A Fresnel imager is a proposed ultra-lightweight design for a space telescope that uses a Fresnel array as primary optics instead of a typical lens. It focuses light with a thin opaque foil sheet punched with specially shaped holes, thus focusing light on a certain point by using the phenomenon of diffraction. Such patterned sheets, called Fresnel zone plates, have long been used for focusing laser beams, but have so far not been used for astronomy. No optical material is involved in the focusing process as in traditional telescopes. Rather, the light collected by the Fresnel array is concentrated on smaller classical optics (e.g. 1/20 of the array size), to form a final image
As a diffraction based optics, its performance is closely bound within certain wavelength band. If observaton is only done in such a band, then no problem, it would be great option because of its cheap price. However, it is usually not the case in astronomical observation. So ,it is not quite often to see any project planning to use Fresnel lens for imaging optics. There was a plan I think in US to fabricate a 50m diameter fresnel lens somewhere maybe in early 2000, but the plan didn't go long.
> a thin opaque foil sheet punched with specially shaped holes
I think astronomers, who crave photons, would be concerned about the light losses - what percentage of the photon flux makes it through the holes?
But the sheer size of the Fresnel unit (compared to standard focusing optics) should still deliver loads of light.
Only roughly 50% makes it through, the other 50% are in the wrong phase and have to be excluded. A satellite TV antenna has been made using zone plates like that using foil stuck on a window, but I can't find a reference to it now.
It does have some advantages
https://en.wikipedia.org/wiki/Fresnel_imager
and does not seem to be the same as a conventional Fresnel zone plate, but more like the equivalent to a sparse array of VLF radio-telescopes. Wikipedia has 10% getting through!
In answer to the the first question. The holes are chosen so that the optical paths from different holes only differs by close to whole wavelengths. Light (or any waves) can't tell the difference between parts any whole number of wavelengths earlier or later. It only works over narrow-ish bandwidths, because of the change in wavelength over the band.
Hi Ronian Siew
The wikipedia article may be right. The name Fresnel array doesn't quite describe the zone plates that I was originally thinking of, but does fit with the wiki description. It makes sense that it would be used with UV, or anything where lenses are difficult. The resolution can be very good, but there could be poor far-out sidelobes but these don't matter in astronomy where the targets have a lot of black around them.
I don't know who writes them. Perhaps retired people with a lot of experience and no commercial reason not to share it?
The ones at my technical level are usually good. I would check them anyway. I find that in the last 15 years the ones I want to learn from have become so up-to-date and with concise terminology that I have trouble understanding them. There probably is a lot of input from university types.
In space a thin very large sheet with holes in it is probably a very good solution, particularly because the tolerance on flatness is a lot larger than the tolerance required for a reflector, where any error in position is nearly doubled in the path error. I assume that any conventional lens is totally impractical because of weight .
- Badges
- Science topic