A Fresnel Lens Based Refracting Telescope
Optical telescopes come in two main varieties: refracting, which use relatively ordinary lenses, like in a magnifying glass, and reflecting, which use parabolic or spherical concave mirrors. Refracting telescopes are extremely expensive compared to reflecting telescopes for the same aperture, but reflecting telescopes use multiple mirrors which must be precisely aligned before satisfactory results can be obtained.
Why not use a fresnel lens? It is extremely lightweight and extremely cheap. I am not the first person in the world to think of this idea, but the only fresnel lens telescopes I am presently aware of are space telescopes. Why hasn't this kind of telescope caught on in the personal use market?
I have no idea why this hasn't become a third kind of consumer-level telescope, and I am highly tempted to build one and see how it performs.