Imaging Without Lenses

Re:Imaging Without Lenses by hliyan on Monday February 05, 2018 @10:32AM (#16309346)

As I understand, one of the biggest costs of optical telescopes is the mirror, which needs to be meticulously ground to a paraboloid. Ever since I was a child, I always wondered if we could use a semi-cylindrical mirror (which is much easier to fabricate) to focus light into a linear sensor array and achieve the remaining focus computationally. If this is possible, we might be able to build much larger, cheaper versions of the James Webb Telescope.

I discovered 5 years back that cylindrical reflectors are not viable, purely optically: https://physics.stackexchange.com/questions/29853/are-cylind...

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Re:Imaging Without Lenses by leggomylibro on Monday February 05, 2018 @05:34PM (#16312862)

Interestingly, vacuum-backed mylar has been used for light 'DIY parabolic mirror' duties. Here's one made out of a trashcan lid:

https://www.youtube.com/watch?v=_8sd9UgjXLE

I'm sure that durabililty would be an issue, and the vacuum would probably leak over time without some significant effort, and it probably wouldn't be very stable under environmental changes, but it's a cool lateral solution.

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Re:Imaging Without Lenses by sp332 on Monday February 05, 2018 @12:17PM (#16310264)

I can't quite visualize the problem. As long as you make the focal length of the first cylinder equal to the distance to the second cylinder + the distance from there to the sensor, and make the focal length of the second cylinder just the distance to the sensor, then both dimensions should converge at the right point?

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Re:Imaging Without Lenses by hliyan on Monday February 05, 2018 @12:29PM (#16310334)

I thought so too. But apparently that doesn't give you a _point focus_, rather a square focus, which doesn't work.

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Re:Imaging Without Lenses by sp332 on Monday February 05, 2018 @12:36PM (#16310374)

Ok, I get it. A circular mirror focuses all directions equally, while two cylindrical ones focus only exactly left-right and exactly up-down correctly, leaving every diagonal out of focus to varying extents.

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Re:Imaging Without Lenses by neuronexmachina on Monday February 05, 2018 @04:27PM (#16312348)

I think what you describe is similar to the notion of a light-field/plenoptic camera: https://en.wikipedia.org/wiki/Light-field_camera

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