A used Dobsonian is the most telescope you can get for your money. The design is simple, there's almost nothing electronic to fail on the basic models, and they turn up for sale all the time. The two things that decide whether a particular one is a good buy are the condition of the main mirror and how easily you can collimate it. Then there's the question nobody thinks about until the day of pickup: how you're going to get it home.
How a Dobsonian works
A Dobsonian is a Newtonian reflector on a simple swiveling base. Light goes down an open tube to a curved main mirror at the bottom, bounces back up to a small flat mirror near the top, and that diagonal mirror sends it out sideways to the eyepiece. The base, usually called a rocker box, lets you push the tube up and down and swing it around. John Dobson popularized the idea in the 1960s as a way to build big, cheap, easy telescopes, and scopes like the Orion XT8 and the Sky-Watcher Flextube range are its direct descendants.
Because the tube is open at the top, the mirror sees more air and dust than the sealed optics of a refractor or Schmidt-Cassegrain. That's the main thing to check.
Reading the main mirror
A telescope mirror is glass with a very thin reflective coating, usually aluminum with a protective overcoat. New, it looks bright silver and reflects a clean image of whatever is in front of it. Over the years, especially in damp, dusty or smoky places, the coating dulls. Dust settles on it, pollen and dew leave spots, and in bad cases the coating develops pinholes, gray patches or peeling.
Ask the seller for a photo looking straight down the tube at the mirror with a flashlight. Some dust is normal and makes little difference to the view, so don't let a few specks put you off. What you're looking for is the overall look of the coating. Bright and even is good. Dull gray, blotchy, or with visible pinholes in a bright light means the mirror is past its best. It will still work, but it won't be as bright as it should be, and in time it will need recoating.
Recoating means sending the mirror to a specialist who strips the old coating and puts on a new one. It's routine work and it brings an old mirror back to as-new brightness, but it means taking the mirror out of its cell, packing it carefully and going without the scope for a while. Factor that in if the coating looks tired.
Asking about storage
The best clue to a mirror's condition is where the scope has lived. A Dobsonian kept indoors with its dust cap on can look nearly new after many years. One kept in a damp garage or shed, uncapped, can look old after a few. Ask the seller where it was stored and whether it has a cap. Ask whether the mirror has ever been cleaned or recoated, and by whom.
Also check the base. Many Dobsonian bases are made of particleboard or plywood, and damp storage makes particleboard swell and crumble, especially around the bottom edge and the bearing pads. A base that's swollen or sticky won't move smoothly, and smooth motion is half the pleasure of a Dob.
Collimation
Every Newtonian needs collimating, which means lining up the two mirrors and the focuser so a star comes to a sharp point. Most used Dobsonians arrive out of collimation simply because they've been moved. That's normal, not a fault, and with practice it takes a few minutes.
Most modern Dobsonians make it easy. The main mirror usually has a small ring or dot marked at its exact center, and adjustment knobs on the back of the mirror cell let you tilt it without tools. The small diagonal mirror has its own adjustment screws. A simple collimation cap, a sighting tube or a laser collimator lets you see whether everything lines up. Faster mirrors, with focal ratios around f/5 or less, need more careful collimation than slower ones.
When buying, ask whether the center mark is there and whether all the collimation knobs and screws are present. A missing knob is a cheap fix. A mirror that has been taken out and put back without care is a bigger job, so ask whether that's ever happened.
Solid tube, truss or collapsible
Solid-tube Dobsonians like the XT10 and XT12 are the simplest. The tube and base come apart for carrying, and the tube holds collimation well because nothing moves. The cost is size, since a solid 12-inch tube is long and heavy.
Truss and collapsible designs, like the Orion XX12i and the Sky-Watcher Flextube models, shorten the tube for transport. The upper cage that holds the small mirror and focuser rides on poles or struts. That makes the scope easier to fit in a car, but every time you set it up the optics have to come back into line, so expect to touch up collimation each night. Check that every strut, clamp and knob is present, because the scope can't be assembled without them.
Push-to and GoTo Dobsonians
Some Dobsonians add electronics. Push-to models have sensors on each axis and a handset that tells you which way to push the tube to find an object. GoTo models add motors that move the tube for you. Both need their handset, cables and power supply, and the GoTo ones need working motors. Treat these like any computerized mount. Ask for a video of the system working, check every cable is included, and check the battery compartment or power supply. If the electronics are dead, the scope still works as a plain manual Dobsonian, which is worth knowing when you weigh a project.
Getting it home
Big Dobsonians are the hardest telescopes to move. A 10-inch is manageable for most people. A 12-inch solid tube is long and heavy enough that you'll want a second person and a big car, and the base is a separate lump of wood. Many sellers only offer local pickup, and that's often the better deal anyway, because you can check the mirror in person and see the scope move.
If it has to ship, the mirror needs the most protection. Ask how it will be packed. Some owners take the main mirror out of the tube and pack it separately, which is the safest way for a long trip, and then it needs collimating on arrival. Check the base is boxed so it can't be crushed. Bring a flashlight, a tape measure and the measurements of your car to any pickup.
A quick checklist
- Flashlight photo of the main mirror, bright and even with no peeling or heavy spotting.
- Where it was stored, and whether it has a dust cap.
- A solid base, with no swollen or crumbling board.
- Center mark on the mirror and all the collimation knobs present.
- For truss and collapsible scopes, every strut, clamp and knob.
- For push-to and GoTo scopes, the handset, cables, power and a working demo.
- A pickup plan that fits the scope and your car.
For more, read how to check a used telescope, or browse every Dobsonian we cover.
