How many radio pictures have you seen? We know about normal optical photos, X-ray photos, infrared photos, but radio photos are not very common. Why not? Because radio cameras, having many ‘pixels’ are big and expensive. For high resolution the camera need to be several wavelengths big and for many pixel, you need many coherent receivers.
Antenna Arrays are basically a radio camera, where you have many small receivers. You can see it as having a pixel per receiver. A normal radio receiver is equivalent to a single pixel camera. However, when you have an array with 100 receivers, you can form a 100 pixel radio image.
You can still make a picture with a single receiver. Below is an image of the sky at 21 cm take by 2 ‘single pixel’ receivers, the the 100-m dish at Effelsberg/Germany and the 64-m Parkes/Australia telescope.
Note that this is an very expensive photo. It took thousands of hours of observation of two of the world’s largest radio telescopes. The telescope had to point to each position in the sky, one after the other.
Furthermore, the resolution is limited by the size of the telescope (Rayleigh criterion). The 100m telescope can not do better than about 0.1 degree resolution.