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by autocorr
3445 days ago
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You're right in the last point, the raw data has such a high rate that they have to process it on the fly, which has its drawbacks. The reason that these instruments output so much data is that the process of correlating the signal between each unique pair of antennas is O(n^2) in the number of antennas. Now 36 antennas (when complete) is not very large, that's approximately the size of the VLA[1], but because of the very unique phased array feeds on ASKAP mentioned in the article, each antenna has not just one receiver, but a little camera of receivers. So if it's an 8x8 feed/receiver array, that's 64 times the data rate of a 36 antenna system. For comparison, I've had projects on the VLA that are about 10 GB/S, but they have only a single feed/receiver. At ASKAP I believe they have to process/"reduce" the data on the fly, which in the most extreme case (likely necessary for the real Square Kilometer Array) is to not store the raw correlations between antennas at all but to convert it to images immediately, which is lossy. Normally this reduction to images is optimized by carefully tuning the imaging parameters and iterating to find the best, but this can't be done if the "visibilities"/correlations have been thrown away. [1] Edit: saying that the Very Large Array does not have a very large number of antennas is actually pretty ironic. |
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