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Comment by A_No_Name_Mouse | Hacker News Reader

> Although Image Stacking puts all the light in one place on the detector, the segments are still acting as 18 small telescopes rather than one big one. The segments need to be lined up with each other with an accuracy smaller than the wavelength of the light.

Why is this necessary? Phase difference shouldn't matter unless the light is coherent, which I wouldn't expect in starlight. Which assumption is wrong?

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You can get interference patterns off of white light: https://www.quora.com/Can-white-light-produce-interference-p...

After all, physicists were producing interference fringes in the 18th century, and they sure didn't have lasers back then: https://en.wikipedia.org/wiki/Young%27s_interference_experim...

For interference the light needs to be able to take two separate paths with different length, which is the case in the oil film example or with two slits. In a telescope, each photon can only take a single path. So what would it interfere with? If it's a different photon then coherence seems to be required

That's a classical-world blunder: single photons can and do interfere with themselves.

Making a measurement of which path a photon took, is forcing the photon to have taken one path. If you don't make the measurement (read, if you don't interact with the photon somewhere along some path it could take), you can get interference between the paths, despite it being a single photon.

> you can get interference between the paths

Yes I understand that: the single photon interference in the double slit experiment. But what two paths could a photon take in a telescope allowing it to interfere with itself? The mirrors are placed so far apart a photon could never bounce off more than one mirror?

Sorry but you didn’t seem to really understand the implications of light being a wave here.

Forget about photons altogether and think of light as a wave and you will understand that yes of course, light bounces on all mirrors and interferes across mirrors. Who cares about the distance between mirrors really, light could (and does) interfere with mirrors miles apart.

Hm? Oil slick iridescence and the original double slit experiment demonstrate entirely classical wave behavior. No QM needed. Same thing with Webb. Light arriving from the same source but in slightly different phases absolutely causes interference patterns explainable in purely classical terms.

When thinking about something like a telescope, you pretty safely ignore the concept of "photons" and think about light entirely as a wave. And waves do interfere with themselves.

Light as something made out of particles is relevant only for the photovoltaic effect and quantum-scale interactions.

Obviously I'm wrong but I'm not sure why. Light bouncing off of two mirrors can interfere constructively or destructively, depending on the phase difference. The phase difference from light bouncing off of two aligned mirrors is random (as it's non coherent light). Adding a constant in the phase difference due to misalignment would still lead to a random difference. So it seems to me the phase shift wouldn't make a difference.

What am I missing?

The phase difference between light bouncing off two aligned mirrors is NOT random. This is what you are missing.

Non-coherent light only means it is made of many wavelengths at the same time (Think of it as the sum of many « coherent lights »). Interference does happen even for incoherent light, within each wavelength.

Example: oil iridescence on water is interference which works on « incoherent » light.

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