Follow any discussion of a beginner asking about a first telescope and it won’t be long before an 8″ Dobsonian is suggested. It might be the most recommended scope for beginners, and not without good reason, though there are several factors that should be seriously considered before buying one.

A Dobsonian is a Newtonian reflector on a simple alt-az mount. Before John Dobson popularized his simple, inexpensive mounting method, German equatorial mounts were commonly used, which were by necessity large and heavy, and thus expensive. By comparison an 8″ or greater Dobsonian becomes relatively affordable and portable.

The first thing to know is that an 8″ Dob is a fairly large telescope and some forethought should be given to storage and transport. At f/6 the tube will be just shy of 4′ long and weigh around 20lbs with the mounting base adding another 20lbs – picture a 40 gallon tank-style water heater and you’ll have a rough idea of the volume. Despite the bulk most people can handle moving the two parts separately without too much trouble, and setup is typically quicker than with a tripod mounted scope of smaller aperture.

Preparing to view is straightforward, just place the mounting base in the desired observing spot then lower the tube onto the base. Add an eyepiece and you’re ready to observe.

Point and Shoot Observing

Observing with any alt-az mount is simple – just point the telescope at the area of the sky you want to view. Compared to an equatorial mount the motions are intuitive, which makes it easy to jump from one part of the sky to another. Unless you spring for a GoTo or PushTo Dobsonian such as Orion’s XT8g or XT8i you’ll need to find your own subjects, which for some might be considered an advantage.

A long tube refractor has a classic look but the eyepiece viewing height varies greatly with changes in the subjects altitude – subjects high in the sky might have you kneeling on the ground while lower subjects may require standing. The Dobsonian’s viewing position results in comparatively little change in eyepiece height, and while an adjustable height viewing chair makes observing more comfortable it is possible to view without one.

Bang for the Buck

With a standard Dobsonian, the simple but well working mount means that the most of the money can go towards the aperture/optics. For this reason a Dob is usually the largest aperture telescope you can buy at a given price point.

Remember that the primary purpose of a telescope is to gather light – all other factors being equal, a larger telescope will give a brighter, more detailed image of every subject. In practice things aren’t quite this simple and while all types of telescopes have their own advantages it does require a lot more money to make up for the aperture that an 8″ Dob will provide.

Care and Feeding

In order to get the best views, especially at higher powers desired for planets and double stars, you’ll need to make sure you Dob is cooled and collimated.

All Newtonian reflectors require collimation from time to time. This is simply the process of aligning the telescopes mirrors for optimal performance and once practiced generally requires less than a minute to touch up before viewing.

Cooling is needed to ensure the temperature of your telescope’s mirror is very close to that of the ambient air. Otherwise thermal currents within the tube will distort the view and you will not see the detail your telescope is capable of providing. Many available Dobsonians have rear mounted fans for this purpose and they are effective.

For the money, there isn’t a scope that will show you more than an 8″ Dobsonian but they aren’t automatically the best choice for every observer. Carefully consider your living situation and observing goals and see how well a Dob might suit you.

-Dave

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