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Comment by cgriswald | Hacker News Reader

I've mostly dealt with amateur and semi-pro equipment and I'm not certain what the state-of-the-art is so someone else might be able to answer this better than me.

These telescopes essentially work by capturing photon counts on a sensor. The individual pixels on the sensor have a limit to the number of photons they can count. You could theoretically subtract the satellite pixel-count values from the photon counts to get rid of the trails. The two problems I see are: 1) You don't know the correct counts for the satellites and I'm not sure how you could get them. 2) The trails will probably saturate the pixels anyway (which can also cause bleeding into other pixels), in which case you just don't have the data of what's 'behind' the trails.

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Tl;dr: Perfect subtraction is impossible due to physics.

Problem is: With photon count the uncertainty in the number of photons also goes up (the relative error goes down). So even if you know that you should have received 100 photons from the satellite (and have not reached the overflow of 256 in this example yet), Poisson statistics means you will actually get anything between 90 and 110 photons. So if you subtract 100 you have an uncertainty of plus or minus 10 photons left. That is deadly if you astronomical source only gave you 2 photons in that pixel in that time.

Are the sensors recording a timestamp with the photon counts? If so, it should be trivial to filter out bright fast moving objects with zero impact on image fidelity.

Photons are not individually time tagged (that is much harder and reduces quantum efficiency due to dead times), but rather are collected for some exposure time (think 30 seconds) and then their count is read.

one gets time signals from photomultipliers, but those are too bulky for traditional pixel sensors. And something as bright as a satellite flare might fry them at an unexpected moment. They are usually built and tuned to be able to observe few to single photons.

could they put an LCD in front of it, and block the relevant pixels as a satellite passes?

That would limit your resolution to the resolution of a LCD panel. Also just not how cameras work

it'd limit your dimming resolution, not your visual resolution.

I mean to use it like auto-dimming mirrors, or smart headlights. Block the bright stuff that you can easily predict, so you can protect the sensitive image sensors from over-saturating on garbage.

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