Looking Down the Revolver
The receivers of the Green Bank Telescope sit in a revolving cabin called the turret. The feed horns poke above the cabin, and their receivers hang beneath them in a suspended building that tilts with the telescope. When an astronomer wants to observe the longest wavelengths at the GBT, the telescope operator spins the largest horn into the beam of radio waves bouncing from the sub-reflector above the turret. The feeds are covered in a radio-clear fabric to keep debris and rain out, and blowers keep the fabric dry and free of obstructions.