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Dobsonian telescopes: big aperture on a simple mount
A Dobsonian is a Newtonian reflector optical tube sitting on a simple wooden or laminate alt-azimuth base that swivels in azimuth and tips in altitude. The design strips cost out of the mount so nearly all your money buys aperture.
A Newtonian on a rocker box
The optics of a Dobsonian are exactly those of a Newtonian reflector: a parabolic primary mirror and a flat diagonal. What sets it apart is the mount. Instead of a tripod and slow-motion controls, the tube rides in a low box that turns on a smooth bearing at the base and pivots on side bearings. You simply push the tube to point it.
This was popularised by John Dobson to make large telescopes affordable, and it remains the cheapest way to put a large mirror under the stars.
Why it dominates deep-sky observing
Because the mount is inexpensive, a given budget buys a much larger mirror than on an equatorial mount. Apertures of 200 mm, 250 mm, 300 mm and beyond are common and affordable in the Dobsonian form. That aperture is exactly what reveals faint galaxies, nebulae and the resolved stars of globular clusters.
For visual deep-sky observing, a Dobsonian gives more light grasp per pound than any other telescope, which is why so many experienced observers own one.
Its limits
A basic Dobsonian does not track the sky, so objects drift out of the eyepiece as Earth turns and must be nudged back, and at high power that nudging is frequent. This makes a simple Dobsonian poorly suited to long-exposure astrophotography, which needs a driven equatorial mount.
Some Dobsonians add motors or a computerised push-to system to help find and follow targets, and truss-tube designs collapse for transport, but the pure visual, non-imaging role is where the design is unbeatable.
Eyepieces for a Dobsonian
A Dobsonian usually has a fast focal ratio, often f/4 to f/6, and a 2 inch focuser. Fast optics reward well-corrected eyepieces and a low-power 2 inch eyepiece for wide views, plus higher-power 1.25 inch eyepieces for planets and tight objects. The scope's focal length still sets magnification with each eyepiece.
Get the exact magnification for your scope
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Sources: Reflecting telescope (Wikipedia, CC BY-SA) ยท Telescope Optics, Rutten and van Venrooij; Sky & Telescope optics primers