# How do ASKAP's focal plane phased array feeds interact with the entire array phasing?

Sky & Telescope's ASKAP Joins the Hunt for Mysterious Bursts says:

A new telescope, the Australian Square Kilometre Array Pathfinder (ASKAP), has joined the search for energetic and elusive fast radio bursts. And in just a few days of looking, it’s already had success!

A new player is now on the scene, however, and it’s already had huge success. ASKAP is a wide-field radio telescope made up of an array of 12-meter antennas. Using phased-array-feed technology, ASKAP is able to instantaneously observe an effective area of 160 square degrees — an enormous field compared to Parkes’ 0.6 square degrees! This capability significantly increases our chances of being able to detect fast radio bursts.

The article links to The Detection of an Extremely Bright Fast Radio Burst in a Phased Array Feed Survey which is not paywalled and contains some discussion of the array.

This sounds really interesting! It is a phased array of dishes, but at the focal plane of each dish there is also a phased array of receivers.

This is different than just adding an array of feed horns and independent receivers as I've discussed in my surprisingly robustly unanswered question What is the highest granularity focal-plane array on a dish radio telescope? Or is this the ONLY ONE? where receivers measure intensity as pixels, in a similar way to a camera (thus the counterintuitively placed astrophotography tag there).

Here, each focal plane is equipped with a true phased array, where if I understand correctly, phase relationships between elements in the focal plane are maintained and recorded.

According to Wikipedia and CSIRO's Australian Square Kilometre Array Pathfinder – Fast Facts there are 36 separate 12-meter dishes in the entire array, and the focal plane of each dish is equipped with a 188 element focal-plane phased-array. 188 $\times$ 36 = 6768 channels, not counting polarizations because I don't know if they are separate channels in this case.

That would require quite a hefty correlater if the analysis was done flat, without any heierarchy in the calculation. This is a pathfinder for the SKA so pushing limits is important, but I'd like to know how this large number of signals is handled. Is there pre-processing?

Question: How do the phased array feeds on each dish of the ASKAP interact with the entire array phasing?

I'm not sure of the "focal plane array" is located precisely at the focal plane or not. If it were, then a given source in the sky would produce signals in only one, or a small group of receivers, and not the whole focal plane array! One normally uses a phased array instead of an imaging system so there must be something more interesting going on here, perhaps only nearest and next-nearest neighbor correlations within the focal plane array but full correlation between the dishes?

How does this thing really work?