People getting involved in astronomy as a hobby want to get the most for their money and the Dobsonian telescope design delivers on that goal in so many ways. I am going to overview the Dobsonian telescope design, often called a Dob. Then I will offer a variety of recommendations which I consider to be the best Dobsonians.
It is key to understand that, in visual astronomy, the aperture is king! Aperture is the measure of the light gathering ability of the telescope. The more light you gather the dimmer the objects you can see and the more detail you can see. If we compare telescopes to cars, the aperture in telescopes is like horsepower in your car.
There are three main designs used for the optical tube of the modern amateur/hobby telescope.
- The refractor that uses a lens to gather light
- The Newtonian reflector that uses a mirror to gather light
- The catadioptric that uses a combination of lens and mirror
Of the three, the Newtonian reflector is the most cost-effective as measured in aperture per dollar, especially once we get larger than 102 mm or 4 inches in aperture. It is much less expensive to develop and manufacture large mirrors than large lenses. The Dobsonian design is based on the Newtonian reflector.
John Dobson in the 1960s created a low cost, easy to build Dobsonian mount for the Newtonian optical tube. Over the years commercial developers have taken advantage of the Dobsonian design to produce quality, cost-effective telescopes making the Dob one of the most popular designs on the market for beginners and experienced observers alike.
How Does Dobsonian Design Works?
The picture shows the typical Dobsonian design with a Newtonian optical tube on a base that swivels left and right like a lazy Susan that you might have on your dinner table. The tube pivots up and down on the side mount supports.
The cutaway shows the full-size optical tube and the last shows it in detail. The starlight comes into the top of the Newtonian optical tube, is reflected by the curved primary mirror on the bottom and angled into the secondary mirror near the top entry point. The flat secondary mirror then turns it 45 degrees, out the side of the tube to the focuser and the eyepiece.
Also shown in the picture is a finder scope, on the top of the tube, that helps you point the telescope. It works in the same way as a sight on a gun. You line up the finder with the target, then look through the eyepiece to see the magnified image.
Advantages of the Dobsonian Design
- A low cost means more of the price can be put into the aperture of the optical tube
- A very stable mount which overcomes the shaky tripods frequently found in low-cost telescopes
- A simple to understand altitude/azimuth design that anyone can use
Computerization In Dobs
Many people think of Dobs as being a manual design. The original home built and most commercially produced Dobs are manual scopes. You have to know where to point it and how to find your targets.
However, today you can also buy Dobs that incorporate computer assistance to show you were to push the scope to find your target. This is called a PushTo system or a DSC, digital setting circle design. The computer guides you but you push the optical tube to find the target. This works very much like a GPS in your car. You put in your destination in the GPS and the GPS tells you where to turn, but you do the driving.
With a PushTo Dob, you put the target into the handset computer and it will show you which way to turn the scope, left and right as well as up and down. This can be very helpful in our increasingly light-polluted sky where fewer and fewer stars are visible to guide us to our objective.
Once you have your target in the eyepiece, you track it by moving the optical tube as the Earth rotates. Some PushTo systems are installed at the factory and some can be added later.
GoTo Dobs are full robotic. These incorporate motors that move the optical tube to the target. Like a self-driving car, you put a target into the computer, usually a handset, and the motors move the optical tube till the target is in the eyepiece.
Normally you would be sitting at your GoTo Dob, but many GoTo Dobs can be controlled by a separate computer and operated remotely. You could have the Dob in a robotic observatory in Montana and operate it over the internet from New York. A camera in the focuser would send the image back to you over the internet.
A nice feature of the GoTo design is that once the target has been centered in the eyepiece, these fully robotic Dobs will track the target so it stays in the center of the field of view. You could walk away and come back 10 minutes later and the target would still be in view. This would not be true of the manual or PushTo designs.
Solid Tube vs Truss Dobsonians
Since mirrors can be scaled up economically, Dobsonian designs can get quite large. It is unusual to see a hobby refractor telescope that is more than 8 inches in aperture. Most catadioptric designs are 16 inches or less. But Dobs of 4 inches to 25 inches or more of aperture are readily available and are very popular.
While the design scales up economically compared to other designs, the optical tubes can get quite large and heavy presenting a handling challenge. An 8-inch aperture Dobsonian, an extremely popular size, typically has an optical tube that is 4 feet long weighing about 25 pounds. The Dobsonian mount, the base that lets you point the tube, would also be about 25 pounds. But the tube lifts easily off the base to make it easier to transport in two pieces. An 8 inch Dob will fit in most cars and easily goes in an SUV, station wagon, or mini-van.
Solid tube Dobs are usually limited to 12 inches in diameter. Beyond that, they become too heavy or too large for easy handling and transport. Enter the truss design.
In the truss design, the optical tube is broken up into the primary mirror box at the bottom and a ring that holds the secondary mirror and focuser at the top. The solid tube that connects them is replaced by a set of poles or trusses. While this design does not always save a lot of weight, it allows the optical tube to be broken down into smaller more manageable pieces.
A 16-inch aperture solid tube Dob might have a tube length of 80 inches, over 6 ½ feet, and weigh more than 100 pounds. It would be difficult to handle and would not fit in the typical car or many SUVs. However, a truss design Dob can be taken apart very quickly and easily so that it will fit in almost any car. You can add a fabric shroud around the truss to block out light and help keep dew from the primary mirror.
A 25-inch aperture Dob, a monster in the hobby market, can fit in the typical SUV, station wagon, and many hatchback cars. Most truss designs incorporate connectors that do not require tools. And some even incorporate mounts that come apart so that the individual pieces are of manageable size and weight.
Recommended Best Dobsonian Telescopes
What follows are Dobs over three ranges:
- Tabletop designs – up to 6” aperture
- Floor standing solid tube designs from 6” to 12”
- Floor standing truss tube designs of 10” to 16 inches.
These are all commercial designs that are readily available and are typically sold as complete telescope systems made up of a Dobsonian mount, an optical tube, focuser, and finder to help you target the scope. Some also include one or two eyepieces to get you started.
Some include collimation tools which will allow you to perform routine maintenance of fine-tuning the optics to ensure the best performance. And some packages include a fan that helps to cool the primary mirror to minimize thermal turbulence around the primary mirror. This helps to optimize optical performance. A fan can also help reduce the accumulation of dew on the primary mirror under humid conditions.
Recommended Tabletop Dobsonians
These typically run in the 3”/76 mm range targeted at children and beginners to about 6-inch designs that grab a lot of light for viewing of deep-sky objects. Most use a single-arm design.
Relative to their floor-standing cousins, most of the tabletops are small and light so they store easily and travel easily. Getting the best tabletop Dobsonian would be great as a first telescope and travel well for family vacations or as a second scope to a larger scope. The thing to remember about tabletop Dobs is that you need a table, stool, box or something else to place the scope at a comfortable height.
1. Celestron First Scope
Latest Amazon Price – $54.95
Children or teens on a tight budget as their first scope
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With a 76 mm aperture it will show you nice views of the Moon, stars, open star clusters, the four bright moons of Jupiter, and its major cloud bands as well as other things in the sky. This link takes you to a kit that includes two eyepieces. I don’t recommend this one for adults who are serious about getting into the hobby.
4. Zhumell Z130
Latest Amazon Price – $499.97
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The Zhumell Z130 offers 130 mm, about 5 inches of aperture. At 21 pounds, this is a serious scope for the more committed hobbyist who wants a tabletop design. It includes a finder, two eyepieces, and an eyepiece rack.
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5. Orion Starblast 6
Latest Amazon Price – Price not available
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6. Orion Starblast 6i
Latest Amazon Price – $499.99
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The Orion StarBlast 6i is probably the largest of the tabletops and it incorporates the Orion Intelliscope system. This PushTo system will help you find your targets. One of the key frustrations for those new to astronomy is the challenge of finding deep sky objects in light-polluted skies. The Orion StarBlast 6i will show you where those jewels are and provide 6 inchs/150 mm of aperture to help you see more than the smaller tabletop designs. If you want some serious aperture and help to find your targets in tabletop design, this is the one for you.
Recommended Floor Standing Solid Tube Designed Dobsonians
When you say Dob, this is what most people are thinking about. These are very cost-effective, offering a lot of aperture for the price. I have included, manual, PushTo, and GoTo Dobsonian models in the set. If you have space and can handle moving the scope, this is where you want to be. I have included models from 6-inch aperture to 12 inches of aperture. Many people start with these scopes.