The focal area of the GBT as seen from the surface of the main reflector, showing outlined in red, bottom to top, (i) the receiver room; with (ii) the L-band feed horn pointing upward at (iii) the 8-m subreflector. Just below the lower edge of the subreflector the lower portion of a screen is visible. This functions to direct feed spillover radiation that would otherwise strike the telescope arm back into the main reflector and onto the sky.