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In the following EHT and GMVA diagram, what do the yellow and blue lines represent? I don't think they're physical communication lines (that would be unfeasible). Do they depict anything specific?

the locations of the participating telescopes of the Event Horizon Telescope (EHT) and the Global mm-VLBI Array (GMVA) Source: ESO/O. Furtak

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I cant see a definitive statement, but I can see some infiormation and make some reasonable suggestions:

  1. The two colours represent the two different collaborations. Blue for the EHT, yellow for the GMVA. We can see this by following the links on the source page of the image. The GMVA page says:

Participating Stations:

in Europe: Effelsberg (100m (Ef)), Onsala (20m (On)), Pico Veleta (30m (Pv)), Metsahovi (14m (Mh)), and Yebes (40m (Ys))

in USA: VLBA (8x25m) - BR, NL, PT, LA, FD, KP, OV, MK (HN, SC are not equipped with 3mm receivers) and the GBT (100m)

ALMA: for VLBI experiments involving phased ALMA the sensitivity depends on the number of antennas which are phased. For N=50 antennas and a typical phasing efficiency of 0.90 a gain of 1.3 K/Jy is obtained, and the effective diameter is ~ 80 m.

This matches the yellow lines.

This page describes the EHT array and more or less matches the blue lines.

So now what are the lines? I'm pretty sure they are "baselines". That is they connect pairs of telescopes whose signals are directly combined in great detail to produce information about the image. The data from the various baselines is then combined in a later step. This wikipedia article describes some of the physics. This paper refers to the individual baselines used for the EHT and their contribution to the image.

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