If we were to make telescopes with Fresnel lenses instead of regular lenses, would it be more practical? Would this also allow for much bigger telescopes to be made?
Here is an example of the type of lens I am talking about from https://commons.wikimedia.org/wiki/File:Fresnel_lens-1.svg
The main advantage of a Fresnel lens is its reduced mass compared to a normal lens. Its disadvantages include difficulty of manufacture and diffraction from the discontinuities in the aperture, these I suspect are killers given the practicality of the alternatives, reflecting telescopes, to a Fresnel lens refactor.
Diffraction will give reduced contrast in the image, already noticeable in a reflector with partially obstructed aperture, but may well be worse with a Fresnel lens. But the difficulty in manufacture may well be the real limit given the relative ease of manufacturing large mirrors.
While I'm at it; it seems as thought making a achromatic Fresnel multiplet might be quite fiddly.
We might also note where there is no alternative, systems analogous to Fresnel lenses are used. The example I have in mind are grazing incidence x-ray telescopes.
It is interesting, it looks like you can combine a "normal" lens with a fresnel to correct for chromatic abberation and have a lighter lens. I don't think it helps with making a bigger refractor though, but maybe a more portable one.
The Nikon Phase Fresnel, and Canon Diffractive Optics are lenses you can buy now with fresnel lenses. They seem to have extra flaring, which you may not want in a astrophotograph.
Phase Frenel lenses:
Canon DO:
In addition to the diffraction issues pointed out by Conrad Turner, a Fresnel lens would suffer from all the other problems that large lenses in telescopes suffer from:
A group at LLNL have worked on it with DARPA funding. They had a folding glass system called Eyeglass in 2003 and a flexible membrane system called MOIRE in 2013. They embraced the diffraction and put it to work, totally unlike refractive optics as we know them.
If we were to make telescopes with Fresnel lenses instead of regular lenses, would it be more practical?
There is a Wikipedia article about using Fresnel zone plates successfully in the visible spectrum. A paper about it called: "First high dynamic range and high resolution images of the sky obtained with a diffractive Fresnel array telescope" was written by Laurent Koechlin et al. in 2011:
"... this concept is well fitted for space missions in any spectral domain, from far UV (100 nm) to IR (25 μm). However, the advantages of Fresnel arrays become outstanding in the UV domain, for which a 6 m size aperture yields milliarcseconds angular resolutions with a very rough manufacturing precision compared to standard optics: typically 50 μm compared to 10 nm.".
From Laurent Koechlin's paper:
"Fig. 1 Top: Fresnel array, close view on the central zones. Bottom: sketch of our prototype, not to scale for clarity. On the real prototype, the Fresnel lens and the achromat doublet are much smaller than the entrance Fresnel array. The distance between the Fresnel array and the field optics is 18 m. The rest of the light path is short (2 m). The zero-order mask blocks the light that has not been focused by the Fresnel array: all diffraction orders are blocked, except one. The achromat forms the final image after chromatic correction by the secondary Fresnel lens.".
Another flat lens is the metalens which uses structures on a flat surface to focus each frequency individually, but "Blade Optics" (prisms) might prove to be the most practical, more below.
Would this also allow for much bigger telescopes to be made?
Yes, they will be bigger; but not in a good way:
"Fresnel arrays of 6m and larger have focal lengths of a few kilometers in the UV; they will require two satellites flying in formation around Lagrangian point L2, but with tolerant positioning in translation (a few centimeters).".
With modern technology that isn't considered completely impractical.
"Blade Optics" have both supporters and naysayers. They have been tested by RASC and a few patents (US 2017 / 0307864 A1) have been issued.
The principle is simple:
NexOptix has a working prototype which is far shorter than an equivalent focal length refractor (straight tube), Newtonian (right angle eyepiece) or even Schmidt-Cassegrain (folded) design:
That squeezes 146 cm (57.48 inches) of focal length into just 12.7 cm (5 inches), though the other dimensions increase (but not proportionally).
Fresnel lenses are being considered for high energy waves, like X-rays. Problem with X-rays is that they are not reflected by mirrors of traditional telescopes : they just pass through.
So a new type of telescopes have been designed for X-ray astronomy, "Grazing Incidence Telescopes". In them the X_rays are reflected at a very small angle, to just change their direction slightly.
And so new difficulties arise form using reflection for X-ray and gamma rays, that Fresnel telescopes are being considered. It could be worth to try to overcome disadvantages as chromatic aberration instead.
Here is some interesting link :
