TYPES OF SCOPES - NEWTONIAN REFLECTORS
Newtonian reflectors use mirrors instead of lenses to capture light.
Named after Sir Isaac Newton, who developed this type, these use a concave
mirror at the back of an open tube to focus light on a smaller, flat,
mirror near the front of the telescope's tube. This mirror, in turn, focuses
the light through a hole in the side of the tube to where an eyepiece
in a focuser receives the light.
This type is almost exclusively used for astronomical viewing. The image
is completely upside down, which doesn't matter on stars, but makes this
instrument unsuitable for land and sea use. Sizes range from 114mm (4.5")
to about 1000mm (~40") for commercially-available scopes. The cost to
aperture ratio is best in this type—you get more size for your dollar.
The largest reflector telescopes in the world are currently the twin Keck
Telescopes on Mauna Kea—these are 10 meters across, or almost 33
feet! There does not appear to be an upper limit for this type, so expect
the Keck telescopes to eventually be superseded by even larger ones.