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The orientation of the interferometer arms at both sites are approximately Northeast-Southwest and Nortwest-Southeast, though I assume that, on account of the Earth's curvature, no pair of arms is exactly parallel. Is there a preferred orientation for the instruments, and is there a benefit in having them in (approximately) the same orientation? My guess on the latter issue is that if an interferometer has blind spots, it is better for those at both sites to be aligned (or at least overlap as much as possible), as a blind spot for one is a blind spot for the instrument as a whole.

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For the German GEO 600, I happen to know that the orientation (and the fact that the two arms do not form an angle of $\pi/2$) is mainly due to building site constraints.

As LIGO seems to be in the middle of nowhere, this might not have played as big a role. However, it seems that your assumption that most interferometers have a similar alignment is wrong - see here: http://www.ligo.org/scientists/GW100916/GW100916-geometry.html - the orientation changes actually quite a lot.

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  • $\begingroup$ Thank you for that informative link, but I am not following the last sentence. Could you explain how the orientation (of the arms, I assume, not of the plane they lie in) affects the triangulation? $\endgroup$ – sdenham Feb 13 '16 at 13:35
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    $\begingroup$ I had a look at it again and I think that this sentence was just false. It would only matter if all experiments were in the exact same spot, which they aren't anyway. The sentence was wrongly concluded from the analysis on the site I linked do. $\endgroup$ – Martin Feb 13 '16 at 14:33
  • $\begingroup$ I have just found this: physics.usu.edu/shane/classes/astrophysics/lectures/… . It mentions '+' and 'x' polarization, but to me, the diagram for one looks like the other rotated $\pi/4$. Diagram C seems to be suggesting that an interferometer aligned with its axes would not detect a signal, and if so, this might be a design consideration. I am thinking of asking another question about the angular and polarization sensitivity of an interferometer, which would be a precursor to this one. $\endgroup$ – sdenham Feb 15 '16 at 14:28
  • $\begingroup$ As the article says, the map of the orientation of the sites is distorted by the projection used. Using Google maps, I measured the angle of the arms of the LIGO sites to the great circle connecting them, and got 28 degrees for the NW-SE arm at Hanford, and 30 degrees for the NW-SE arm at Livingston. The sites are 1883 miles apart, so the planes of the two interferometers are at an angle of about 0.476 radians (27 degrees) to each other. $\endgroup$ – sdenham Feb 15 '16 at 16:08
  • $\begingroup$ Yes. If the wave is orthogonal to the signal, you won't detect anything. This is why I thought this might be the reason for them being NOT aligned - and it is true that you mustn't have two times the same experiment with exactly the same orientation. However, this will never be the case in reality anyway, because the two experiments lie on different points of the planet, so it will not be too huge a design concern. $\endgroup$ – Martin Feb 16 '16 at 15:04

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