Calibrating the Array Arrays must normally be calibrated to reduce errors caused by atmospheric and instrumental instabilities. Imagine trying to see a car a distance down the road on a hot sunny day. The hot air rising off the surface of the road wavers, causing the car's image to shimmy. Similarly, variations in atmospheric conditions cause stars to appear to twinkle when we look up at night.
Atmospheric instabilities make it difficult and sometimes impossible to obtain useful data.
Adding to this difficulty, spurious signals in the array's complex instrumentation can drown out the incoming signals from space. Doing so requires calibration. During calibration, the array is pointed at a familiar, simple and intense radio source such as a quasar . Knowing the magnitude of such errors, astronomers can subtract them from the data collected during an observing run. The goal of course is to image radio sources of unknown complexity as precisely as possible.