correlator

In astronomical interferometry, the correlator is the device or software that interferes the wave-signals to produce the interference pattern.

At visual wavelengths, interference is typically carried out optically, but for long enough wavelengths, the signal can be converted to an analog electrical signal of the wave's oscillations, which can be processed (combining the oscillations in a manner to produce the interference) by analog electronics, or can be digitized and fed to software or digital hardware to do the processing. The device, digital or analog, doing this processing is termed the correlator. The processing may be done in real time, or for very-long-baseline interferometry, it may be done later, after bringing together (from the sites of the various receivers) the signal-data consisting of digital files that include very precise timing of the reception.

For most directions (those not perpendicular to a straight line between the antennas), the waves reach the antennas at different points in time, the antenna further from the source receiving each wave crest last, the period of time being termed a geometric delay. The identical delay can be imposed on the signal received by the nearer antenna using electronics, and the correlator can amplify those signals (voltage levels) that are received by both by multiplying the two, the correct delay producing the maximum output signal. The timing of the rise and fall of the resulting signals as the Earth turns (due to the receiving antennas' directions of highest sensitivity reaching and passing the actual direction of the object) offers the most precise data regarding position in the sky, i.e., is what yields interferometry's extremely high angular resolution: angular distances are derived from the time periods over which the changes occur.