Dobsonian telescope
A Dobsonian telescope is an altazimuth-mounted Newtonian telescope design popularized by John Dobson in 1965 and credited with vastly increasing the size of telescopes available to amateur astronomers. 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, deep-sky objects such as nebulae and galaxies. This type of observation requires a large objective diameter of relatively short focal length and portability for travel to less light-polluted locations.
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 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.
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 1965 pointed out that "for hundreds of years, wars were fought using cannon on "'Dobsonian' mounts". Dobson 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. 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 a very large, inexpensive, easy to use, portable telescope, one that could bring deep-sky astronomy to the masses.Dobson's design innovations
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. 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 of times past, which typically utilized massive German equatorial mounts. John Dobson's telescopes combined several innovations to meet these criteria, including:- Nontraditional alt-azimuth mount: Instead of a standard rotating mount using axial bearings, Dobson opted for a very stable free-standing mount 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.
- * The classical Dobsonian mount consists of a flat horizontal "ground board" platform on top of which are attached three of the seven supports. Upon these three supports rests a box construction called a "rocker box". A loose center-bolt 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. Each depression has a widely spaced pair of supports installed in the cut. The telescope optical tube assembly has two large round trunnions, secured on the left and right sides. Their common axis intersects the center of gravity of the telescope OTA. The trunnions rest atop the aforementioned four supports in the top cutouts of the rocker box. To raise the telescope, just lift the tube and the trunnions will slide over the four supports. To move the telescope left or right, push or pull the top rim of the OTA so that the pivoting rocker box slides over the ground board's three supports.
- * 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 Teflon 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 Formica. 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.
- * Dobson's use of Teflon over textured material and gravity-produced wedging forces 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. 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. Only the Ball-Scope mount invented later can rival the steady smoothness of a Dobsonian.
- Thin mirrors: Instead of costly Pyrex mirror blanks with the standard 1:6 thickness ratios 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 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 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.
- A square "mirrorbox": 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.
Characteristics
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, 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 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.
- 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.
- *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.
- 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, 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.
Derivative designs
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, 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.Modifications to the altazimuth mount (rocker box)
The main attribute of 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.Compact “rocker box” mounts
Many designs have increased portability by shrinking the altazimuth 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 the weight loading of telescope tube from the use of heavier eyepieces or the addition of cameras etc.Overcoming the limitations of the altazimuth mount
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.- 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 the coordinates of the telescope direction. These systems not only give the user a digital read-out for right ascension declination, they also interface with digital devices such as laptop computers, tablet computers, and smartphones using live ephemeris calculating / charting planetarium software to give a current graphical representation of where the telescope is pointing, allowing the user to quickly find an object.
- Equatorial platform: The use of equatorial platforms fitted under the altazimuth mount has given users limited equatorial tracking for visual and astrophotographic work. Such platforms can incorporate a clock drive for ease of tracking, and with careful polar alignment sub-arc second precision CCD imaging is entirely possible. Roeser Observatory, Luxembourg have contributed hundreds of astrometric measurements to the Minor Planet Center using a home-built 20" dobsonian on an equatorial platform.
Commercial adaptations
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 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 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 due to the size of the tube. Truss Dobsonians of 12 to 18 inches 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 aperture and greater. Truss Dobsonians are the largest telescopes commercially available today. A massive 36 inch aperture Hybrid model from New Moon Telescopes was displayed at the 2018 Northeast Astronomy Forum. In 2019, a huge 50 inch aperture folded Newtonian from Canadian based Optiques Fullum was installed in New Jersey.
The Dobsonian's effect on amateur astronomy
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 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.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 has also opened up the field of high precision asteroid astrometry to the amateur wishing to contribute minor planet positions to the Minor Planet Center.