Hacker News Reader

Comment by flyinglizard | Hacker News Reader

Can that be overcome with computational photography?

Optical interferometry has been used to synthesize very large apertures in ground based experiments (e.g., https://en.wikipedia.org/wiki/Palomar_Testbed_Interferometer). That was in the mid 1990s.

Operational ground interferometers have obtained 5 mas resolution, which corresponds to sub-mm resolution from an orbital platform.

You still have atmospheric turbulence, but do we know how to get around the naive diffraction limit.

No matter what you do, you still have to deal with diffraction [1], which means the resolution is inversely proportional to the aperture. I think the Trump/Iran release was estimated to be pretty close to the diffraction limit.

[1] https://en.wikipedia.org/wiki/Angular_resolution#Explanation

How about optical interferometry? IIRC, this can beat the diffraction limit.

Optical interferometry means you have a wider telescope. Same limit formula.

Yes, but the aperture of the interferometer is no longer limited in the same way, so we could say that interferometry gets around the “naive” diffraction limit.

Well, but if you have a bunch of these in orbit, able to image the same site, at the same time...

Yep.

  angle = k (wavelength / diameter)

And the diameter is going to be limited by the launch vehicle. But multiple satellites could improve this.

Just took a radars course and we definitely spent a whole section on just atmospheric effects / defraction. My only guess about how you could maybe lower the error is by monitoring and measuring the condition of the atmophere and building a model that can use that data to apply corrections and reduce error to your image.

If there was a known object in an image could a high resolution image of the known object be used to compensate for atmospheric diffraction effects?

One example is a laser guide star: https://en.m.wikipedia.org/wiki/Laser_guide_star

Yes. Ssshhh.

As I mentioned above, there is a technique called lucky imaging, where you take lots of short exposures and only use the best ones. In astronomy, this can be used to reach the diffraction limit. You need a bright source, however, and it provides a narrow field of view.

Another technique is to use multiple camera sources spread out over an area. The angle between the cameras is known, so differences in the images caused by diffraction and atmospheric aberrations can be computed out to some degree.

Can you expand on "the Trump/Iran release was estimated to be pretty close to the diffraction limit"? What is this in reference to?

https://mobile.twitter.com/realDonaldTrump/status/1167493371...

https://www.npr.org/2019/08/30/755994591/president-trump-twe...

I think your question was misunderstood, as the other replies do not go into any details on diffraction limit at all.

I too would be interested in knowing if this image is showing all we got, or because someone else looked at it and said "sure you can release it, we've got better than that anyway."

Can multi-camera/multi-view photogrammetry defeat this limit and create super-resolution imagery?

I'm neither an astronomer nor an imaging expert, but I've read that ground-based telescopes use "lucky imaging" plus deformable mirrors to work around atmospheric distortion. Maybe something analogous works in the reverse direction.

For deformable mirrors they shine a control beam into the sky. A spy satellite would have to shine it downwards.

Can you use a known object which is illuminated by the sun?

Given that people have called this 'wavefront based correction' I would guess you need light source where all photons are 'in phase' like a laser. The sun is not such a source.

However, I do not know near enough about physics to say the above with any degree of confidence.

Reply on news.ycombinator.com