Interferometer delay & line light spreading

In summary, optical interferometers use collimated light from farther telescopes that is sent down the delay line and then into a beam combiner. However, the articles only show light entering along the axis, not at a small angle. The light may spread out over the 100m long delay line, but this is accounted for with extra lenses and mirrors. Lord Rayleigh's investigation of diffraction dictates that a large primary mirror is required for a long focal length. This means that the beam coming out of the telescope may be wider than expected, but it is still able to be reflected perpendicular to the output.
  • #1
In every article that I've read about optical interferometers they explain that collimated light from farther the telescope is sent down the delay line, and then into a beam combiner with other beam. Like in the photo below

41586_1998_Article_BF26172_Fig1_HTML.png

But they all only show light that enters the telescope along the axis. Not the light that would enter with some small angle, let's say from other star in the view. So what is happening in the delay line? The light must be spreading out. You can see my exaggerated situation below, with large angles.

beams.png


You can see that the light from small angle will go out the eyepiece on some angle, and after some length it would go past the, let's say some mirror that somewhere far to the right. I understand that we are using interferometer because what we want to see is separated by very very small angle, but still... Keck telescope delay line is around 100m long, I would imagine light would spread out over that distance.

Do they have to take care of this by using extra lenses and mirrors, or am I overthinking and light would not spread out that far?
 
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  • #2
Each telescope has a very long focal length, and the delay line is used as part of that. So the operation of the telescope is basically unaltered by the delay line. Of course, there is a maximum focal length that can be used, due to diffraction, as investigated by Lord Rayleigh. So a large primary mirror is required.
 
  • #3
Then the beam coming out of telescope should be pretty wide? I mean, right at that mirror that reflects light perpendicular (in my first diagram)
 

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