Dobsonian telescope
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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.
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[edit] 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.
[edit] The original “Dobsonian” design
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 clusters, nebulae, 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:
- Thin mirrors. Mirrors normally have to be extremely thick so they won’t flex and sag out of shape under their own weight. Standard Pyrex mirror blanks are also very expensive. The original Dobsonian designs got around this by using thin mirrors made out of ship porthole glass from salvage yards. Since the telescope has an alt-azimuth mount the mirror only had to be supported in a simple cell with a backing of indoor/outdoor carpet that evenly supported the weight of the mirror.
- A hard telescope "tube". The tubes were originally made from Sonotube (used in construction to pour concrete columns). These tubes were inexpensive compared with commercially available telescope tubes and available in extremely large sizes. And since the Dobsonian is intended to be transported out to dark sky locations, Sonotubes had the advantage of being extremely durable (as opposed to a commercial aluminum or fiberglass tubes which tend to dent or shatter with the slightest impact). The Sonotube had the added advantages of being thermally stable and non-conductive so it also prevented handling of the tube assembly from introducing unwanted convection currents into the optical light path.
- A square "mirrorbox" or a plywood box tube base/mirror housing, sometimes hinged in the back to allow easy mirror removal. This gave a rigid flat surface to attach the "carpet" support, and made it easy to attach the altitude bearings (sometimes just an easy to procure PVC toilet closet flange).
The features of the Dobsonian's mount are:
- An alt-azimuth “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.
- The azimuth (side-to-side) motion is provided by a combination of Teflon blocks turning on a flat Formica covered surface.
- The altitude (up-and-down) motion is provided by a large diameter axle (like the above mentioned "closet flange") turning on Teflon bearing blocks attached to the altitude cutouts.
The use of Teflon on all bearing surfaces and the large diameter of the bearings create 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 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.
[edit] Applications
[edit] 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. This ratio also ensures that per inch of aperture, a Dob will weigh less and require less space than other types and is thus the most portable design.
- Ease of Use: As an altazimuth mounted telescope that appears very much like an oversize cannon, it is very intuitive to point a Dobsonian. All well constructed Dobsonians have bearing assemblies that move smoothly under finger pressure with minimal backlash. Setting up for hard tube dobs simply involves placing the mount on the ground, and setting the tube on top.
- Excellent Deep Sky Performance: As the original “light bucket” the large aperture and fast focal ratio make these telescopes ideal for deep sky objects. On nights with good seeing, the larger Dobsonians (18 in +) can render most Messier objects in extraordinary detail across the entire field of view of a high-power eyepiece, and can reach fainter objects in excess of 15th magnitude.
- Good to Excellent Planetary Performance: Although originally intended for deep sky objects, a Dob with high-quality well-collimated optics can perform admirably on planetary objects as well. As with all Newtonian telescopes with a central obstruction, the image contrast will be inferior to a refractor. One way for larger Dob owners to get around this is to stop down their telescope with an off-axis-aperture mask to create an unobstructed telescope (in essence, a Herschelian telescope) with theoretical performance comparable to a similar size refractor.
[edit] Limitations
- Non equatorial drive: 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. So 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. However, since the late 1990s the use of Poncet Platforms as well as computerized stepping motors that can move the telescope along both axes have begun to negate this disadvantage. For visual astronomy these systems work quite well. Newer systems with high-grade stepping motors and a field de-rotator can provide accurate enough tracking for CCD work; however, most serious astrophotographers would likely prefer the inherent stability of an equatorial or fork mount. This limitation also once meant that setting circles could not be used with a Dobsonian, however newer digital setting circles specifically designed for Dobsonian scopes have completely eliminated this constraint.
- 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.
[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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 amateur astrophotography an equatorial mount is needed. Placing the basic Dobson mount onto a 'Poncet Platform' is one way to achieve this at low cost.
[edit] 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[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 inch (600 mm) to 30 inch (750 mm) range (hard tube and other reflector designs typically stop at 18 inch (450 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 inch (375 mm to 450 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 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, Meade and others.
[edit] 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 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.
[edit] Books on Dobsonians and other published information
- John Dobson, How and Why to Make a User-Friendly Sidewalk Telescope edited by Norman Sperling ISBN 0-913399-64-7 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.
- David Kriege, Richard Berry, The Dobsonian Telescope: A Practical Manual for Building Large Aperture Telescopes, 1997, Willmann-Bell, ISBN 0-943396-55-7
