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It's been a little while since I dabbled in radio astronomy, but arrays like the VLA use interferometry to produce high resolution images. However, the high re

It's been a little while since I dabbled in radio astronomy, but arrays like the VLA use interferometry to produce high resolution images. However, the high resolution is only along the directions between pairs of telescopes. Since you don't have telescopes in all directions, there are some directions with low resolution and others with high resolution. (In the technical parlance they talk about "UV-coverage" and filling the UV plane".) This produces patterns in the Fourier domain that look like this: http://www.aoc.nrao.edu/~vlbacald/idx/snap_uv.gif http://www.aoc.nrao.edu/~vlbacald/idx/snap_uv.gif. [1] As the object moves in the sky, these dots move around and sort of "fill out" the image in Fourier space, so you get coverage that could look like this: http://www.aoc.nrao.edu/~vlbacald/idx/lowd_uv.gif http://www.aoc.nrao.edu/~vlbacald/idx/lowd_uv.gif but it's still not 100% full. The advantage of a telescope that is a single large dish like Arecibo is that this plane is completely filled. The resolution is more or less the same in all directions. The main disadvantage, of course, is that it is embedded in the ground and can only look at what is directly overhead. (They can maneuver the secondary dish to look at things a little bit away from directly overhead, but I seem to remember it only being a few degrees.) [1]: See this page for those images and more details: http://www.aoc.nrao.edu/~vlbacald/read.shtml http://www.aoc.nrao.edu/~vlbacald/read.shtml

mumblemumble 6y ago
Could this characteristic be mitigated by arranging the telescopes in a random pattern rather than using simple, regular pattern like the VLA follows? That seems like it might be what's going on with the UV coverage diagrams being demonstrated at the bottom of page 3 in this paper about the ATA: https://www.seti.org/sites/default/files/ATA-memo-series/memo15.pdf https://www.seti.org/sites/default/files/ATA-memo-series/mem...
- petschge 6y ago
  You can improve UV coverage a bit but removing redundant duplicate baselines. Which is what pseudo-random patterns do. And these are used e.g. by Lofar. But the problem of lacking very small baselines to fill in very large scales on the sky remains. Interferometer are simply not very good for imaging extended objects. And of course the other thing is that you might have less reflective area in total and thus lower sensitivity for dim things.
- watersb 6y ago
  It isn't a random pattern, but yes. A logarithmic expansion of baseline distances seems to be the first thing you want in a sparse array. Ideally you want to be able to look in all directions with about the same amount of sensitivity and angular resolution. But the UV coverage of even an asymmetrical, relatively sparse array like the VLBA is surprisingly good, because as the Earth rotates, the antennas describe sweeping arcs on the sky. The regular UV coverage of the VLA is great, though, for sensitivity at short duration observations. But Arecibo was great for pulsar work. :-(

mumblemumble 6y ago

Could this characteristic be mitigated by arranging the telescopes in a random pattern rather than using simple, regular pattern like the VLA follows? That seems like it might be what's going on with the UV coverage diagrams being demonstrated at the bottom of page 3 in this paper about the ATA: https://www.seti.org/sites/default/files/ATA-memo-series/memo15.pdf https://www.seti.org/sites/default/files/ATA-memo-series/mem...

petschge 6y ago
You can improve UV coverage a bit but removing redundant duplicate baselines. Which is what pseudo-random patterns do. And these are used e.g. by Lofar. But the problem of lacking very small baselines to fill in very large scales on the sky remains. Interferometer are simply not very good for imaging extended objects. And of course the other thing is that you might have less reflective area in total and thus lower sensitivity for dim things.
watersb 6y ago
It isn't a random pattern, but yes. A logarithmic expansion of baseline distances seems to be the first thing you want in a sparse array. Ideally you want to be able to look in all directions with about the same amount of sensitivity and angular resolution. But the UV coverage of even an asymmetrical, relatively sparse array like the VLBA is surprisingly good, because as the Earth rotates, the antennas describe sweeping arcs on the sky. The regular UV coverage of the VLA is great, though, for sensitivity at short duration observations. But Arecibo was great for pulsar work. :-(