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Newtonian reflectors: the most aperture for the money
A Newtonian reflector uses a curved primary mirror at the base of the tube and a small flat secondary near the top to send the image out to a side-mounted eyepiece. It is the design that gives the most aperture for the money.
How a Newtonian works
Light enters the open tube and falls on a concave primary mirror at the bottom. That mirror reflects the light back up the tube to a small flat diagonal mirror set at forty-five degrees, which turns the cone of light out through the side of the tube to the focuser and eyepiece. Because the light is folded by mirrors rather than passed through a lens, a mirror can be supported across its whole back and made large at modest cost.
This is why a Newtonian delivers more aperture per pound than any other design. A 200 mm Newtonian costs a fraction of a 200 mm refractor and gathers the same light.
Strengths on deep-sky objects
Aperture is what shows faint galaxies, nebulae and globular clusters, and the Newtonian's low cost per millimetre of aperture makes it the classic deep-sky instrument. A 150 mm to 250 mm Newtonian on a simple mount is the workhorse of amateur observing.
A fast Newtonian, around f/4 to f/5, also gives a wide, bright field well suited to sweeping the Milky Way and to deep-sky imaging, though fast mirrors demand good eyepieces to stay sharp at the field edge.
The collimation trade-off
A Newtonian's two mirrors must be aligned to each other, a quick adjustment called collimation. The alignment can drift with transport and temperature, so a Newtonian owner learns to check and tweak it, usually with a simple sighting tool. It takes minutes once learned and is the main routine that a refractor owner avoids.
The open tube also means the optics need occasional dusting and the tube takes time to cool to the night air before images settle.
Focuser and eyepieces
Most Newtonians use a 2 inch focuser that also accepts 1.25 inch eyepieces through an adapter, giving access to both wide-field 2 inch eyepieces and the larger range of 1.25 inch ones. The scope's focal length sets magnification with any given eyepiece in the usual way.
Get the exact magnification for your scope
Open the Fit My Telescope toolMatch eyepieces to your Newtonian's focal lengthFind newtonian reflector telescope on eBayaffiliateRelated
Sources: Reflecting telescope (Wikipedia, CC BY-SA) ยท Telescope Optics, Rutten and van Venrooij; Sky & Telescope optics primers