Newton built the first working reflecting telescope in 1668 to avoid the colour fringing that plagued the refractors of his day, and the design he arrived at is still the commonest amateur telescope in the world three and a half centuries later.
What Newton was solving
Seventeenth century refractors suffered badly from chromatic aberration, and the only known remedy was to make them absurdly long. A mirror reflects all colours identically, so a reflecting telescope has no chromatic aberration at all. Newton's contribution was the flat secondary that brings the light out of the side of the tube where an observer can reach it.
Why it survived every later design
Because it is the cheapest way to make a large accurate optic, and that has never stopped being true. Only one surface needs figuring, the glass quality barely matters, and the mirror can be supported from behind. Every increase in amateur aperture over three centuries has come through this design or its descendants.
What the modern version adds
Better coatings, which raise reflectivity from Newton's polished speculum metal at around sixty per cent to over ninety, and the dobsonian mount, which made large apertures affordable to carry as well as to build. The optical arrangement itself is unchanged.
Questions people ask about telescope
What is a newtonian telescope?
A reflecting telescope with a concave primary mirror and a flat secondary that directs light out of the side of the tube to the eyepiece.
Who invented the reflecting telescope?
Isaac Newton built the first working one in 1668, to escape the colour fringing of the refractors of his time.
Is a newtonian reflector good?
It is the standard amateur telescope for good reason: the most aperture per pound of any design, with occasional collimation as the only real upkeep.