Dobsonian telescope
Encyclopedia
Altazimuth mount
An altazimuth or alt-azimuth mount is a simple two-axis mount for supporting and rotating an instrument about two mutually perpendicular axes; one vertical and the other horizontal. Rotation about the vertical axis varies the azimuth of the pointing direction of the instrument...
mounted newtonian telescope
Newtonian telescope
The Newtonian telescope is a type of reflecting telescope invented by the British scientist Sir Isaac Newton , using a concave primary mirror and a flat diagonal secondary mirror. Newton’s first reflecting telescope was completed in 1668 and is the earliest known functional reflecting telescope...
design popularized by the amateur astronomer
Amateur astronomy
Amateur astronomy, also called backyard astronomy and stargazing, is a hobby whose participants enjoy watching the night sky , and the plethora of objects found in it, mainly with portable telescopes and binoculars...
John Dobson
John Dobson (astronomer)
John Lowry Dobson is a popularizer of amateur astronomy. He is most notable for being the promoter of a design for large, portable, low-cost Newtonian reflecting telescopes that bears his name, the Dobsonian telescope. The design is considered revolutionary since it allowed amateur astronomers to...
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 optimized for visually observing faint deep sky objects such as nebula
Nebula
A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases...
e. This type observation requires a large objective
Objective (optics)
In an optical instrument, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image. Objectives can be single lenses or mirrors, or combinations of several optical elements. They are used in microscopes, telescopes,...
diameter (i.e. light-gathering power) of relatively short focal length and portability for travel to relatively less light polluted
Light pollution
Light pollution, also known as photopollution or luminous pollution, is excessive or obtrusive artificial light.The International Dark-Sky Association defines light pollution as:...
locations.
Dobsonians are intended to be what is commonly called a "light bucket" operating at low magnification
Magnification
Magnification is the process of enlarging something only in appearance, not in physical size. This enlargement is quantified by a calculated number also called "magnification"...
, and therefore the design omits features found in other amateur telescopes such as equatorial
Equatorial mount
An equatorial mount is a mount for instruments that follows the rotation of the sky by having one rotational axis parallel to the Earth's axis of rotation. This type of mount is used for astronomical telescopes and cameras...
tracking. Dobsonians are popular in the amateur telescope making
Amateur telescope making
Amateur telescope making is the activity of building telescopes as a hobby, as opposed to being a paid professional. Amateur telescope makers build their instruments for personal enjoyment of a technical challenge, as a way to obtain an inexpensive or personally customized telescope, or as a...
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 whole range of large-aperture Newtonian reflectors that use some of the basic dobsonian design characteristics, regardless of the materials from which it was 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 the 1950s, 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 glassPorthole
A porthole is a generally circular, window used on the hull of ships to admit light and air. Porthole is actually an abbreviated term for "port hole window"...
, and mountings constructed from plywood
Plywood
Plywood is a type of manufactured timber made from thin sheets of wood veneer. It is one of the most widely used wood products. It is flexible, inexpensive, workable, re-usable, and can usually be locally manufactured...
, Teflon strips and other low-cost materials. Since he built these telescopes as aids in his avocation
Avocation
An avocation is an activity that one engages in as a hobby outside one's main occupation. There are many examples of people whose professions were the ways that they made their livings, but for whom their activities outside of their workplaces were their true passions in life...
of instructional sidewalk astronomy
Sidewalk astronomy
Sidewalk or Street Corner Astronomy refers to the activity of setting up a telescope in an urban setting on a for profit or non-profit basis as an entertainment or for public education.-Overview:...
, 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 innovations
Star cluster
Star clusters or star clouds are groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars which are gravitationally bound, while open clusters, more loosely clustered groups of stars, generally contain less than...
s, nebula
Nebula
A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases...
e, 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
Dark-sky movement
The dark-sky movement is a campaign by people who want to reduce light pollution so people can see the stars, to reduce the effects of unnatural lighting on the environment, and to cut down on energy usage....
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 PyrexPyrexPyrex is a brand name for glassware, introduced by Corning Incorporated in 1915.Originally, Pyrex was made from borosilicate glass. In the 1940s the composition was changed for some products to tempered soda-lime glass, which is the most common form of glass used in glass bakeware in the US and has...
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-azimuthAltazimuth mountAn altazimuth or alt-azimuth mount is a simple two-axis mount for supporting and rotating an instrument about two mutually perpendicular axes; one vertical and the other horizontal. Rotation about the vertical axis varies the azimuth of the pointing direction of the instrument...
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 altitudeAltitudeAltitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...
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 FormicaFormica (plastic)Formica is a brand of composite materials manufactured by the Formica Corporation now based in Newcastle, Tyne & Wear, a division of the New Zealand company Fletcher Building. In common use, the term refers to the company's classic product, a heat-resistant, wipe-clean, plastic laminate of paper or...
covered surface. For the altitudeAltitudeAltitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...
(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
- Maximized objective diameter
- 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.
- 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" telescope: The Dobsonian design of maximized objective diameter combined with portability makes the design ideal for observing dim 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.
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 mountEquatorial mountAn equatorial mount is a mount for instruments that follows the rotation of the sky by having one rotational axis parallel to the Earth's axis of rotation. This type of mount is used for astronomical telescopes and cameras...
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 does not allow for the use of conventional setting circlesSetting circlesSetting circles are used on telescopes equipped with an equatorial mount to find astronomical objects in the sky by their equatorial coordinates often used in star charts or ephemeris.-Description:...
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 zenithZenithThe zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e...
, 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, finderscopeFinderscopeA Finderscope is a small auxiliary telescope mounted atop the main astronomical telescope and pointed in the same direction. The finderscope usually has a much smaller magnification than the main telescope can provide and therefore can see more of the sky. This helps in locating the desired...
s or even unusually heavy eyepieceEyepieceAn eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as telescopes and microscopes. It is so named because it is usually the lens that is closest to the eye when someone looks through the device. The objective lens or mirror collects light and brings...
s 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
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 SUVSport utility vehicle
A sport utility vehicle is a generic marketing term for a vehicle similar to a station wagon, but built on a light-truck chassis. It is usually equipped with four-wheel drive for on- or off-road ability, and with some pretension or ability to be used as an off-road vehicle. Not all four-wheel...
, hatchback
Hatchback
A Hatchback is a car body style incorporating a shared passenger and cargo volume, with rearmost accessibility via a rear third or fifth door, typically a top-hinged liftgate—and features such as fold-down rear seats to enable flexibility within the shared passenger/cargo volume. As a two-box...
, 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 trussTruss
In architecture and structural engineering, a truss is a structure comprising one or more triangular units constructed with straight members whose ends are connected at joints referred to as nodes. External forces and reactions to those forces are considered to act only at the nodes and result in...
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
Serrurier truss
A Serrurier truss is used in telescope tube assembly construction. The design was created in 1935 by engineer Mark U. Serrurier when he was working on the Mt. Palomar Hale telescope. The design solves the problem of truss flexing by supporting the primary objective mirror and the secondary mirror...
, 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 main attribute of a Dobsonian's mount is that resembles a "gun carriage" configuration with a "rocker box" consisting of a horizontal trunnionTrunnion
A trunnion is a cylindrical protrusion used as a mounting and/or pivoting point. In a cannon, the trunnions are two projections cast just forward of the centre of mass of the cannon and fixed to a two-wheeled movable gun carriage...
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 (rocker box) 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 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 Dobsonian style altazimuth mount.- Digital setting circles: The invention of microprocessorMicroprocessorA microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
based digital setting circles has allowed any altazimuth mounted telescope to be fitted or retrofitted with the ability to accurately display what are the coordinates of the telescope direction. - Equatorial platformEquatorial platformAn equatorial platform or equatorial table is an equatorial telescope mount in the form of a specially designed platform that allows any device sitting on it to track astronomical objects in the sky on an equatorial axis. They are used to give equatorial tracking to any device sitting on them, from...
: The use of equatorial platforms (such as the Poncet PlatformPoncet PlatformA Poncet Platform or Poncet mount is a type of equatorial platform that uses a simple polar pivot and an inclined plane. The motion of the mount allows any device sitting on that platform to track the apparent motion of the stars in the night sky...
) fitted under the altazimuth mount has given users limited equatorial tracking for visual and astrophotographicAstrophotographyAstrophotography is a specialized type of photography that entails recording images of astronomical objects and large areas of the night sky. The first photographs of an astronomical object were taken in the 1840s, but it was not until the late 19th century that advances in technology allowed for...
work. Such platforms can incorporate a clock driveClock driveIn astronomy a Clock drive is a regulatory mechanism used to move an equatorial mounted telescope along one axis to keep the telescope in exact sync with the apparent motion of the celestial sky...
for ease of tracking.
Commercial adaptations
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 in (254 to 304.8 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
Obsession Telescopes
Obsession Telescopes is an American telescope and optics manufacturer that specializes in Dobsonian telescopes. It was started in 1989 by David Kriege and is based in Lake Mills, Wisconsin.-Products:...
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 inches (609.6 mm) to 30 inches (762 mm) range (hard tube and other reflector designs typically stop at 18 inches (457.2 mm) due to the size of the tube). As of March 2010, Orion Telescopes & Binoculars
Orion Telescopes & Binoculars
For the unrelated British telescope manufacturer, go to Orion OpticsOrion Telescopes & Binoculars is an American-based retailer of foreign manufactured telescopes and other equipment for astronomy that also sells some equipment for other areas of interest, such as birdwatching...
offers the largest commercially available trus tube Dobsonian, a 50" aperture model. 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 in (381 to 457.2 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.
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 partiesStar party
A star party is a gathering of amateur astronomers for the purpose of observing the sky. Local star parties may be one night affairs, but larger events can last up to a week or longer and attract hundreds or even thousands of participants. Many regional star parties are now held annually and are an...
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
New General Catalogue
The New General Catalogue of Nebulae and Clusters of Stars is a well-known catalogue of deep sky objects in astronomy. It contains 7,840 objects, known as the NGC objects...
objects, the amateur astronomer of today can routinely observe objects in the IC
Index Catalogue
The Index Catalogue —also known as the Index Catalogue of Nebulae, the Index Catalogue of Nebulae and Clusters of Stars, IC I, or IC II— is a catalogue of galaxies, nebulae and star clusters that serves as a supplement to the New General Catalogue...
, Abell
Abell catalogue
The Abell catalog of rich clusters of galaxies is an all-sky catalog of 4,073 rich galaxy clusters of nominal redshift z 18.0*Galactic-Latitude: Areas of the sky in the neighbourhood of the Milky Way were excluded from the study because the density of stars in those fields—not to mention...
, Perek Kohoutek, Minkowski
Minkowski
Minkowski is a surname, and may refer to:* Eugène Minkowski , French psychiatrist* Hermann Minkowski Russian-born German mathematician and physicist, known for:** Minkowski addition** Minkowski–Bouligand dimension...
and other obscure catalogues once considered the domain of professional astronomers, thanks in part to Dobsonians.
Books on Dobsonians and other published information
- John Dobson, How and Why to Make a User-Friendly Sidewalk Telescope edited by Norman SperlingNorman SperlingNorman Sperling is an author, editor, publisher, teacher, and telescope designer living in San Mateo, California.Sperling received a BA from Michigan State University after graduating from Montgomery Blair High School in Silver Spring, Maryland. He followed that with an MA in History of Science...
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