The highest resolution camera is the Hubble's Advanced Camera for Surveys' High-Resolution Channel instrument, with a pixel resolution of 0.025 arcseconds. Suppose a giant star with a diameter 2000 times that of the Sun was a mere 120 light years away or so. The HRC would see that star as 20 pixel diameter circle. This is adequate to measure the star's diameter to within 10% accuracy. The HRC could resolve such a star as being more than a spot (larger than one pixel in diameter) at ten times that distance. For a star the size of the Sun, the star would have to be less than 0.61 light years away for the HRC to resolve it as being more than a spot.
Astronomers can do better with arrays of telescopes. Milliarcsecond resolutions are now feasible. At the milliarcsecond level, a sun sized star remains larger than a spot even at 15 light years distance.
The largest ground based visible light interferometers are around 650m baseline so have an angle resolution of 1:10^9 or 0.2mas.
You can resolve a nearby large star like Betelgeuse with a 4m telescope it has a diameter of around 55mas and is 600lyr away so an interferometer like SUSI would be able to resolve it at a distance 275x as great or about to the edge of the galaxy.
I looked up the SUSI website which seemed to be up-to-date and it says that although there is a baseline capacity of 650m, they currently only go up to 80m.
I guess that in the next few years they will probably upgrade and be able to measure large bodies at the edge of our galaxy as mentioned.
If SUSI currently has a maximum baseline of 80m then (after browsing the web) the CHARA possibly has the greatest resolution (0.5mas) with a baseline of 330m?
Most interferometers COAST,VLTi,Keck and Chara are aimed at imaging - so you have more than 2 telescopes.
SUSI was the only large 2 dish system I could think of.
These can take actual pictures rather than just measure diameters although they need more signal and so aim at nearer/brighter/larger stars and currently have smaller baselines. There is no real theoretical limit to the baseline it's just a question of engineering and cost.
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