Comment by skykooler | Hacker News Reader

Comment by skykooler | Hacker News Reader

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Yes, you'd need to get quite far from the Sun to use it as a lens - about 650 AU is where it starts becoming practical. It would also not be re-orientable, so any "telescope" launched to that location would only be able to observe a single target. (But, notably, it is far easier to send a probe to a far point in our own solar system than it is to send it to another star entirely.) There's a paper that goes into much more detail at https://arxiv.org/pdf/1604.06351

So why not use interferometry instead? Well, it has some significant drawbacks. For example: the Event Horizon Telescope used radio telescopes - and pretty much had to, due to how interferometry works: you need to be able to compare the phase shifts between the multiple telescopes, which means you need to be able to sample the signal faster than the radio frequency you're using and record it. The EHT records 64 gigabits per second for each telescope, and then all this data needs to be combined to compute the resolved image. This amount of data would be problematic for space-based telescopes - even on Earth, it was not practical to send multiple petabytes over the internet, so it was saved to hard drives which were shipped by truck instead. This isn't practical in space, so you would need to transmit the data by radio, which means you'd end up with some crazy ratio of thousands of hours of transmitting for every one hour you spend recording.

"It would also not be re-orientable, so any "telescope" launched to that location would only be able to observe a single target."

I just presume that such observation spacecraft makes sense to be made as a rather long term investment than some one-time fly-by thing like New Horizons, so the sensible thing to do is to have it in an orbit around the Sun at that (≥650AU) distance and to take multiple observations. Of course, the easiest solution may be for the orbit to be elliptical one, with the useful position only around its aphelion, in which case it will be able to observe only a single target indeed. But, I also presume that past the first few such elliptical orbit experiments the aim will shift to have the spacecrafts set on distant (and most likely circular) orbits that make possible observations at any time and thus its target would be something akin to a horizon rather than a single point, wouldn't it?

650 AU is quite far away. To get there in a reasonable time frame (a few decades), you have to be going quite fast, on an escape trajectory out of the solar system. If you wanted to be on an elliptical orbit, the orbital period would be over five thousand years (and a circular orbit would be over sixteen thousand years). It's so far out that it's just not practical to try and move laterally.

The other nice thing about being on an escape trajectory is that because of how gravitational lensing works, rather than a focal point you get a focal line. So as the probe continues moving away from the sun, it can continue imaging its target (and the image quality may get better as it gets further out, since the ring gains more separation from the surface of the Sun.)

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