[MrGLaDOS]

Arecibo and FAST are giant immovables dishes (essentially a parabolic valley covered by some electrically conducting material). The receivers suspended above these dishes are somewhat moveable, but that is about it. That means they can only observe a small region of the sky exactly overhead (this region changes together with the rotation of the earth). VLBI requires multiple telescopes significantly separated from each other in physical space (think thousands of km) to track the same object on the sky during an observation. Thus VLBI is hard/impossible for giant immovable dishes like FAST :)

[watersb]

> VLBI requires multiple telescopes significantly separated from each other in physical space (think thousands of km) to track the same object on the sky during an observation. Thus VLBI is hard/impossible for giant immovable dishes like FAST. :)

Good point... But the VLBI people I know are "here, hold my beer..." engineers. And astronomers will use Vegemite for thermal paste, if it lowers their Tsys in a tight spot.

If FAST-class dishes are cheap enough, you could build an assortment of them, pointing in different (fixed) directions. The primary azimuth would still train close to zenith, but the hard part is that suspended receiver assembly, the moving bits, a relatively heavy platform (that eventually fell into Arecibo's dish, after decades of minimal budget and maintenance:-( ... Dang, the Canadians have proposed large dirigibles, tethered like weather balloons, for the primary focus assemblies.

I miss that crowd. My first drive was the NRAO VLBA correlator, 23 (!) years ago. I'm out of the loop now.