# How did Jocelyn Bell aim her radio telescope (phased array)?

In 1967 Jocelyn Bell discovered pulsars using the Cambridge University radio telescope - a two hectare field of posts and wires.

The pulsar was found to be in the constellation Vulpecula. How did she locate the pulsar? In other words, how do you aim a two hectare radio telescope consisting of posts and wires?

EDIT. I'll try to make my question clearer. I'm not a physicist and know zilch about radio telescopes. However, I've driven past the Mullard Radio Astronomy Observatory near Cambridge several times and seen their radio telescopes - a bunch of mini Jodrell Bank-type dishes. Now even I can imagine how you can aim one of these dishes towards a particular point in the sky. You crank the handle and point the dish to your target star or whatever, like you'd set up a TV satellite dish. What I cannot understand is how Jocelyn Bell was able to use a field of posts and wires to accurately locate a pulsar in Vulpecula. I've read that "The [radio telescope] output appeared on four 3-track pen recorders, and between them they produced 96 feet of chart paper every day." How did she know what portion of the sky these chart results referred to? I'm not asking for a technical explanation, just a plain English description of how she could home in on a particular point in the sky. Don't worry about making it too simple.

## 3 Answers

Jon Custer gave a correct technical overview; I'll have a go at a bare-bones conceptual description. The idea is to record the amplitude at each receiver in the array at a series of times, and then to offset the signals from the different receivers in time and look for a correlated signal. From the offset applied to get a correlated signal you can use simple geometry to get the angle to the source. I'll walk through it with a diagram:

On the left I show three time snapshots of a source emitting some signal (red dot). The source is a bit further from receiver 2 (green) than receiver 1 (blue), so the signal arrives a little bit later at R2. This is shown in the amplitude-time graph on the right. If the blue curve is offset to the right it will eventually overlap with the green curve, which would show up as a strongly correlated signal. The phase offset is, in other words, the time delay between the arrival of the signal at the two detectors. The time delay multiplied by the speed of light is the difference in the distances to the source seen by the two receivers. If this difference is zero then the source must be directly "above" and between the two detectors; the larger the difference, the larger the angle from this reference direction to the source.

Scale this up to more receivers spread out in two dimensions and you can locate a source on the sky. Of course there are some "technical details" which make this a bit harder to actually implement, but the principle is basically as above.

• So Bell would have first looked for an interesting signal and then when she'd found one would then look for the various offset "copies" of that signal in order to locate the source in the sky? Is that a fair summary? May 23, 2018 at 14:03
• @Peter4075 More likely the phase offsets would be "scanned" to "point" the telescope to different sky locations, and then look for interesting correlations at each position. This is more practical because a real signal has noise, so it's hard to pick out an "interesting" signal from a single feed. But basically, yes. May 23, 2018 at 14:16
• @Peter4075, that would be practical with modern software, but the sheer volume of data involved means it probably wouldn't be used in 1967. Rather, you'd dial in an offset corresponding to a specific part of the sky, and if there was something there, the individual small signals would add up to produce a single large signal. If there wasn't something there, the offset mismatches mean that signals from elsewhere would cancel out to near-zero.
– Mark
Jul 12, 2018 at 1:22

A phased array is an array of antennas which emits a beam of radio waves which are steered to point in different directions by adjusting the phase alignment of different rows and columns. But the same equipment can receive signals similarly.

Usually, a phased array sends pulses from different dipoles at phase differences, which eventually causes the beam produced to point in a certain direction. Kyle Oman's answer describes the way the Interplanetary Scintillation Array, which was used by Bell, uses the phase difference between signals to process a direction.

A misconception which I think led to the question is the idea that the dipole antennae were angled physically like cable TV dishes to focus on a particular region in the sky. The whole telescope wasn't physically turned and angled to focus on a particular region. It was designed to be able to scan a certain region of the sky, i.e. it was designed such that it gets the best look (highest sensitivity) at areas with a declination of approximately +30°. This was 'sweet spot' set up by using a reflective screen. There's nothing particularly specific to that: the position of the reflective sheets allows you to send the beam from the dipoles to different regions, but the sensitivity decreases as you deviate from +30°.

After this, the reverse of phase shifting processing techniques lets you find the direction of the received beam. The wikipedia page on phased arrays tells you about how a 2 dimensional array lets you sweep the areas with multiple degrees of freedom.

In essence, the telescope used a static structure of dipoles, and then used signal processing techniques to compare the results from different individual dipoles to locate the region of activity.

Other useful reading: https://en.wikipedia.org/wiki/Interplanetary_Scintillation_Array

@lalala pointed out in a comment that one way to understand how phased arrays work is by comparing them to the way ears deduce the direction of a certain sound. See Wikipedia: Interaural time difference

• This must be exasperating for you. Sorry. But, when you say "signal emitters", do you mean that the signal from outer space is picked up by one part of the telescope and then emitted by another part of the telescope? May 26, 2018 at 14:35
• Haha I evidently didn't edit my old one perfectly... I thought I was pretty thorough. The dipoles are essentially signal emitters in usual phased arrays, but here they receive signals in the reverse process. Kyle's answer discusses what they do with the signals.
– user191954
May 26, 2018 at 15:10
• Another inane question! Figures 1 and 2 in the original paper (researchgate.net/publication/…) show a single pulsating signal. What's happened to all the offset signals that Kyle talked about? May 26, 2018 at 16:26
• @Peter4075 My pleasure :) Those graphs show the equivalent of one of the lines (eg. the green line only) in Kyle's graphs. Both of Kyle's lines show the same waveform but with a phase difference between them. The source has emitted a particular waveform, but the same thing reaches the 2 observing dipoles at slightly different times. Since it's the same waveform, there's no point in publishing them in the paper: you'll end up creating a muddling mess. The phase offset Kyle highlights is only relevant when you want to find the location of the body, not the nature of the radiation.
– user191954
May 26, 2018 at 16:37
• Maybe to add, same principle is also used for our ears to locate sound (there are some other tricks, because with the phased array you cannot distinguish.front an back). Jul 11, 2018 at 14:54

The answer can be found in the original pulsar discovery paper, A. Hewish et al., Nature 217 709-713 (1968), where the array is discussed. The most relevant bits are:

The aerial consists of a rectangular array containing 2,048 full-wave dipoles arranged in sixteen rows of 128 elements. Each row is 470 m long in an E.-W. direction and the N.-S. extent of the array is 45 m. Phase-scanning is employed to direct the reception pattern in declination and four receivers are used so that four different declinations may be observed simultaneously. Phase-switching receivers are employed and the two halves of the aerial are combined as an E.-W. interferometer. Each row of dipole elements is backed by a tilted reflecting screen so that maximum sensitivity is obtained at a declination of approximately + 30°, the overall sensitivity being reduced by more than one-half when the beam is scanned to declinations above + 90° and below - 5°. The beamwidth of the array to half intensity is about ± 1/2° in right ascension and ± 3° in declination; the phasing arrangement is designed to produce beams at roughly 3° intervals in declination.

• That's way above my head. I've edited my question to, hopefully, make it clearer. May 21, 2018 at 15:34
• Basically, they set it up as a phased array, using time difference between signal reception from different portions of the antenna to get direction. The reflecting screens behind antennas helped increase directional sensitivity. May 21, 2018 at 17:32