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Assuming that it's possible to get the equipment (including accurate clocks) there in the first place, and that it's possible to get the data back afterwards, is it possible to do very long baseline interferometry in any useful wavelength range (radio is presumably easiest) over distances on the order of 1AU or more between the telescopes? How precisely do we need to track the telescopes' positions in space and time to be able to make it work?

A secondary question I have is whether it's feasible over interstellar scales, again assuming that you've already solved the problem of how to get the payload there in the first place. e.g. if we can insert a Kepler-like device into A. Centauri orbit, would that allow us to build a virtual telescope with a "baseline" of $~4.4ly$?

This question was originally inspired by watching Acapella Science's history of exoplanets while writing a sci-fi story involving (STL) interstellar travel and FTL comms, but I'd like to know whether there's any theorectical obstacle to doing it in standard physics, i.e. without the cheat of getting the data back to Earth nigh-instantaneously. The scenario I was envisaging involved a "fast" (i.e. $>0.95c$) spaceship - I don't know if that makes tracking the position of the telescope to the precision required infeasible for some reason.

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There is a space based inteferometer operating with a baseline about 2x the distance to the moon Spektr-r.

At wavelengths where you can measure the incoming signal phase directly (ie radio) and so do the correlation later digitally there is no real limit on the baseline. We used to do VLBI by post - shipping the data tapes back and having the photons interfere in the computer.

You do need to know the baseline, but as long as you can also detect an unresolved point source as well you can bootstrap the accurate distance.

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  • $\begingroup$ Could you expand on that last point? I'm reading "unresolved point source" to include any locally visible star(s), so that suggests that you should be able to determine the distance accurately merely from looking around yourself at the fixed stars, which seems a bit of a unrealistic "magic bullet" to me. $\endgroup$ – redroid Apr 10 '17 at 18:05
  • $\begingroup$ @redroid You need to know the spacing between the antenna to a fraction of wavelength. If you can't measure this directly (because they are millions of km apart) you can find the last fraction of a wavelength part by simultaneously measuring the pattern from a perfect dot (ie star) and working back from that - details are complicated! $\endgroup$ – Martin Beckett Apr 10 '17 at 18:34
  • $\begingroup$ Don't you need three receivers and then you can use phase closure techniques without having to know the exact position of each receiver. $\endgroup$ – ProfRob Nov 17 '20 at 17:50

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