A reflecting telescope gathers light with a curved mirror rather than a lens, and that one substitution is why most amateur aperture is in reflectors. Mirrors are cheaper to make large, they weigh less than the equivalent glass, and they bend no colours.
How the newtonian arrangement works
Light travels down an open tube to a concave primary mirror at the bottom, reflects back up to a small flat secondary tilted at forty five degrees, and leaves through the side of the tube near the top, where the eyepiece sits. That is why you look into the side of a reflector rather than the end, which surprises people who have only seen refractors.
Why it is cheaper per millimetre
Only one surface has to be figured to optical precision, the glass need not be transparent or free of bubbles, and the mirror is supported across its back rather than held at the edge, so large mirrors are practical. A 200mm mirror is an ordinary object; a 200mm apochromatic objective is a specialist item costing many times as much.
The two costs of the design
The secondary mirror and its supports sit in the light path, which slightly reduces contrast and puts faint spikes on bright stars. And the mirrors can shift, so the instrument needs collimating occasionally, which takes five minutes with a cheap cap once you have done it twice.
Questions people ask about telescope
What is a reflecting telescope?
One that uses a curved mirror as its main optic instead of a lens. The newtonian is the common amateur form, with the eyepiece in the side of the tube.
Is a reflector good for beginners?
It is the usual recommendation, because the money buys aperture. The two things to know are that it needs occasional collimation and that you look into the side rather than the end.
What is a newtonian reflector?
The specific reflecting design with a flat secondary sending light out of the side of the tube, invented by Newton and still the commonest amateur telescope.