Radiospectrographs do not produce images, however, and so they cannot be used to locate features spatially. This can make it very difficult to understand where a specific component of the solar radio emission is coming from and how it relates to features seen at other wavelengths. Producing a radio image of the Sun requires an interferometer, which in radio astronomy means an array of many telescopes that operate together as a single telescope to produce an image. This technique is a sub-type of interferometry called aperture synthesis. Beginning in the 1950s, a number of simple interferometers were developed that could provide limited tracking of radio bursts. This also included the invention of sea interferometry, which was used to associate radio activity with sunspots.