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Why aren't refracting telescopes built using Fresnel lenses?
The world's largest refracting telescopes are pathetically small compared with the largest reflectors, for obvious reasons. I mean, imagine driving the truck carrying the lenses up that mountain in Hawaii. I'm reading an Arthur C. Clarke book about saving the world using a huge Fresnel lens in space. Fresnel lenses can be as thin as you like. Why don't we use existing technology to build both Earth- and space based Large Fresnel lens refracting telescopes?
I've just spotted a minor error in my question; a Fresnel lens is, of course, a diffracting, not refracting device.
4 Answers
- John WLv 79 years agoFavorite Answer
Fresnel lenses approximate a lens in incremental steps. This would be fine if the grating are relatively small in relationship to the frequencies that you're talking about hence it may work for radio telescopes but would be impractical for the visible spectrum. Using fresnel lenses in an optical telescope would be like looking through a Kaleidoscope.
- JoshLv 69 years ago
They are not as clear, it would only work well in space where the rotation of the Earth wouldn't mess with the exp osier time it would take to get a good/ better picture than the ones in Chilly. here is where the problem lies and that is plastic will expand and contract with temp change and any change/warp will destroy to image, In space when the telescope is exposed to the sun it could heat up by over 40 degrees, sun shield need to be 30% bigger than the diameter of the lens.. Example ..Kepler
a Fresnel lens would have to be 300ft in diameter to be better than Hubble so you know how big the heat shield would have to be. The James Web is the best bet for now until we get the advanced Kepler2 under way, it is said Kepler 2 to will be 50,000 times as powerful as Kepler 1
- QuadrillianLv 79 years ago
Fresnels are good at simple focusing tasks, but introduce far too much distortion to be much use for imaging.
They basically present a great many edges and discontinuity into the light stream, and these prevent detailed images being formed at anything other than the lowest powers.
Cheers!
