 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.
• Balance Issues: 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, finderscopes or even unusually heavy eyepieces. 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.

A modified Dobsonian from 1983 that features a collapsible open tube assembly with integrated bearing surface and a very compact "rocker box" mount.

Two Dobsonians that combine a truss tube, compact "rocker box", large radius altitude bearings, and a collapsible design.

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.

Commercially mass-produced 10" Dobsonian

Commercially custom-built 16" truss Dobsonian

The original intent of the Dobsonian design was to provide an affordable, simple, and rugged large-aperture instrument at low cost.[18] These same attributes facilitate their mass production. One of 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.[19][20] 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.[19]

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 Obsession Telescopes in 1989.[21] Later American manufacturers included StarStructure[22], Webster Telescopes[23], AstroSystems[24], Teeter's Telescopes[25], Hubble Optics[26], Waite Research[27] and New Moon Telescopes[28]. 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.[29]

In 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.[19] In 2017, Sky-Watcher introduced its line of large Stargate models.[30]

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 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) Hybrid model from New Moon Telescopes was displayed at the 2018 Northeast Astronomy Forum.[31] In 2019, a huge 50 inch (1270 mm) folded Newtonian from Canadian based Optiques Fullum was installed in New Jersey.[32]

The Dobsonian design is considered revolutionary due to the sheer size of telescopes it made available to amateur astronomers.[33][34][35] 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 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 the 1970s, today 16-inch systems are common, and huge 32-inch systems not all that rare.[36]

In combination with other improvements such as narrow-pass filters and improved eyepieces, 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 NGC objects; thanks in part to Dobsonians, the amateur astronomer of today can routinely observe dim objects listed in obscure catalogues, such as the IC, Abell, Perek Kohoutek, Minkowski, and others once considered the domain only of professional astronomers. The Dobsonian (mounted on an equatorial platform) has also 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.

• List of telescope types

• Moser, Don (November 1, 2005) 35 Who Made a Difference: John Dobson – Smithsonianmag.com
• Moser, Don (April 1989) "A Salesman for the Heavens Wants to Rope You In", Smithsonian Magazine.
• Mobberley, Martin (2004). The New Amateur Astronomer. Springer Science & Business Media. p. 21. ISBN 978-1-85233-663-9.
• Albert, David H. (1999). And the Skylark Sings with Me: Adventures in Homeschooling and Community-based Education. New Society Publishers. p. 92. ISBN 978-0-86571-401-4.
• Woodruff, John. "Dobsonian". Firefly Astronomy Dictionary. p. 60.
• Ferris, Timothy (2003). Seeing in the Dark: How Amateur Astronomers Are Discovering the Wonders of the Universe. Simon and Schuster. p. 37. ISBN 978-0-684-86580-5.
• Kriege, David; Berry, Richard (1997). The Dobsonian Telescope: A practical manual for building large aperture telescopes. Willmann-Bell. ISBN 0-943396-55-7.

Wikimedia Commons has media related to Dobsonian telescopes.

• Sidewalk Astronomers Organization
• San Francisco Sidewalk Astronomers' telescope building page
• Complete Plans for Building a Dobsonian Telescope
• The Largest Portable Dobsonian Telescope in the World (42 inch - 107 cm)
• World’s Largest Backyard Telescope