Inside the RF-container lies all the electronics that does the signal processing. Because there is so much of it, and it is so close to the antenna array, it is crucial that it is shielded to prevent interfering signals from escaping from these electronics to feedback into the system. Now that we've started taking routine calibration and test data, it is important to keep the RF-contain sealed against unwanted radio leaks.
The notice on the door reads: "INTERFERENCE HAZARD - The equipment in this RF-container can disrupt telescope observations. Do not open this door without first checking with the Operations Group."