In astronomical Interferometry, the Correlator is the device or software that interferes the waves to produce the interference pattern.
At visual Wavelengths, interference is typically done optically, but at longer wavelengths, the signal can be digitized to the point where digital signal processing in hardware or software can do the work. It may be done in real time, or for Very-long-baseline Interferometry (VLBI), it may be done by bringing the signal data (necessarily including precise timing) together as digital files later.
Back End Instrument
Front End Instrument