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.
Accurate calibration is essential for producing high-quality images.
The image on the right was produced with uncalibrated data and contains no discernible features. Calibrated data was used to generate the left-hand image in which the overall shape of M82 is clearly visible.