This is the LVDT (Linear Voltage Displacement Transducer) which measures the position of the dome relative to the antenna. This is the way we know were the dome is. This transducer is located under the sidecab, and can be seen easily from the first floor. If you ever have to move the antenna off of a dome-telescope limit switch, using the hand paddle, then you'll want to place this transducer in the center of its travel.