A dobsonian is a newtonian reflector on a mount that a woodworker could build, and that plainness is the point. John Dobson's insight was that everything spent on gears and tripods is aperture not bought, and the design has dominated value on this shelf ever since.
The mount is the whole idea
Two bearings, one turning in azimuth and one tipping in altitude, made of plywood and low friction plastic. It is stable, it damps vibration quickly because it is heavy and low, and it costs almost nothing. Point it by pushing the tube. That is the entire user interface, and it is the reason a dobsonian puts more mirror in front of you than any other design at the same price.
Tabletop models, and the thing to know first
A tabletop dobsonian is the same design shrunk, and it needs something roughly waist high to stand on. That is not a defect and it does catch people out: a garden chair is too low and a table is rarely where the sky is. Owners usually settle on a plastic storage crate or a folding camping table, and once solved it never comes up again.
What a dobsonian will not do
It does not track, so objects drift and you nudge. It has no computer, so you find things yourself. And it cannot do long exposure photography, because both of those things are prerequisites. If any of those is a requirement, the design is not for you, and if none is, nothing else is better value.
Questions people ask about dobsonian
What is a dobsonian telescope?
A newtonian reflector on a simple alt-azimuth box mount. The design maximises aperture per pound by spending nothing on gears or tripods.
Is a tabletop dobsonian worth it?
Yes, if you have something to stand it on. The 130mm tabletop at $305.00 in this index is the cheapest instrument here that shows deep sky objects convincingly.
Where can I buy a dobsonian?
Sky-Watcher and Celestron both sell them direct with printed prices, and every specialist dealer stocks them. The second hand market is also good, because there is very little to go wrong.