Astronomy And The Dobsonian Telescope
10" Dobsonian Image
Dobsonian Truss Tube Design
Size Comparisons To Man
Dobsonian Solid Tube Design
Breakdown of Dobsonian Module Design
10" Dobsonian Image
Dobsonian Telescope Primer
What is a Dobosnian Telescope and how does it differ from other designs? What are the advantages of this type of telescope, and why might I purchase one instead of a Reflector or Refractor ? These are the questions I will answer here, in the hope that I may interest you in the hobby of Astronomy. Astronomy is a wonderful hobby that can be a family oriented activity with many rewards. Not many hobbies have the ability to be educational, inspiring and interactive in a way that offers bonding and memories for family and friends.
Look deeply at the possibilities Astronomy can offer. You can include your telescope with many other activities you are involved in with your family. Camping, fishing, hunting, beach combing and clam digging. Just bring along the telescope for a wonderful nighttime activity to enjoy with everyone on such occasions. If you go dirt biking and camping in the desert, your nights together can be filled with the wonder of viewing the night skies.
OVERVIEW OF DOBSONIAN DESIGN:
A Dobsonian is an Alt-Azimuth mounted telescope design using the Newtonian features of a basic reflector telescope. It was popularized by the amateur astronomer John Dobson in the mid-1960s, and featured simple mechanical design modifications that made it easy to manufacture. Certain main component parts, such as the primary mirror, mounting system and optical tube characteristics were altered and made more easy to manufacture and more cost effective to produce. The object was to get an observatory grade optical reflector telescope into the hands of amateur astronomers at an affordable price.
The Dobsonian telescope's inherent simplicity brought a large following in the early 1970s, as users were attracted to the large aperture sizes available. Since it offered very effective cost advantage and an ability to be fairly easily transported to dark sky locations, like the desert and mountains, amateur astronomers embraced it and they became very popular. This resulted in a proliferation of large telescopes which would have been too expensive to buy minus the new design options of the Dobsonian. In that an 8" Newtonian Reflector would have been considered very large 30 years ago, today brings 16" and 32" dobsonian systems that are relatively affordable. This allows for deep space observation and the inherent joy of becoming an amateur professional astronomer/astrophotographer. The night sky is open to us in all of it's beauty.
The Dobsonian telescope was originally designed to cut cost and make very large aperture telescopes more affordable to the general public. The aperture of any telescope is the optical tube diameter, usually expressed as the actual mirror or objective lens diameter, rather than the actual optic tube inside diameter. In this, a 10" Dobsonian telescope has a 10" primary reflector mirror. In this size, the actual tube diameter can be 11-½" to 12" in diameter. A very large instrument in mid-1960s era of amateur astronomy. The new designs offered a focal lengths/plane characteristic of f/4 - f/5 or longer. This provided a wide viewing angle capturing more of the dark night sky in a given view.
One of the first companies to offer Dobsonian designs commercially was the Coulter Optical Company. (1980's - 1990s) This company developed 10" (254mm) X 50" (1270mm) f/5 and 17.5" (444.50mm) X 72" (1829mm) f/4.5 primary mirror models using Dobson's original design concepts. The Truss Tube version above (first image) has a 10" Aperture (primary mirror) with an f/5 focal plane. A Dobsonian with an open inline optical tube design utilizes what is called a truss tube assembly. Some Dobsonians use the solid optical tube design as the 10" Celestron Solid Tube (second image) shows above. The truss tube is an attempt to keep cost down, and has a few drawbacks that will be explained later in this article.
The Dobson design changed the playing field dramatically. The truss tube assemblies are easily broken down into modules for easy transport to dark sky locations. The mount side-stepped the traditional equatorial mount design and embraced what is called the alt/azimuth design, which is essentially a take-off from a cannon gun-mount. As can be seen above, the mount design allows vertical altitude swivel for up and down alignment. The base rotates commonly on a roller track bearing assembly that provides dozens of support points for smooth rotation/swivel positioning. Two base plates commonly enclose a spec-plate design that has holes in it to accept Teflon coated ball-bearings for smooth rotation to the desired telescope azimuth sweep position. Both azimuth and altitude adjustments are accomplished smoothly by hand.
These telescopes do not provide the same level of computerized mount capability and star-finder positioning/tracking found in equatorial mount designs. Dobsonian designs are manufactured with Go To features however, which allow you to position the telescope on a celestial object. These telescopes do not auto-track an object for long photographic exposures because the mount design is inherently not capable of following a meridian from the equator to polar zenith as the earth rotates.
DOBSONIAN DESIGN INNOVATIONS:
- THIN MIRRORS: Normal Newtonian mirror specifications called for the use of Pyrex glass with a standard ratio of 1:6. That equates to 1" of thickness for every 6" of desired mirror diameter. Therefore, a 6 inch Newtonian Pyrex mirror would be 1 inch thick, a 12 inch would be 2 inches thick, etc. The Dobson design used a much more cost effective product to make the mirrors. The glass used was marine-porthole glass applying a 1:16 thickness ratio. In this ratio, a 16 inch mirror only needed to be 1 inch thick. To keep cost down, modern manufactures use the same ratio, although BK7 optical quality glass is commonly used for the primary mirror and thicker glass ratios are also available.
- MIRROR WEIGHT LESSENED: The lower the weight of a mirror design, the less the mirror sags in it's mount or moves during transport, effecting the collimation of the primary and secondary mirrors. Collimation is shown by the focal plane image below. It is the alignment of the focused beam of primary mirror collected light onto the center of the diagonal secondary mirror that is effected. This misalignment has to be corrected when a Dobsonian is re-assembled after being transported.
- MIRROR BOX ASSEMBLY: The early design was a box located at the lower end of the optical tube housing the primary mirror. Because the Dobson mirror design was thinner, the need to support the mirror strength wise was needed. The early design used common carpeting to line the mirror box. Modern manufacturers use a fully round truss or solid optical tube with a mirror mount bracket at the base of the tube. The mirror bracket is made to be easily adjusted for collimation of the mirrors.
- OPTICAL TUBE MADE MODULAR: The optical tube assembly of the Dobsonian design is made modular for easy transport. A solid tube is simply slid together and set screws tightened at a remote viewing site. A truss assembly is broken into three parts or more as indicated in the images below. The alt/azimuth base assembly is easy to take down and re-assemble also, making this design optimal for transporting the telescope to dark sky locations. It also makes it easy to store the telescope in a closet or garage when necessary. Note that not all Dobsonian solid tube assemblies break down. For the ones that do not, you can still transport them easily by removing the optical tube from the base assembly, which sets up quickly at a new site.
ADVANTAGES OF A DOBSONIAN DESIGN:
- GOOD/EXCELLENT IMAGE PERFORMANCE: As originally intended for deep sky observation and large mirror size, the Dobsonian delivers high quality visual images of planetary objects. When the mirrors are properly collimated and the manufacturer has provided quality optic ability, these telescopes are top of the line. As with any Newtonian telescope, the image contrast can be wanting when compared to a Refractor telescopes capability.
- EXCELLENT DEEP SKY PERFORMANCE: The original concept of using large light gathering mirrors and a fast focal ratio, make these type scopes ideal for deep space observations. On nights with good viewing conditions, a large Dobsonian (18" or larger) can render most objects in extraordinary detail. (See images below) It offers a wide field of view through a high powered eyepiece, and can render astounding images of faint objects when other telescopes cannot.
- EASY TO USE: As an alt-azimuth mounted telescope that looks like a common cannon, the Newtonian Dobson is easy to point and manipulate by hand. The mount design can be moved by hand in fine increments with relative ease. The bearing assembly is smooth under minimal pressure with little or no backlash. Setting up a solid hard tube Dobsonsian is especially easy because all you have to do is set the mount on the ground and place the scope tube in the cradle.
- RELATIVELY COMPACT SIZE: A Dobsonian is structured to be taken apart easily and re-assembled in a dark sky area like the desert. Because of this the size of each element is easily packed away for transportation. Even though you have a very large telescope aperture size, this design feature brings relative balance. Other telescope designs using an equatorial mount, are not able to be taken apart easily and transported.
- UNIT OF COST: As Dobson intended, the cost of producing a telescope using this design would bring serious cost advantages. You can have a very large professional telescope at a very reasonable cost. The ratio of cost versus aperture size is amplified by a weight and storage space advantage coming through it's design, that make it the most portable telescope design available in amateur astronomy.
LIMITATIONS OF THE DOBSONIAN DESIGN:
- NON-EQUATORIAL DESIGN: The Dobsonian design is optimized to be a portable large aperture, inexpensive deep sky telescope. An expensive clock driven equatorial mount was left out of the design. In this, a user must nudge the scope in timed sequence to the rotation of the earth and rise the azimuth/elevation up or down as needed. This is required to keep your planetary object in view and centeed in the eyepiece. However, modern Dobsonian telescopes do offer 'Poncet Platforms' using computerized stepping motors to move the telescope along its axis with fair accuracy. These designs do make it possible to use a Dobsonian for Astrophotography, but inherent limitations (below) in the mount design usually push the more serious photographer to a more stable equatorial mount system.
- ZENITHAL HOLE: Alt-Azimuth mounts are known for being hard to point at objects that are at or approaching zenith. Zenith is defined as straight overhead, and at the extreme polar direction. This is because a large movement of the azimuth axis is needed to move the scope even a small amount. Improvements in the azimuth bearing material and design can and has minimized this problem, but has not solved it. Equatorial telescope mounts also have problems aligning on objects at the extreme celestial pole, but to a lesser extent than a Dobsonian.
- BALANCE ISSUES: Since a Dobsonian telescopes tube is usually fixed in relationship to the azimuth bearing system, adding or subtracting equipment such as a camera effects to upset the balance of the scope. A counterweight system can be used to compensate for this problem, and common sense will overcome it. Counterweight systems are commercially available to use for this purpose.
Overcoming the limitation of manual operation is not an exact science with the Dobsonian. A number of efforts have been made by amateur and commercial organizations to computerize and motorize the mount with a basic Go-To ability. However, for any serious astrophotography an equatorial mount is needed. Placing the Dobsonian on a Poncet mount is one way to achieve a reasonable solution at a low cost, but even that solution is not perfect. In short, the Dobsonian is a wonderful telescope that will show you astounding views of all deep space objects. This along with the cost advantages make this telescope option a fine choice for even a beginner in astronomy. Photography is possible at a less perfect level, but the aperture size and viewing pleasures weigh pretty evenly overall. With all these advantages you should consider this telescope type a great buy with wonderful viewing capabilities.
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Typical 10 Inch Primary Pyrex Mirror
Focal Plane of Typical Dobsonian Reflector
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10" Dobsonian Image
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