I've learnt that a setup of radio antennas could possibly be used as a radio telescope:

"A linear array of 10 radio antennae is set up as a radio telescope to view the sky at a wavelength of λ = 21 cm. Each antenna receives radiation uniformly from all directions towards the sky. Adjacent antennae are a distance d apart and feed signals to the same receiver via a mixer and identical cables of equal length."

My task is to: "assume the Fraunhofer approximation, obtain an expression for the angular distribution of the intensity, I(θ), received from a distant source of radiation whose line of sight is perpendicular to the array, where θ is an angle measured with respect to the normal to the array from its centre."

While this is from a homework problem, the trouble I am having here is understanding the concepts and description behind this problem. It would be great it someone could explain to me:

• How can this setup of radio antennae be used as a radio telescope?
• What exactly does the problem mean when they say "line of sight is perpendicular to the array"? (Wouldn't this automatically mean θ=0?)

Imagine you have three antennas $A,\, B$ and $C$ with separation $d$.
However you will note that the signals from each antenna are not in phase with one another except when $\theta = 0$.
This means that the size of the combined signal from each antenna depends on the angle $\theta$.